Today this story in ScienceNOW tumbled across my feed:
What a terrible headline!
I mean, really, what were they thinking? Of all the mistakes of science writing, this is the worst -- sensationalizing a story with a pseudoshocking "question" to which the answer is obviously "no". It misleads readers about the process of natural selection.
Science, have you lost your mind and become Buzzfeed?
Likewise, BBCNews gets the story wrong, illustrating it with a photo of a bearded George Clooney and Ben Affleck:
The ebb and flow of men's beard fashions may be guided by Darwinian selection, according to a new study.
The more beards there are, the less attractive they become - giving clean-shaven men a competitive advantage, say scientists in Sydney, Australia.
Hello? Trends in beard lengths today are not caused by Darwinian selection, they are caused by culture. This isn't novel science, it is Alfred Kroeber circa 1918. These trends occur on a much faster timescale than a single generation. Of course, our attentiveness to trends might be a product of natural selection in ancient humans.
But that isn't the subject of the current beard study.
Let me step off the fainting couch about the headlines and look a bit deeper into the articles. The study described in the ScienceNOW story by John Bohannon should make a large segment of evolutionary psychologists very nervous. It demonstrates that research subjects score attractiveness in ways that may depend on incidental
The setup: they had a bunch of guys grow beards, taking photographs along the way. That resulted in a large array of photos of the same men, bearded, stubbly and clean-shaven. They showed the photos to a large sample of women and men (presumably undergraduates in psychology classes, although the study itself is not yet online). The experimental condition:
But there was a catch: The frequency of beardiness varied in each set of photographs, ranging from rare to common. Some subjects viewed sets of photos in which most of the men were clean-shaven, while others saw mostly the heavily stubbled or bearded versions, and others saw intermediate ranges of stubble and beardiness. If frequency-dependent selection plays no role in facial hair trends, the context shouldn’t matter.
But the context did matter. When facial hair was rare among faces, beards and heavy stubble were rated about 20% more attractive. And when beards were common, clean-shaven faces enjoyed a similar bump, the team reports online today in Biology Letters. The effect on judgment was the same for men and women.
OK, so people score relative novelty higher in attractiveness. That means whenever they are judging photos on "attractiveness" of a trait, they are likely to be filtering their assessments through the frequency of the trait. Our views on attractiveness are subjective, and depend on what else we're looking at.
So what's the problem? For years, psychologists have been examining "attractiveness" by asking undergraduates to look at photos. Sometimes the photos have been manipulated by averaging together the pixel values of a large number of portraits -- resulting in an "average" face. In other experiments, photos have been manipulated to represent more "masculine" or "feminine" forms, or to have slight asymmetries. The assumption underlying this research is that mate choice was very important in human evolution -- so important, that very slight psychological preferences toward a trait might be strongly selected.
The beard study gives a clear reason why this assumption is flawed. The effect of the environment on "preference" for bearded or unbearded men is everything. In the study, the environment is manipulated by the experimenter. In human societies, the relevant environment is manipulated by culture. If the environmental variance is so high, the evolvability of such preferences will be very low.
Others have pointed out that there is very little evidence that the preferences reported in such experiments actually correlate with mating behavior by the same population of undergraduates, let alone ancient humans.
The beard study points to an unrecognized frequency effect in this kind of research. Why do research subjects show a slight bias toward one kind of photo as opposed to others? We should now suspect that a slight variability in the frequencies of distractor variables might explain the significant but weak preferences exhibited in many such studies.
In any event, culture creates a powerful background to a person's responses to photographs in an experimental context. That background fluctuates rapidly on the timescale of individual lifetimes. The environmental variance that results from such shifting preferences makes it very difficult to imagine a dedicated adaptation to preferences about beards operating subconsciously in people today.
Popular Mechanics asks, "How Many People Does It Take to Colonize Another Star System?". The basic problem is that a multigenerational star voyage requires the trekkers to mate and reproduce many times while maintaining a limited population size. Too few people, and the colonists will rapidly lose genetic diversity by genetic drift.
The article starts by noting the work of anthropologist John Moore on the question. Moore concluded that the social structure necessary to prevent inbreeding was essentially that of clans or extended tribes of hunter-gatherers -- strong kin avoidance rules to prevent inbreeding and a population size of 150-300 people.
A new paper by Cameron Smith focuses instead on the worst case scenarios, concluding that the "safe" population size would be much higher:
Entire generations of people would be born, live, and die before the ship reached its destination. This brings up the question of how many people you need to send on a hypothetical interstellar mission to sustain sufficient genetic diversity. And a new study sets the bar much higher than Moore's 150 people.
According to Portland State University anthropologist Cameron Smith, any such starship would have to carry a minimum of 10,000 people to secure the success of the endeavor. And a starting population of 40,000 would be even better, in case a large percentage of the population died during during the journey.
A number as large as 40,000 people would enable the mission to approximate the effective population size of the entire human population of earth before 100,000 years ago or so. For reasons I've discussed many times (for example, "Cultural impedance, demographic growth, effective population size"), the effective population size of humans does not mean that the actual number of people in the ancestral human population was very small. With Pleistocene people, there were many processes that reduced the genetic diversity (and hence our estimates of effective population size) within a population of a relatively large actual population size -- on the order of a few hundred thousands of people.
Forty thousand is pretty small, but on a random-mating voyage of a hundred generations should basically approximate the Wright-Fisher population model. Smith further examines scenarios in which "catastrophic" events may affect the mission, greatly reducing genetic variation (or eliminating it). In these scenarios, a population dispersed across multiple "ships" would create a buffer, but each of those units has its own small population size issues, arguing for a bigger mission.
I'll take a deeper look at Smith's upcoming paper after the AAPA meetings. These future scenarios really help us think about the limits that existed in past human populations, which were less constrained in some ways but more so in others. Moore's approximations for a future "generation ship" mission incorporated social dynamics in ways that have clear parallels in the past (his ethnographic work focused on small village societies of Southeast U.S. native peoples). Smith's simulations refer to a larger-scale aspect of genetic drift.
The interaction of these two factors does not easily reduce to equations, but creates the most interesting anthropological questions. How much social control is necessary to maintain the viability of a colonizing population, not only genetic viability but also cultural viability? What is the balance between shared goals and practical needs?
I question the general assumption that such a mission would "need" to maximize the genetic diversity of the colonists. In fact, many potential groups of interstellar colonists might prefer to reduce their genetic diversity.
Imagine a small group of people with the sufficient motivation to divorce themselves from humans on Earth, launch across interstellar space for thousands of years, forcing their descendants to live within a tiny habitat, with the expectation that their common offspring will colonize a new planet a hundred generations hence. The kind of internal discipline necessary to motivate such a scheme is more like a cult than an open society.
Cults enforce cooperation by means of social isolation,
By increasing the relatedness of the population, they could enhance the incentives for cooperative behavior. In effect, people boarding the voyage on Earth would be assured that their descendants would not merely be notional descendants but in fact strongly genetically similar to them. Such groups don't want to board this ship with a random selection of humanity; they want to board with their cousins. That reduced level of genetic variation would generate a larger genetic payoff for each individual launching from Earth.
They are not going to create a microcosm of Earth's genetic variation. They're going to create a colony of clones.
UPDATE (2014-04-05): A number of Twitter commentators have suggested that you don't need to have so many people if you have a store of frozen sperm and egg cells. In essence, you could create a human version of the Long Term Evolution Experiment run by Richard Lenski. By unfreezing the eggs and sperm of previous inhabitants -- or unrelated eggs and sperm brought from Earth -- the colonists could add whatever genetic variation is required, or "rewind" the colony to a previous gene pool.
