Header image

john hawks weblog

paleoanthropology, genetics and evolution

Photo Credit: 3D printing a Homo naledi cranium. John Hawks CC-BY-NC-ND

From Paige Madison, a historical note on the public revelation of Raymond Dart’s examination of the first australopith: “Announcing the Taung Child to the World”:

The Nature publication was set to appear February 3, 1925. However, as the date approached, there was no word from the journal. The Star finally contacted Nature and asked about the publication date, and Nature replied that Dart’s claims of the missing link were so unprecedented that the paper had been sent to various experts in England for review. But the reviewers had not yet gotten back to Nature. The Star informed Nature that they would not withhold their release, and would publish their story as planned on February 3.

Plus ça change…

I was doing some searching through the abstracts for the upcoming AAPA meetings, and I found that the system gave me abstracts for past meetings as well, so I didn’t notice at first when I was seeing some cool undescribed fossil results from years past.

In 2012, Jennifer Conaway and William Kimbel had an abstract discussing A.L. 1901-1, a Hadar mandible with supernumerary molars: Australopithecus afarensis mandible with supernumerary molars from the Hadar site, Ethiopia”. The presentation was withdrawn but the finding is an interesting data point:

Within the Hadar sample of A. afarensis mandibles containing a complete molar row, 2 out of 21 (9.5%) specimens hold supernumerary M4s. Although the sample size is modest, this frequency is consistent with those reported for large-bodied extant hominoids.

These are not uncommon in gorilla and orangutan samples, but they are very uncommon in human populations today. Maybe the more interesting observation is that this is another Hadar mandible complete enough to have both sides of the dentition preserved. Unfortunately, this specimen does not seem to have been published anywhere yet, but maybe in the upcoming few years we’ll see it.

Andy Farke did a short interview with Kelsey Stilson, an author of a recent study on the paleopathology of rhinocerotids: “Author Interview: Kelsey Stilson on Gnarly Rhino Bones”. The study itself is interesting and I’ll be reading it carefully. But I wanted to point to this quote on the process of science:

I saw that you put your data–photos, observations, etc.–on MorphoBank. That’s awesome for the field, but must have been a ton of work. What tips do you have for other researchers who might want to follow in your footsteps and upload their own massive data sets?
Yes, it was a ton of work, but as a scientific community we have to upload everything we have (including metadata and associated documentation) for the future. Floppy discs degrade, computers crash, and notebooks get recycled. Even NASA accidentally erased the original Apollo 11 mission recordings. No matter how sloppy or out of focus you think your data is, it might be important some day and at the very least no one will have to redo that work again. My advice is to let any insecurities go and upload your data. That way there are at least two copies of it in the world. Also, include the specimen number when you rename your photos. That will save you a lot of grief.

She has some great advice for young scientists from the point of view of someone who began this research as an undergraduate research project.

Wilton Krogman, in a footnote to his review, “Fifty years of physical anthropology.”

I have told this story to a few friends, and I repeat it here. In 1933 I became a consultant in skeletal identification to the FBI, and in 1938 I wrote a "Guide," published in the FBI Law Enforcement Bulletin. During World War II, I was in the Washington office of the FBI and an agent approached me and asked me in knew a Professor Franz Boas, and what did I know about him? I asked why. He told me that Boas was under investigation, and was to be brought up for interrogation because of personal identification with so many "un­American" and "pinko" groups. I told him in no uncertain terms of the personal integrity of the man, adding that so sincere was he in his liberal views that I was quite sure that his name on an organizational letterhead meant most often that he "lent" his name in sympathy rather than in 100% endorsement. Hoover called off his "birddogs."


Krogman, W. M. (1976). Fifty years of physical anthropology: The men, the material, the concepts, the methods. Annual Review of Anthropology, 5(1), 1-15.

On the subject of long-running experiments in biology, Atlas Obscura has a story about a 137-year-old experiment in the germination of common weeds: “The world’s longest-running experiment is buried in a secret spot in Michigan”.