That can be done with today's technology. We may question whether freezing is really a viable strategy across 1000 years. As yet we only know that freezing works over 20 years or so, and we don't have good statistics yet about whether germ cells or embryos frozen over longer time periods have any increased chance of mutations or other long-term effects. Still, the level of risk already faced by interstellar voyagers is likely much larger than a slight increase in risk from long-term freezing of germline tissue.
It would make perfect sense to have a large store of adaptive variants available to deal with whatever challenges the colonists face on their new world. Imagine that they settle a world with only two-thirds of Earth's sea-level atmospheric pressure. An influx of frozen Tibetan sperm would bring in genes to adapt the colonists to their hypoxic world.
Of course, we may also consider that a starship 50 or 100 years from now will be leaving an Earth with vastly greater genetic engineering potential than we currently possess. Colonists after the ninety-eighth generation might vastly prefer a bit of genetic tinkering to their own gene pool, instead of unfreezing vastly different DNA from an Earthling stranger. In that sense, the colonists will not need either a large population or a giant frozen sperm vat. They can build as they go.
This brings us back to social dynamics. The colonists must maintain their motivation and ability to put the colonization plan into motion as they arrive at their destination. Death of the colony is not the only risk; their culture may slowly devolve until they are nothing but interstellar lotus-eaters. We don't know how large a cultural group is necessary to maintain the necessary traditions over a thousand-year voyage.
That seems like an interesting problem.
Zach Zorich has written an interesting article for Nautilus, about the optical illusions caused by firelight flickering across parietal art: "Early Humans Made Animated Art".
From the article:
When Lascaux cave was discovered in 1940, more than 100 small stone lamps that once burned grease from rendered animal fat were found throughout its chambers. Unfortunately, no one recorded where the lamps had been placed in the cave. At the time, archeologists did not consider how the brightness and the location of lights altered how the paintings would have been viewed. In general, archeologists have paid considerably less attention to how the use of fire for light affected the development of our species, compared to the use of fire for warmth and cooking. But now in Lascaux and other caves across the region, that’s changing.
We can take the idea of "getting into the mind of the artist" overboard. One of the main techniques to depict motion in artwork is "superposition" -- drawing a body part of an animal in two or more different positions, to imply the motion from one position to the other. That's a technique commonly used in cartoons today by artists. But it's also done by children doodling.
Much of the impression of power and mystery comes from place. Ancient humans added to this substantially with their artistic sense.
In February, I revisited the 1964 definition of Homo habilis by Louis Leakey, Philip Tobias and John Napier: "Leakey, Tobias and Napier on the definition of our genus". The paper is remarkable because it represents the first major attempt to enlarge the anatomical definition of our genus. In that review, I referred to Bernard Wood's work on the Koobi Fora fossils. I also pointed to Wood's later essay, published with Mark Collard, that attempted to shrink the definition of Homo, expelling Homo habilis from our genus entirely.
I still have a post in the queue reviewing that paper closely. But in the meantime, Bernard Wood has published a new essay in Nature commemorating the fifty-year anniversary of Leakey, Tobias, and Napier's paper: "Human evolution: Fifty years after Homo habilis".
Wood's essay also touches on the issue of enlarging our genus. He adds a perspective on the role of OH 5 ("Zinjanthropus") on this problem of the scope of Homo:
Because Nutcracker Man was found in the same layers as the stone tools, the Leakeys assumed that it was the toolmaker, despite its odd appearance. But when Louis announced the discovery, he was not tempted to expand the definition of Homo. That would have eliminated any meaningful distinction between humans and australopiths. Instead he erected a new genus and species, Zinjanthropus boisei (now called Paranthropus boisei), to accommodate it
That is an important idea. I should note that the reduction of Australopithecus into Homo was not an insuperable barrier. By this time, the surfeit of genus names had begun to embarrass those anthropologists who were trying to adopt the ideas of the modern synthesis. Increasingly, Paranthropus, Telanthropus, Atlanthropus, Cyphanthropus, and the like were rejected by the field's young vanguard. At the time that Mary Leakey found Zinjanthropus, only a minority had begun to use Homo erectus in the place of Pithecanthropus. Only eight years earlier, Ernst Mayr had staked out the position that all hominins should be lumped into Homo. That was the range of views, with a larger, more expansive Homo slowly gaining ground among theorists. But that argument would be much easier with the extremely humanlike reconstruction of OH 7 in 1964, than with OH 5 in 1959.
Wood recounts his own experiences analyzing the Koobi Fora fossil collection, during which he became convinced that the Homo habilis sample actually includes two species. He was not the scientist who named Homo rudolfensis, but his analysis was the first to give it teeth (literally), as he staked out a collection of additional mandibular and cranial specimens in addition to KNM-ER 1470.
Today, Wood believes that the Homo habilis and Homo rudolfensis samples should be placed into yet a third genus.
Although H. habilis is generally larger than A. africanus, its teeth and jaws have the same proportions. What little evidence there is about its body shape, hands and feet suggest that H. habilis would be a much better climber than undisputed human ancestors. So, if H. habilis is added to Homo, the genus has an incoherent mishmash of features. Others disagree, but I think you have to cherry-pick the data8 to come to any other conclusion. My sense is that handy man should belong to its own genus, neither australopith nor human.
I don't necessarily disagree about the mishmash of features. But I don't know which specimens Wood has in mind when he says that H. habilis is larger than A. africanus, unless he is referring to brain size and not body size.
At any rate, proposing a name for such a third genus is probably fruitless under the rules of taxonomic nomenclature. The species name habilis was erected as a species within Homo, while rudolfensis was initially suggested as belonging to Pithecanthropus (which almost everybody considers to be synonymous with Homo, and has the type specimen of Trinil, now part of Homo erectus for most scientists. Homo habilis and Homo rudolfensis may not even be sister species, so it would be nonsensical to name a new genus just based on the assumption that they are monophyletic. Wood and Collard did not provide a new genus name for them; they preferred to put H. habilis and H. rudolfensis into the existing genus, Australopithecus. And Wood acknowledges that many anthropologists think that these two species should be collapsed into one -- and some think they both should be collapsed into Homo erectus.
Wood does not mention in his essay the one significant species with a mosaic of australopithecine-like and Homo-like anatomy: Australopithecus sediba. To me, this is the most interesting comparison with the classic Homo habilis definition. Whatever we include in Homo habilis (or break off into Homo rudolfensis) we have no comprehensive anatomical sample across the skeleton. We don't have any idea what a Homo habilis pelvis would be like, nor can we exclude that known pelvic remains like KNM-ER 3228 may be Homo habilis (or Homo rudolfensis). The pelvic anatomy of A. sediba is more Homo-like than the pelvis of A. africanus or A. afarensis, the hand has features that are more Homo-like than the type specimen of Homo habilis. We don't know what a Homo habilis femur looks like. Many have argued that the OH 62 skeleton is Homo habilis -- with its fragmentary proximal femur -- but that depends on an argument about its similarity with STW 53, a specimen that is only Homo habilis by the most generous stretch of taxonomic liberalism.
Consider the problems posed by the Malapa sample in 2010, compared to the Olduvai Homo habilis sample in 1964. The Homo habilis sample presented a larger brain size than known australopithecines that was nonetheless smaller than any known sample of Homo, a human-like hand and foot with some primitive features, and teeth with a handful of features that distinguished them from the known australopithecine (A. africanus, A. robustus and A. boisei) sample. The next thirty years showed that the hand and foot evidence were at best uncertain, leaving the teeth and brain. Malapa initially produced two skeletons, with an australopithecine like brain size, body size, limb proportions and foot, but a series of Homo-like details in the pelvis, teeth, and skull. We now know that both samples represent anatomical mosaics of primitive and derived characteristics. Leakey, Tobias and Napier used their small set of evidence of shared features to enlarge the definition of Homo. Berger and coworkers declined to further enlarge the definition of Homo on the basis of new shared features, instead emphasizing the overall australopithecine-like adaptive pattern represented by the brain and body size.
What a mess early Homo is! Wood draws from this mess the conclusion of repeated divergence and speciation with widespread parallelism.
The ongoing debate about the origins of our genus is part of H. habilis's legacy. In my view, the species is too unlike H. erectus to be its immediate ancestor, so a simple, linear model explaining this stage of human evolution is looking less and less likely. Our ancestors probably evolved in Africa, but the birthplace of our genus could be far from the Great Rift Valley, where most of the fossil evidence has been found.
Far from the Great Rift Valley. Hmmm...
Wood B. 2014. Human evolution: fifty years after Homo habilis. Nature 508:31-33. doi:10.1038/508031a
Becca Peixotto has two updates on the Rising Star Expedition blog today, describing some of the excavation activities this week. In "What's new at this week's excavation", she highlights the smaller scale of the dig and gives some insight about how the cave has changed at the end of this rainy South African summer.
The late summer in the Cradle of Humankind this year was remarkably rainy. In karst regions like this, rainwater does not stay long on the surface in creeks or rivers. Instead it quickly seeps through cracks in the dolomite.
The water sometimes pools in the caves, like the chilly puddle at the narrowest squeeze in the Postbox belly-crawl. No staying dry in that one! The water may hang in the air like it does in the final chamber where there is no standing water but where the humidity registers on our air monitors at 99.9%, or the rainwater may become part of an existing drip, dissolving the dolomite and slowly depositing calcium carbonate as a stalactite or other speleothem.
Becca's second post, "Young Visitor Helps Recover First Top Jaw From the Site", describes the visit of a young "Reach for a Dream" participant to the site, where he helped direct the excavation through the comms.
She then turns to this week's progress:
We accomplished our initial goal of recovering the maxilla (the part of a skull containing the upper teeth) and long bone that have been calling out to us since their initial uncovering four months ago.
This long bone was one of the earliest pieces to be uncovered, but its size and orientation prevented easy removal throught out the November dig because with each bit revealed, other bones were found on top or adjacent to it.
That accomplishes the first of the week's excavation goals, and further new fossils have come out.
Ten years ago I published a paper on the failure of cladistics to resolve questions of early hominin relationships. My study used computer simulation to produce a very large number of small "fossil" samples drawn from populations that evolved entirely under random genetic drift, with every anatomical character accurately measured and independent of every other character. This scenario was unrealistically good in many respects compared to the real fossil record, where characters are not independent, can be distorted by postdepositional processes, and often evolve in parallel under natural selection. What I found is that many of the small samples in the hominin fossil record are not good enough to test hypotheses about their phylogeny.
The tests in this paper show that parsimony recovers a correct phylogeny in nearly 100% of cases where either sample sizes or the number of independent characters are large. But for the foreseeable future, most hominid taxa will be known from only very small samples, and there can only be a very limited number of independent characters observable on fossil skeletal remains. This paper shows that simple parsimony in such cases will often fail to obtain correct results, and the lack of statistical tests for sample adequacy in phylogenetics has meant that until now, paleoanthropologists have not commonly known to what extent their phylogenetic models are falsely influenced by the factors examined here. Many paleoanthropologists may understand that small sample size, correlations among characters, heterogeneity of samples, and other issues pose barriers to phylogenetic research, but nevertheless may feel that cladistics analyses of fossil hominids provide successively better approximations of the truth. However, the results of this paper show that the output of parsimony analyses does not follow the innate statistical instincts that most researchers may have developed in other analytical contexts; indeed, they can be paradoxical, as discussed below.
Large samples work. Small samples mislead. Worse, including small samples into a study with large samples can lead to incorrect arrangements of the large samples.
This positivist outlook is reflected by a common, but fallacious, perception: that phylogenetic research has been converging on the “correct” answers, with the “problem” preventing stable evolutionary trees to be drawn being the continual appearance of new specimens and species. But if the samples available to test hominid phylogenetic hypotheses were statistically sufficient, then analyses would be very unlikely to change when new specimens or species were added. Recent discoveries of early hominids confirm the substantial possibility of change in the current most parsimonious phylogenetic hypotheses. For example, the possible addition of Kenyanthropus (Leakey et al., 2001) as a sister taxon to H. rudolfensis would either remove H. rudolfensis from the Homo clade or it would remove the Homo clade as a sister to Australopithecus. In any event, the topology of basal nodes in the phylogeny (including relationships that are in complete consensus among pre-1999 cladistics studies) could be completely rearranged. That this might occur on the basis of the few apparently derived similarities between two specimens, KNM-WT 40000 and KNM-ER 1470, is strong proof of the statistical weakness of the data. It also implies that even the interrelationships of relatively large samples such as those assigned to A. afarensis and H. habilis may be contingent on the most parsimonious arrangement of other quite small samples. We can expect that other new hominid taxa, including Orrorin, Ardipithecus, Australopithecus garhi, and possibly Sahelanthropus, will therefore further disrupt our previous understanding. With the addition of each new taxon, the number of possible hominid phylogenies grows exponentially greater, and with this number grows the number of ways that phylogenies may be in error.
Since 2004, many paleoanthropologists have done better acknowledging the weaknesses of parsimony analysis. Most substantial discoveries (for example, Ardipithecus ramidus in 2009 and Australopithecus sediba in 2010) have been published with cladograms placing them among known hominin samples. But results have been very cautiously discussed in these cases, emphasizing the drawbacks of other, less-complete specimens in earlier studies of hominin phylogeny. In these cases, specimens that preserve both cranial and postcranial remains have shown how biased the study of purely cranial characteristics can be.
What these examples do not present -- at least not yet -- is more than one or two specimens for most characters. So they underrepresent the variability within species, preventing us from telling with characters are fixed, and which vary. Small sample size remains a severe constraint on our ability to test hypotheses of relationships. What we know about early hominins depends disproportionately on the Hadar, Sterkfontein and Swartkrans samples -- and the attendant assumption that each of these samples mostly represents a single species assemblage. In each case, the variability represented is very extensive, showing us the limits of understanding mixed-species assemblages like that represented in the Turkana basin between 2 million and 1.5 million years ago.
Our knowledge of large hominin samples is very good, and we can be fairly confident about their relationships. But even in those cases, there is ambiguity. For example, is A. africanus closer to Homo than A. afarensis? That depends on how we constitute the samples and which characters we include. The latter question seems deceptively simple -- include everything! But the more we include, the more we must rely on singular specimens.