In the fall of 1879, Dr. William James Beal walked to a secret spot on Michigan State University’s campus and planted a strange crop: 20 narrow-necked glass bottles, each filled with a mixture of moist sand and seeds. Each vessel was “left uncorked and placed with the mouth slanting downward so that water could not accumulate about the seeds,” Beal wrote. “These bottles were buried on a sandy knoll in a row running east and west.”
In the spring of 2000, under cover of night, current WJ Beal Botanical Garden curator Dr. Frank Telewski and his colleague Dr. Jan Zeevaart crept out to the same secret knoll and dug up the sixth-to-last seed bottle—completing the latest act in what has become the world’s longest continually monitored scientific study.

Contrary to the headline, it is not the world’s longest-running experiment. But it is a very long one for biology.

Skeletal remains from Milner Hall, a new excavation area in Sterkfontein Caves

This month, the Journal of Human Evolution has published a short paper from Dominic Stratford and colleagues describing two hominin fossils from Milner Hall, a new excavation area within Sterkfontein Caves. One of the authors is my UW-Madison colleagues, Travis Pickering. If you know Sterkfontein, Milner Hall stretches from the area near the Silberberg Grotto across toward the underground lake.

The deposits sampled here are near the base of the fossil-bearing strata in Sterkfontein. Stratford, Grab and Pickering in a 2014 paper described the geological context of the excavation in Milner Hall. The excavation area includes at least three talus deposits that have sloped in from different sources, and have ages ranging from some of the oldest at Sterkfontein up to the last half million years. The hominin material comes from nearest the surface, in the T1 talus deposit. This talus was formed as miners early in the 20th century worked on a large stalactite nearby; this mining caused a relocation of a mixture of sediments from Member 5 and Member 2 into the T1 talus. According to the new paper describing the hominin material, the two hominin pieces most probably come from Member 5 sediments, which include some Oldowan flakes and a core. The age is uncertain.

StW 668 and StW 669
Figure 2 from Stratford et al. 2016, showing StW 669 on the left (a) and StW 668 on the right (b).

The proximal hand phalanx StW 668 is within the size range of humans but is more curved than human proximal phalanges. Its curvature is a bit greater than that seen in the proximal phalanges from Malapa or Dinaledi, but within the range of Hadar proximal phalanges. It is a larger phalanx than any seen at any of the South African fossil sites.

The tooth gives a bit more to compare:

StW 669, with its small occlusal area (116.5 mm2), tall vertical sides and widely spaced cusp apices, is grossly more similar to the M1s of Homo than to those of Australopithecus and Paranthropus. In overall shape and size, it compares most favorably to the Olduvai (Tanzania) M1 OH 6 (MD = 12.5 mm; BL = 12.3 mm) which is traditionally assigned to Homo habilis and to UW 101-1688, an M1 of the newly proposed South African species Homo naledi (MD = 12.4 mm; BL = 12.0 mm; Fig. 3a). In addition, like the majority of M1s traditionally assigned to early Homo (and especially to H. habilis, as well as to South African early Homo), StW 669 falls very near to the line of occlusal symmetry; in contrast, the vast majority of M1s of australopith-grade taxa (including Paranthropus) fall farther away and noticeably below this line.

Stratford and colleagues judge it to be a first molar, which is reasonable based on the cusp morphology. But the tooth lacks a distal interproximal facet, and I wonder if it may be a second molar instead. For some of these species (including H. naledi), the first and second molars can be hard to tell apart, without comparing a sample of them directly.

The tooth is small relative to most Australopithecus first molars, which means it is even smaller for a second molar of most species. But the size of the tooth does not distinguish species easily because are upper first molars attributed to Au. afarensis, Au. africanus, and even Au. robustus that have the same mesiodistal length as StW 669. These australopith species, however, have a broader buccolingual dimension than StW 669, which plots close to Homo first molars. The areas of the cusps are also more like Homo than Australopithecus, although the StW 669 paracone is yet smaller than the H. habilis examples. The two South African species that are most similar to this tooth are H. naledi and Au. sediba.

On the surface, the tooth looks like a very reasonable match for H. naledi first molars, and I think the match is even better for second molars. But it is not a bad match for the MH1 specimen of Au. sediba, either. We haven’t yet finished the analysis of cusp areas of the H. naledi molars, and on the basis of just this one tooth, I’m not sure we would be able to say StW 669 is very different from H. habilis, the KNM-ER 62000 specimen attributed to H. rudolfensis, or Dmanisi H. erectus.