At an extreme, we turn to features like the upper-to-lower limb proportion. This would seem to have strong adaptive relevance, and the lower limb is clearly relatively longer in humans and Homo erectus compared to earlier hominins. Many scholars have argued that the upper-to-lower limb length ratio in AL 288-1 (Lucy) is more humanlike than in several later australopithecine skeletal specimens (including OH 62, often attributed to Homo habilis). But until recently this was the only skeleton with both upper and lower limb elements sufficiently preserved to estimate length. To compare other "species", researchers were forced to compare the dimensions of joint surfaces, or to estimate bone length based on regressions from joint dimensions or small portions of bone shafts. The discussion of OH 62 has been particularly protracted, with some scholars arguing for humanlike proportions and some for more apelike proportions, on the same bone fragments. In other words, the question comes down to "character analysis" -- the detailed consideration of how the character develops, how it varies within samples, and how it should be scored on fossil specimens. As long as we are counting characters independently in our cladistic study, without considering sample sizes for those characters or the confidence in the character analysis for those characters, our comparisons will be limited to the accuracy of the smallest samples.
Often people have argued that previously-unknown results are credible if a study replicates other results on which prior work largely agrees. That is, credibility can be judged as a function of consistency with earlier work. I considered this issue in my 2004 paper:
It was no surprise, for example, that A. robustus and A. boisei were grouped as sisters in most cladistic analyses, or that A. afarensis was an outgroup to later hominids, or that H. habilis and H. rudolfensis were often grouped with later Homo. The original descriptions of the fossils pointed out the derived resemblances in each of these cases, and there has been relatively little disagreement on any of these points since the fossils were unearthed. Although the inclusion of these well-documented sister-group statements may be a minimum standard of credibility for a cladogram, they convey no necessary confidence in the results of the method for new, unknown, or disputed relationships. Different cladistic analyses of fossils do not sample different possible worlds in the same way as the simulations presented in this paper; they apportion a single set of observations in different ways. Because the observations are the same, the results must agree—absent differences in character analysis or parsimony assumptions—and we should expect unanimity of analyses even if they are statistically inadequate.
Small samples may lead to wrong results, and they are likely to lead to the same wrong results no matter how many times we look at them. The only way to do better is to increase the sizes of samples.
None of this means we shouldn't use parsimony approaches. But we should pay much more attention to the results from analysis of larger samples. And we should be very critical of the composition of those samples. Particularly bad are the surface lag deposits representing landscapes that may have had multiple species on them. In these cases, each specimen may have hundreds of thousands of years of uncertainty in its provenience, and may be attributed to a "species" based on nothing more than the local abundance of dental remains across a half-million year span.
Hawks J. 2004. How much can cladistics tell us about early hominid relationships? American Journal of Physical Anthropology, 125(3), 207-219. doi:10.1002/ajpa.10280
Ann Gibbons reports from a recent conference in Spain about new work that has sequenced a whole genome from a 45,000-year-old femur from Siberia: "Oldest Homo sapiens Genome Pinpoints Neandertal Input". The femur as yet is a context-free find from a riverbank, so it isn't correct to call it an "Upper Paleolithic" specimen, though its radiocarbon date puts it into that time frame in this region of the world. The overall genome of the specimen is similar to living people rather than Neandertals, and the investigators (led by Svante Pääbo) are calling it the earliest modern human specimen to produce a whole genome so far.
Because it is a report on a conference presentation, there are very few useful details. This is the most interesting of the results reported:
Because all living people in Europe and Asia carry roughly the same amount of Neandertal DNA, Pääbo's team thought that the interbreeding probably took place in the Middle East, as moderns first made their way out of Africa. Middle Eastern Neandertal sites are close to Skhul and Qafzeh, so some researchers suspected that those populations were the ones that mingled. But the team's analysis favors a more recent rendezvous. The femur belonged to an H. sapiens man who had slightly more Neandertal DNA, distributed in different parts of his genome, than do living Europeans and Asians. His Neandertal DNA is also concentrated into longer chunks than in living people, Pääbo reported. That indicates that the sequences were recently introduced: With each passing generation, any new segment of DNA gets broken up into shorter chunks as chromosomes from each parent cross over and exchange DNA. Both features of the Neandertal DNA in the femur suggest that the Ust-Ishim man lived soon after the interbreeding, which Pääbo estimated at 50,000 to 60,000 years ago.
Without details, there's not much I can say. There are a lot of controls I'd like to see, but as described here it is not an unexpected result. A slight increase in the representation of the Neandertal DNA coupled with more resolution on the timeline of introgression.
I will point out that the methods used to detect chunks of Neandertal DNA work better with longer chunks. So the result described here is a rather tricky one. Much here seems to depend on the assumption that there was only one time that Neandertals contributed DNA to later populations.
That's not the assumption I would start with.
I'm not in South Africa this week but I am following closely as a small team of excavators is underground in the Rising Star site. I've posted the agenda for the week's work at the Rising Star Expedition blog: "A critical piece of the hominin puzzle".
The initial goal of this work is to recover a hominin maxilla that is exposed in the original “puzzle box” excavation area. As the November expedition was drawing to a close, the excavation team uncovered this maxilla. When they were working that area, the excavators carefully cleared around each fragment of bone before removing it. That’s how they first uncovered the maxilla. Although they could carefully work around it, they couldn’t bring it out of the sediment because of the overlying bones. It broke everyone’s hearts to leave that piece in situ, but at the time, we estimated it would be at least two additional full days of careful excavation work to bring it out.
The team will soon see whether that was right, or whether it was an underestimate!
The short excavation this week is helping to lay the groundwork for the upcoming May workshop, which will produce the initial descriptions of the fossil sample. More than 25 early career scientists have accepted positions in the workshop, from at least 11 countries (and I have a feeling I am missing one or two countries in there). It is an accomplished group and I am looking forward to seeing them working with the fossils!
The rapidly changing field of ancient DNA has settled into a kind of normal science, as several teams of researchers have coalesced around a set of approaches to discover the genetic relationships among ancient peoples. Ewen Callaway this week in Nature profiles some of the key investigators and their recent work: "Human evolution: The Neanderthal in the family".
The headline is driven by Neandertals and the successful sequencing of even more ancient DNA from Sima de los Huesos. But the quest for the most ancient DNA is maybe the less interesting of the two developments discussed in the article. The other is the theoretical paradigm that attempts to break down the genomes of living and ancient people into parts that come from different original populations:
A few years ago, David Reich discovered a ghost. Reich, a population geneticist at Harvard Medical School in Boston, Massachusetts, and his team were reconstructing the history of Europe using genomes from modern people, when they found a connection between northern Europeans and Native Americans. They proposed that a now-extinct population in northern Eurasia had interbred with both the ancestors of Europeans and a Siberian group that later migrated to the Americas6. Reich calls such groups ghost populations, because they are identified by the echoes that they leave in genomes — not by bones or ancient DNA.
Ghost populations are the product of statistical models, and as such should be handled with care when genetic data from fossils are lacking, says Carlos Bustamante, a population geneticist at Stanford University in California. “When are we reifying something that's a statistical artefact, versus when are we understanding something that's a true biological event?”
In the case of the putative ancestral connection between European and Native American groups, the ancient Mal'ta specimen from near Lake Baikal appears to confirm the hypothesis that an ancient group really did exist that contributed to both present-day groups. The advantage of the "ghost population" approach is that it does make clear predictions that can be tested with ancient DNA. Probably the most famous at this moment is the hypothesis that a very ancient "ghost population" must account for some fraction of the ancestry of the Denisova genome.
I think in many cases that "ghost population" approach is too simplistic. It is always possible to split a population into some number of dissimilar parts, but it's not obvious what the most parsimonious scenario should be. One possibility is two "pure" ancestral populations that mixed together, but there are many other possibilities -- including a single geographically dispersed population that coalesced across its range. Mathematically, the two "pure" population scenario is simpler, and it does capture some parts of the evolutionary divergence of populations. But simpler math doesn't necessarily make a more parsimonious hypothesis. When we ignore the archaeological and skeletal records in favor of math, we miss lots of information that might help shape these hypotheses.
But then, that's where anthropologists are important to understanding the past. It is interesting to see the jockeying among geneticists for new results, but you start to notice how they are describing models that don't describe any archaeological reality!
Ed Yong, writing for the new Wellcome Trust-sponsored science publication, Mosaic, has gone to Thailand to follow the development of artemisin-resistant malaria: "How malaria defeats our drugs".
Nosten thinks that without radical measures, resistance will spread to India and Bangladesh. Once that happens, it will be too late. Those countries are too big, too populous, too uneven in their health services to even dream about containing the resistant parasites. Once there, they will inevitably spread further. He thinks it will happen in three years, maybe four. “Look at the speed of change on this border. It’s exponential. It’s not going to take 10 or 15 years to reach Bangladesh. It’ll take just a few. We have to do something before it’s too late.”Hundreds of scientists are developing innovative new ways of dealing with malaria, from potential vaccines to new drugs, genetically modified mosquitoes to lethal fungi. As Nosten sees it, none of these will be ready in time. The only way of stopping artemisinin resistance, he says, is to completely remove malaria from its cradle of resistance. “If you want to eliminate artemisinin resistance, you have to eliminate malaria,” says Nosten. Not control it, not contain it. Eliminate it.
The article has a fascinating story about the work of a physician and a researcher, both with years of experience in Southeast Asia fighting malaria outbreaks.
One of my video lectures is titled, "Everything I know about human genetics I learned from malaria", reflecting the importance of the parasite in human evolution. As I read this article, I wondered about other aspects of human-malaria interactions not covered by artemesin resistance, especially the importance of the hemoglobin E variant in this part of the world.
I was curious about the use of Homo ergaster over time. It seems to me that fewer and fewer paleoanthropologists have been using it over the last few years. My perception (I'll share later) is not borne out by the Google Scholar statistics: More scientific papers during the last five years have mentioned Homo ergaster than in any previous period.
As I was looking, I checked out Google Ngram Viewer and found this alarming result:
Google Ngram Viewer tracks the use of words or phrases in books, and goes only up to 2008. It's pretty striking to see Homo ergaster had nearly overtaken Australopithecus afarensis in book mentions. These are not only popular books, they are also textbooks and technical books, so it's hard to generalize about the causes of the trends, which are overall very small compared to other hominin species (such as Homo erectus).
Here is another interesting comparison, considering that the main descriptive papers about Ardipithecus ramidus were not released until 2009:
Some of the decline in Ardipithecus ramidus after 2000 may have been caused by the availability of Orrorin tugenensis and Sahelanthropus tchadensis as "first hominins" -- few popular books about human evolution can avoid naming the earliest known hominin fossils.
Homo habilis and Homo erectus dwarf the popular usage of any of these other species names. Books have used Homo habilis three times more than Australopithecus afarensis at its peak, and Homo erectus appears six times more often.
The rise of Homo ergaster after 1990 is in part explained by the rise of popularity of cladistics in paleoanthropology. That trend is also evident in two other species names:
By 1980, the names Homo neanderthalensis and Homo heidelbergensis were at the nadir of a five-decade-long decline. Homo neanderthalensis was discarded mainly for the non-taxonomic "Neanderthal" (or "Neandertal"), while Homo heidelbergensis lost ground partly to "early Homo sapiens", and later (after 1970) to "archaic Homo sapiens". But after 1990, cladists had revived those formal taxonomic names, arguing that both represented real species of Pleistocene humans. The "out of Africa" perspective on modern human origins helped to promote the hypothesis that the formation of new species was a common mechanism of evolutionary change throughout the Pleistocene. Homo heidelbergensis was the foremost beneficiary of this viewpoint. My readers know that I don't like either of these names and prefer the non-taxonomic equivalents.
The same rise of cladistic methodology and its accompanying philosophy has spurred the use of the term "hominin" instead of the earlier term, "hominid". By 2008, "hominin" was used approximately one third as often as "hominid" according to the Ngram Viewer.
Archaeologists and agronomists have accomplished a lot to understand the relationships of today's domesticated crops and their wild progenitor species. But the people who domesticated plant species in the early Holocene were not working with today's atmospheric CO2 and temperature regime.
That's important when considering the effectiveness of early domestication efforts. The end of the last glaciation saw a worldwide increase in atmospheric CO2 level, and a general warming of the climate in the regions where agriculture first appeared. The wild progenitors of today's crop species may have had very different productivity with the lower atmospheric CO2 and lower temperatures that characterized the climate 15,000 to 10,000 years ago.
Dolores Piperno and colleagues (2014) investigated the productivity of teosinte raised artificially in greenhouses with controlled CO2 and temperature regimes. They replicated the lower level of CO2 and lower temperature
It, therefore, becomes of considerable interest to ask if, during the late-glacial and early Holocene periods (c. 16–10 ka) when people first encountered, exploited, and cultivated many of the wild progenitors, the plants differed from modern wild populations, influencing crop plant evolution in ways that have been little considered. The last hunters and gatherers and first farmers worked with the phenotypes they saw, and it can be imagined they were attuned to and interested in the phenotypic variability they encountered on natural and cultivated landscapes. Unfortunately, chronologically-coarse and often geographically-uneven archaeo-botanical records do not adequately capture the range of phenotypic attributes that early cultivators experimented with. Moreover, the macrofossils (seeds, fruits, stems) that can best inform this question are often poorly preserved and have as yet to be recovered from Late Pleistocene and early Holocene records for many wild progenitors and earliest cultivars, including Zea (e.g., Piperno et al., 2009 and Piperno, 2011). Thus, modern representatives of crop plant ancestors constitute the basis for much of the morphological and genetic study of proto-domesticates and their wild ancestors.
Piperno and colleagues found two very interesting results:
The teosinte was less productive under the low CO2, low-temperature regime that approximated the Late Glacial climate of Mexico.
The teosinte raised under Late Glacial conditions had some phenotypic characteristics that resembled maize, that are typically not found in teosinte grown today or in those grown under Early Holocene conditions.
The second is the more newsworthy result. Teosinte is phenotypically very different from maize. The transformation of this wild crop into maize has been one of the most fascinating problems in studying the domestication of plants anywhere in the world. The idea that wild teosinte may have been more like maize in the past is pretty provocative.
This result is what had me reviewing the paper carefully. What the authors describe is a response rooted in phenotypic plasticity, in which a small proportion of the teosinte plants grown under Late Glacial conditions are more maize-like in some characteristics:
In every grow-out some of the plants in the chamber with late-glacial conditions (hereafter, referred to as the LGC) were complete maize-like phenotypes in branching architecture and inflorescence sexuality; like maize, they had a few, very short (non-measure-able) lateral branches tipped by female ears instead of tassels, that were attached directly to a single main stem tipped by a tassel (Fig. 3A) (hereafter, these plants with maize-type branching and inflorescence sexuality traits are referred to as “maize-like phenotypes” or MLPs). A total of six plants out of 33 from all the grow-outs combined representing every population studied had these characteristics (one plant from population 3 in 2009; one from pop. 4 in 2010; and one each from pops. 1 and 2, plus two from pop. 3 in 2011).