But in the South African context, it seems like another example of a non-africanus specimen at Sterkfontein. And both Au. sediba and H. naledi are so far known from only a single site each, so demonstrating that they were present at a second site might really help us to understand when and how long these species existed.


Stratford D, Heaton JL, Pickering TR, Caruana MV, Shadrach K. 2016. First hominin fossils from Milner Hall, Sterkfontein, South Africa. Journal of Human Evolution 91:167-173. doi:10.1016/j.jhevol.2015.12.005

Stratford D, Grab S, Pickering TR. 2014. The stratigraphy and formation history of fossil- and artefact-bearing sediments in the Milner Hall, Sterkfontein Cave, South Africa: New interpretations and implications for palaeoanthropology and archaeology. Journal of African Earth Sciences 96:155-167. doi:10.1016/j.jafrearsci.2014.04.002

Notable paper: Stefanie Grosser, Nicolas J. Rawlence, Christian N. K. Anderson, Ian W. G. Smith, R. Paul Scofield, Jonathan M. Waters. 2016. Invader or resident? Ancient-DNA reveals rapid species turnover in New Zealand little penguins. Proceedings of the Royal Society B doi:10.1098/rspb.2015.2879

Synopsis: Grosser and colleagues studied mtDNA from ancient remains of two species of little penguins on New Zealand, the New Zealand endemic Eudyptula minor and Eudyptula novaehollandiae, which occurs both in New Zealand and Australia. They found that all of the remains dating to before 1600 represent E. minor, and they conclude that the appearance of E. novaehollandiae happened only after human-mediated decline in the E. minor population, both because of human predation and because of the human introduction of dogs and rats.

Interesting because: Ancient DNA is increasingly useful to determine the causes of population turnover. Prior to this large sample of aDNA, some biologists had speculated (on the basis of comparison of genetic data from living birds) that E. minor and E. novaehollandiae had coexisted in New Zealand for as long as 200,000 years. Retrospective population genetics just isn’t very accurate when it comes to the time of population divergences and demographic parameters. Ancient DNA is vastly better with questions like this one.

Cool figure: I really like the way they have illustrated the haplotype network and changes in frequencies over time:

Figure from Grosser et al showing haplotype network for penguin mtDNA
Figure 2 from Grosser et al. 2016. Original caption: Temporal haplotype network reconstruction of New Zealand (blue) and Australian little penguin (red) mitochondrial control region sequences from southeastern New Zealand (Otago). Each layer represents a particular time period as indicated in the inset. Circle size is proportional to haplotype frequency. Edges between haplotypes represent single mutation steps. Unobserved haplotypes are indicated by small black circles. Grey circles connected by dashed lines represent haplotypes that occur within the population but not the particular temporal layer. The insets show sampling localities in Otago. Circle colour in insets indicates species affiliation of specimens of the particular site. Blue and red borders interpolate Otago-wide distribution of E. minor and E. novaehollandiae, respectively. H, haplotype diversity; n, sample size; NZ, New Zealand (E. minor).

The National Post of Canada has a long article by Sharon Kirkey on the rise of obesity: “The shape of the future: Is obesity a crisis or just the latest stage of evolution?” It’s a nice piece that covers both genetics, the history of how we approach fat, and the changing views of fatness in society. I’ll be assigning it to my introductory students this semester.

This nice quote about the complexity of the relationship of FTO to obesity could have used a bit more explanation:

But many researchers are convinced basic genetics — not how we (or our mothers) behave — is the biggest driver of obesity, accounting for as much as 80 per cent of the risk of carrying excess weight. Their challenge is to tease out which genes among the 21,000 that make up the human body play a major role in “food-seeking behaviour,” satiety, cravings, and how our body stores and distributes fat.
The leader today is the FTO, the fat mass and obesity-associated gene that regulates appetite. People who inherit one copy of the FTO mutations (possibly as many as one in six people of European descent) have a 30 per cent higher risk of obesity; two copies, and the risk increases 7o per cent.
Even more remarkable, that risk appears to have changed over time. In a study published in 2014, researchers found people born before 1942 did not show an association between the risk variant and obesity. Those born later did.

The gene by environment interaction here is one of the most fascinating topics in human genetics.