The maize-like phenotypes also exhibited a seed maturation strategy characteristic of maize, with most of their seeds maturing at the same time. In contrast, in other plants in the LGC and MCC, as in the wild, seeds matured sequentially from the tips of the branches to the base over a period of about two months, requiring several “harvest” trips to collect them before they began to fall off the plant soon after maturation.
Many models for the domestication of plants in the Old World have focused on the incidental effects of collecting. For example, many wild grain species have shattering ears -- their grains easily pop off the ears to seed naturally near the mother plant. Those seeds would be much less likely to be collected by humans who were collecting the wild grains. If those people had extra seeds at the end of the winter and planted them, those seeds would preferentially represent plants with non-shattering ears. In this way, selection for non-shattering varieties could have been an incidental effect of human collection.
If Late Glacial collectors in Mexico could have selected incidentally on traits that were already present in the wild population of teosinte, the genetic variation that correlated with such phenotypic plasticity would likewise be subject to incidental selection from human collection.
Piperno and colleagues then conducted an experimental treatment in which the atmospheric conditions mimicked the Early Holocene, around 10,000 years ago. In this atmospheric treatment, the seeds of the maize-like phenotype plants from the earlier Late Glacial treatment continued to be more likely to produce maize-like phenotypes.
There are some unresolved genetic issues here. Today's wild teosinte population likely does not preserve all the genetic variation that once existed across the range where maize was domesticated. The original local variants that gave rise to the first maize may therefore have had phenotypes lacking in today's wild teosinte. When we look at the variability of today's teosinte in the face of atmospheric variation, then, we might be looking at an axis of phenotypic variation that falls short of the phenotypes of teosinte during the Late Glacial.
The other major finding of the study is that the plants were less productive under the Late Glacial atmospheric treatment. Also found by other studies, that has become a key observation about the productivity of wild plants in the time leading up to domestication.
People became intensive collectors in some parts of the world in the wake of the Last Glacial Maximum. Yet the domestication of crop plants and transition from collecting to agriculture did not happen until substantially later. Why the delay? Social factors don't seem a sufficient explanation: Some areas had a long history of sedentism and relatively high population density before domestication, others did not.
One answer is that the productivity of some wild grains increased at the transition to the Holocene, making it more possible to reseed from collected stores, and less necessary to depend on a broad array of less-preferred plants. So agriculture may have had not only a general environmental trigger -- the end of the last Ice Age -- but also a more specific environmental trigger: the phenotypic plasticity of wild plant species in the face of atmospheric conditions.
Piperno DR, Holst I, Winter K, McMillan O. 2014. Teosinte before domestication: Experimental study of growth and phenotypic variability in Late Pleistocene and early Holocene environments. Quaternary International (in press) doi:10.1016/j.quaint.2013.12.049
Next Monday I'll be delivering the 2014 Kalb Lecture for the Department of History at Rice University. The lecture will review the ways that genetics is adding a "deep time" component to the study of human history. I'll be talking about some recent results from ancient DNA and their intersection with some traditional problems in archaeology and human biology.
Does biology matter to history? Come and find out!
"How genetics is expanding and deepening history"
Monday, March 31, 2014
4:30 PM to 6:00 PM
309 Sewall Hall
6100 Main St
Houston, Texas, USA
The recovery of ancient DNA samples from Neolithic, Bronze Age and later peoples has begun to transform our understanding of migrations and connections among these ancient populations. At the same time, DNA evidence from living people has added new understanding to the causes of population growth and local adaptations to environments. We can begin to show how Tibetan peoples became adapted to high altitude, and how mixture with early Chinese farmers affected their population across the last few thousand years. We can see the replacement of early Arctic peoples by later ones, the effects of pre-classical Eurasian dispersals upon southern African indigenous peoples, the surprising influence of South Asians upon Aboriginal Australians. We know that today's Europeans are largely derived neither from early European hunter-gatherers nor from the first farmers, but from subsequent waves of population movement. These stories of perihistoric and prehistoric peoples are new narratives, some describing populations that no longer exist. Genetics additionally gives us ways to examine how environments and human interactions may have caused episodes of population growth and migration in the past.
I've been doing a good amount of reading about Neandertal diets lately, and have some stuff to report. First, a recent paper by Hervé Bocherens, Gennady Baryshnikov and Wim Van Neer examines a Paleolithic forensic case. There are Black Sea salmon bones in several Middle Paleolithic sites in the Caucasus, of enough quantity to suggest they were an important part of the Neandertal diet in this region. There are no Neandertal bones in the relevant sites to yield stable isotope evidence, so the importance of these salmon to the overall diet cannot be directly assessed.
But there are cave bear and cave lion bones in the same caves, and some archaeologists have suggested that these carnivores might be in part responsible for accumulating salmon bones in the caves. So Bocherens and colleagues set out to test this possibility, by doing stable isotope analysis of the carnivores at one of the sites, Kudaro 3. They found that the lions and bears were not eating the salmon:
The carbon, nitrogen and sulphur isotopic composition of cave lion and cave bears in the Middle Paleolithic cave site Kudaro 3 strongly suggests that these large carnivores were not consuming salmon and therefore were not responsible for the accumulation of salmon bones in the cave. It seems that hominins, in this case most probably Neandertals, were using this food resource. When the environment was providing direct access to abundant aquatic food resources such as anadromous salmon, it seems that Neandertals were not ignoring these resources. This conclusion is in agreement with recent work based on use-wear analyses of lithic artefacts that exhibit a broad-based subsistence for Neanderthals including fish consumption (Hardy and Moncel, 2011). Further isotopic investigations of Neandertal bone using sulphur isotopic composition in addition to carbon and nitrogen may help to document directly and to quantify the consumption of marine food resources in archaic hominins.
So that buttresses the case that Neandertals in the Caucasus were using salmon. It seems that we are moving beyond the demonstration that Neandertals occasionally used marine or aquatic resources, toward showing a consistent use of those resources in some areas of the Neandertal world.
Bocherens H, Baryshnikov G, Van Neer W. 2013. Were bears or lions involved in salmon accumulation in the Middle Palaeolithic of the Caucasus? An isotopic investigation in Kudaro 3 cave. Quaternary International (in press) doi:10.1016/j.quaint.2013.06.026
Barbara King devoted a recent NPR blog post to highlighting some professional acrimony in Current Anthropology: "Did Humans Evolve On The Savanna? The Debate Heats Up".
In the Current Anthropology exchange, Manuel Domínguez-Rodrigo discusses the "savanna hypothesis" for the origin of hominin bipedal locomotion. The commentary of several experts follows the article, a regular feature of Current Anthropology. One of those experts is the noted paleoanthropologist Tim White, who argues strongly that the savanna hypothesis has been disproven by his work in the Middle Awash, on the habitat preferences of Ardipithecus.
In King's view the resulting published exchange between White and Domínguez-Rodrigo crosses the line into unprofessional acrimony.
My view? Welcome to paleoanthropology.
I want to focus on a different aspect of the exchange. White's comment on the paper includes the following passage:
Consequently, the simplistic narrative that hominid origins were initiated in open savannas created by climate change stands largely abandoned. Which available ecological habitat(s) among Africa’s diverse landscapes was favored by the earliest hominids (Ardipithecus subsumes Orrorin and Sahelanthropus; Haile-Selassie, Suwa, and White 2009)?
Wait a minute. This claim should be surprising to anyone following the literature on early hominins. Is it really presently viable to think that Ardipithecus should include both Orrorin and Sahelanthropus?
Origin of the claim
The claim originated in a paper by Yohannes Haile-Selassie, Gen Suwa and Tim White in 2004. There, they wrote about the variation in teeth attributed to Ardipithecus, noting that the hominin-like features of these teeth were shared with other fossil samples from the latest Miocene.
Metric and morphological variation within available small samples of late Miocene teeth attributed to A. kadabba, O. tugenensis, and S. tchadensis is no greater in degree than that seen within extant ape genera. Despite claims of molar enamel thickness differences among these late Miocene fossils, we question the interpretation that these taxa represent three separate genera or even lineages. Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus.
That was the final paragraph of the paper, in some ways a shocker. Haile-Selassie, Suwa and White were claiming that the apparent diversity of early hominin fossil samples was mostly illusory. These different fossils, from Kenya, Chad and Ethiopia, all differed from Australopithecus and living apes in basically the same ways. Why not recognize that they are the same thing?
What they omitted from the paragraph is that some other samples of Miocene apes also differ from Australopithecus and living apes in similar ways. Primate paleontologists recognize the diversity of those lineages because they have other features that cannot be easily shoehorned into a single dental genus, and because many of them represent more of the skeleton than the teeth.
The weakness of the early hominin record (in 2004) was that except for the teeth, none of the available samples replicated the same parts. In 2004, the only parts of the Ardipithecus anatomy that had been described in any detail were the teeth. Orrorin had some teeth and a mandible along with three partial femora and a partial humerus. Sahelanthropus had a skull, with its teeth. So when Haile-Selassie, Suwa and White (2004) asserted that these samples were consistent with the variation in a single genus or species, that assertion was really only testable based on what they wrote about the teeth.
That situation changed in 2009, as White and colleagues published a series of descriptions of the cranial and postcranial anatomy of Ardipithecus ramidus. Those descriptions are obviously relevant to the question of whether we can count these different samples as part of the same genus or species. Moreover, there has been a subsequent literature on some aspects of the anatomy of Ardipithecus, Sahelanthropus and Orrorin that offer additional points of comparison.
Lovejoy and colleagues (2009) described the femur of Ardipithecus, ARA-VP 1/701, emphasizing several morphological similarities with the Orrorin femora, in particular BAR-1002'00. The following paragraph gives a series of comparisons between the Ardipithecus femoral specimens and other early hominins, including BAR-1002'00:
In ARA-VP-1/701, the medial border of an obvious hypotrochanteric fossa homolog converges with the spiral line to form a markedly rugose, elevated plane on the posterior femoral surface, but their further course is lost to fracture. A similar morphology is visible on the ARA-VP-6/500 specimen. A broad linea of low relief is also clearly present in ASI-VP-5/154, assigned to Au. anamensis (34). Its morphology is reminiscent of that of A.L. 288-1, in which the linea is still notably broad, but contrasts with that of MAK-VP-1/1, which is more modern in form at 3.4 Ma (31). Because most of the length of the ASI-VP- 5/154 shaft is preserved, its moderately elevated linea (~11.5 mm in breadth) is distinct and imparts a prismatic cross section at midshaft. Specimen BAR-1002'00 (Orrorin tugenensis) (35) presents obvious homologs to these structures. Moreover, both BAR-1002'00 and ASI-VP-5/154 exhibit an obvious homolog to the third trochanter, and neither shows any evidence of a lateral spiral pilaster.
The ARA-VP-1/701 and ARA-VP-6/500-5 femora from Aramis present few points of comparison because of the state of preservation. I cannot find the ARA-VP-6/500-5 femur illustrated anywhere except for the montage that shows the entire ARA-VP-6/500 skeleton. There is no photo of the femur fragment anywhere in the article by Lovejoy et al. 2009 or the supplementary information. Both pieces are described in the text as "proximal femur fragments" but neither preserves the proximal end; the better-illustrated ARA-VP-1/701 fragment ends just below the lesser trochanter.
The information from the Lukeino (Orrorin) femora is more complete: BAR-1002'00 includes a nearly complete proximal end and much of the shaft, and BAR-1003'00 replicates much of the anatomy of this specimen except for the head. These specimens show several features that link them with later hominins, including an elongated and anterio-posteriorly flattened femur neck, intertrochanteric line and obturator externus groove. The cortical bone distribution of the neck is
It is notable that the Aramis femoral fragments do not preserve the specific traits that most strongly link the Orrorin femur BAR-1002'00 to later hominins. As Lovejoy and colleagues (2009) discuss, there are similarities in the preserved portions between BAR-1002'00 and ARA-VP-1/701. The most striking of these is the "obvious homolog to the third trochanter" noted by Lovejoy and colleagues (2009). This is a marked, laterally projecting tuberosity for the attachment of the ascending tendon of the gluteus maximus muscle, on the lateral and posterior surface of the femur approximately at the same level as the lesser trochanter. Almécija and colleagues (2013) discuss this trait:
A different mode of bipedalism (from that of modern humans) practised by Orrorin (and Ar. ramidus) is consistent with the possession of a laterally protruding gluteal tuberosity below the greater trochanter (that is, small third trochanter, for insertion of the ascending tendon of the gluteus maximus) in combination with a 'broad proto-linea aspera'. The latter arrangement indicates that a modern human configuration, consisting in a more posteromedial translation of the gluteus maximus insertion (functionally related to hypertrophy of the quadriceps at the expense of the hamstrings), had not yet occurred in the above-mentioned taxa.
The morphology of the gluteal tuberosity is shared by some early hominins but also with a number of Miocene apes. Pickford and colleagues (2002) emphasize that the form of this feature in BAR 1002'00 and BAR 1003'00 is different from Miocene apes except Ugandapithecus in being a relatively vertical crest, coalescing with the crest leading distally toward the linea aspera. They compare the form most closely with the Swartkrans femora.
Both Lovejoy and colleagues (2009) and Almécija and colleagues (2013) briefly discuss the "broad proto-linea aspera" morphology present in both the Aramis and Lukeino femora. This is the most hominin-like trait apparent in the Aramis femoral fragments, and the strongest argument that the femora reflect a functional or facultative bipedal ability in this primate. What weakens that argument for similarity is that we do not know the morphology of the common ancestors of hominins and chimpanzees. Almécija and colleagues (2013) note that the anatomical configuration represented by Miocene apes may have been closer to the pattern of the hominin-chimpanzee common ancestor than are living great apes:
Overall, our results agree with Napier’s viewpoint, according to which the morphological pattern of the proximal femur displayed by some Miocene apes is more likely to have been co-opted for bipedalism than the derived femoral morphology that is displayed by extant great apes. This does not necessarily imply a particularly close phylogenetic link between hominins and some of the Miocene apes included in our analyses. Rather, it would imply that Miocene apes would closely resemble the femoral morphotype of the last common ancestor between African apes and hominins (Fig. 5), whereas the morphology displayed by extant great apes would be much more derived—and probably homoplastic—as an adaptation for enhanced abduction during suspensory behaviours.
This is a similar argument to that proposed by Lovejoy and colleagues (2009): namely, that the anatomical pattern of living great apes is derived away from the hominin-chimpanzee common ancestor, and therefore those living apes are not good comparisons for early hominin locomotor anatomy. The implication of this argument is that we cannot assume that features shared by Lukeino and Aramis femoral fragments, but lacking in chimpanzees, are signs of common ancestry of these samples. They may reflect the anatomical inheritance of the common ancestors of hominins and other apes.
Kuperavage and colleagues (2010) considered the Orrorin femora in the context of the early hominin record, including Lovejoy and colleagues' (2009) description of the Ardipithecus femoral anatomy:
At this stage in our knowledge of the first half of the human evolutionary record, approximately 6–3 Ma, there are more gaps than data points, and the data points that do exist mark populations that are at best known incompletely. The phrase “sparse and fragmentary” often is applied to this period, with complete justification. The situation is complicated still further by the non-comparable and otherwise unknown aspects of the fossils that have been discovered. Evidently much of a femoral diaphysis is known from deposits purportedly 7–9 Ma from Chad, but the specimen is still undescribed. The BAR 1002’00 and BAR 1003’00 femoral fragments comprise much of the proximal portion, with the region of the lesser trochanter well preserved. The next more recent sample, represented by ARA-VP-1/701, is not only separated by more than 1.5 Ma, but lacks nearly all of the lesser trochanter and more proximal portions. No os coxa accompanies BAR 1002’00 and 1003’00, while a heavily crushed specimen of this bone is represented by ARA-VP-6/500.
The absence of descriptions or publication of the Chad material makes it impossible to assess whether Sahelanthropus may conflict with the Ardipithecus or Orrorin femoral anatomy. I wrote about the purported Sahelanthropus femur in 2009: "Sahelanthropus: 'The femur of Toumaï?'"
If we cannot compare the Sahelanthropus femur, we can certainly examine the cranium compared to the Aramis reconstruction. The Sahelanthropus and Ardipithecus crania look quite different from each other, even in the CT-based reconstructions. Here is TM 266-01-60-1 (abbreviated TM 266 as in Zollikofer et al. 2005), the skull of Sahelanthropus on the left (Zollikofer et al. 2005), compared to the composite reconstruction of the Ardipithecus cranium (Suwa et al. 2009) on the right. The two are approximately to scale. Suwa and colleagues (2009) give a series of linear measurements of the Ardipithecus cranial reconstruction, and provide equivalent estimates from the TM 266 reconstruction (in some cases estimated from photographs). I have aligned the two reconstructions approximately on the front of the foramen magnum. As is visually apparent, the TM 266 skull is substantially longer than the Aramis reconstruction. The two have approximately the same palate breadth, as estimated by Suwa and colleagues (2009).
Suwa and colleagues (2009) had to match the 3-dimensional configuration of the semicircular canals of the two temporal bones of the ARA-VP-1/500 specimen to find the relative position of these bones, and that information underlies the reconstruction of the cranial base here. The Aramis reconstruction has a slightly narrower cranial base than TM 266, but the TM 266 skull is overall much longer. Some of that greater length can be attributed to the extremely long nuchal plane of TM 266, which is extraordinary compared to later fossil hominins. Wolpoff and colleagues (including me) (2006) made the long nuchal plane one element of our argument that the Sahelanthropus skull morphology is outside the range of known hominins. That might appear to be a very important difference between the two crania. But in reality, the Ardipithecus cranial base and vault elements come from two different specimens, one of which (ARA-VP-1/500) was scaled to fit the other (ARA-VP-6/500). Neither specimen preserves the nuchal plane, which is a blank spot in the reconstruction.
If TM 266 should really be placed in Ardipithecus as suggested by White (2014), it is interesting to consider the Aramis cranial reconstruction with a nuchal plane oriented like TM 266. Below is a lateral view of both reconstructions, from Suwa et al. (2009) and Zollikofer et al. (2005):
It is not obvious from the visualization of the 3-d reconstruction how much of the curvature of the ARA-VP-6/500 parietals is actually present in the specimen, and how much of the appearance of curvature is the product of reconstruction. In lateral view, the other reason for the greater length of the TM 266 reconstruction is apparent: The skull has an extensive supraorbital torus which projects substantially anteriorly, which is very different from the Aramis reconstruction. By contrast, the Aramis reconstruction has a much more prognathic face, from the midface downward and particularly through the procumbent incisors. The profile of the TM 266 face is much more orthognathic. TM 266 is reconstructed with a broad, triangular nasal aperture with its superior border inferior to the orbits, and the Aramis reconstruction has a narrow nasal aperture with a rounded floor that extends superiorly above the base of the orbits.
There are, in other words, many visually striking differences between these reconstructions that reflect the basic architecture of the two skulls. We should keep in mind that they are reconstructions of crania that were highly distorted. The anatomical evidence makes it inevitable that TM 266 has a very long nuchal plane, for instance, and does not rule it out for the Aramis reconstruction. Both crania have been reconstructed with the biporion line approximately even with basion, and in both cases the interpretation of the reconstructions has favored a more vertical habitual posture.
It is not obvious to me that the two cranial reconstructions represent the same species or genus. At the same time, they are not obviously more different than two crania assigned to different species of Australopithecus, such as STW 505 and A.L. 444-2. It may be useful to think through the consequences of taking this assertion seriously. What would the Aramis reconstruction look like with a long, flat nuchal plane as in TM 266? What can we conclude about the anterior dentition of TM 266 if we use the Aramis dentition as a model?
Are Sahelanthropus and Orrorin sunk?
One way to tell whether this is a serious hypothesis is to examine the behavior of its authors. William Kimbel and colleagues, including Suwa and White (2014) recently published a study of the cranial base of Ardipithecus, in which they compared the ARA-VP-1/500 specimen with a number of other early hominins and living great apes. In this study, they somehow neglected to compare the specimen to the Sahelanthropus cranial specimen TM 266. Neither the word "Sahelanthropus" nor "Toros-Menalla" (the locality that produced the TM 266 skull) appears in the text.
This omission seems like a fairly obvious problem with that paper. Either the authors don't really believe that the TM 266 skull belongs in Ardipithecus, or they believe it does belong in Ardipithecus and omitted half the basicranial sample. In either case, the TM 266 skull is obviously relevant; it is the only complete basicranium in the five million year span preceding the Aramis specimen. What should we conclude about the authors' real opinion about Ardipithecus and Sahelanthropus?
I have worked with casts of the Orrorin material, and I have worked extensively with the published descriptions of the Sahelanthropus and Ardipithecus material. The Lukeino and Aramis femora are not identical, but the preserved portions of the Ardipithecus femoral specimens are not sufficient to demonstrate that they share the same derived features that apparently link Orrorin with later hominins. There cranial reconstructions from Toros-Menalla and Aramis are quite different from each other in both broad architectural features and in many details.
But only a handful of paleoanthropologists have access to accurate casts or three-dimensional CT-based models of the Ardipithecus and Sahelanthropus crania. Some of the relevant bone elements have never been illustrated in any journal, so it is impossible to verify some claims about them. The published descriptions have notably omitted many standard details, such as standard dental measurements (I discussed this in 2009: "Whoa, who stole the data?"). We are in some ways no further along than we were in 2004 when Haile-Selassie, Suwa and White first published their claim that the dentitions of these samples represent a single species or genus. Some people know a lot about the anatomy of these samples, but they have not presented enough evidence for other scientists to evaluate their claims.
Almécija S, Tallman M, Alba DM, Pina M, Moyà-Solà S, Jungers WL. 2013. The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins. Nature Communications 4:2888. doi:10.1038/ncomms3888
Domínguez-Rodrigo, M. 2014. Is the “Savanna Hypothesis” a Dead Concept for Explaining the Emergence of the Earliest Hominins? Current Anthropology 55:59-81. doi:10.1086/674530
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