Yes, there’s also some stuff about the thrifty genotype and caveman diets in there as well.

A laboratory at Kyoto University has been maintaining a long-term evolution experiment on fruit flies that started in 1954. Now the flies are adapted to living in pitch black darkness:

To keep the flies away from light, they are reared in vials kept in a large pot painted black on the inside and covered with a blackout cloth. When the vials and food need to be changed, the researchers tend to the flies in the pitch dark, then use a feeble red light to check on their work. Fruit flies can’t see this light because the species lacks those light receptor proteins that absorb red wavelengths.
When [Syuiti] Mori retired, he passed on the precious fly stocks to his colleagues at Kyoto University, who have maintained them continuously to this day. The stock of flies has now spent more than 1,500 generations without light. In human terms, that would be like sequestering generations of our ancestors in the dark for 30,000 years.

It’s an interesting type of experiment, similar to the Long Term Experimental Evolution project that has kept E. coli cultures under a constant environmental regime for more than 64,000 generations. The populations adapt to their environmental conditions, different from the natural situations in which their ancestors evolved.

Culture, mathematical models, and Neandertal extinction

I am not philosophically opposed to building a mathematical model of Neandertal populations. Some of my best work has involved mathematical model-building. Models have an important place in helping us to understand evolutionary history. But when it comes to understanding Neandertal and modern human interactions, we have had lots and lots and lots of models and few testable predictions.

When you assume that modern human populations grew faster than Neandertal populations, you will conclude that modern human populations could have out-reproduced the Neandertals. This is not a very deep piece of circular logic. and so I get a little frustrated at the number of papers that really say nothing more than this.

Find x meme

Modeling is a start, but cultural systems are complicated. Clever innovations can help a population grow, but a population can co-evolve with its culture, yielding not only more growth but also greater difference in growth between populations. A cultural innovation can be tied to a particular landscape or raw material substrate, making it difficult to apply outside the context where it was invented.

The populations that we call “modern humans” really did out-reproduce the Neandertals. That’s why living people have only a small fraction of Neandertal ancestry today. But is culture a sufficient explanation? Were modern humans just smarter than Neandertals? Or were other factors important to the interactions between these populations?

Differential equations and Neandertals

I’m taking up this subject today in response to a paper by William Gilpin, Marcus Feldman and Kenichi Aoki, who have investigated a differential equation model for population growth with feedbacks from competitive interactions. The abstract summarizes the paper well:

Archaeologists argue that the replacement of Neanderthals by modern humans was driven by interspecific competition due to a difference in culture level. To assess the cogency of this argument, we construct and analyze an interspecific cultural competition model based on the Lotka−Volterra model, which is widely used in ecology, but which incorporates the culture level of a species as a variable interacting with population size. We investigate the conditions under which a difference in culture level between cognitively equivalent species, or alternatively a difference in underlying learning ability, may produce competitive exclusion of a comparatively (although not absolutely) large local Neanderthal population by an initially smaller modern human population. We find, in particular, that this competitive exclusion is more likely to occur when population growth occurs on a shorter timescale than cultural change, or when the competition coefficients of the Lotka−Volterra model depend on the difference in the culture levels of the interacting species.

The Lotka-Volterra system of differential equations is one in which two components of a system change over time, in such a way that the amount of change in one component depends upon the magnitude of the other component. It has been most famously applied as a model for predator and prey populations, where predator population growth is coupled to prey population size. The behavior in this system can be cyclical—for example, if a predator population crashes, the growth of a prey population can resume, driving subsequent growth in the predator population.

This kind of feedback between components of the system is determined by the differential equation coefficients. The utility of this kind of system is that varying the coefficients allows us to investigate the conditions under which the system can exhibit stable, cyclical, or degenerate behavior.

Gilpin and colleagues assume the Neandertal and modern human populations to have been in a competitive interaction, where the rate of growth of the Neandertal population is smaller when the modern human population is larger, and vice-versa. Each population grows logistically up to a carrying capacity, which is determined by a parameter that they refer to as “culture level”. This “culture level” increases with population size, and it increases faster for the modern human population than for Neandertals—in other words, in this model Neandertals are stupid.

In this system, modern human populations grow faster when they wipe out the local Neandertals. They innovate faster when the population gets larger. If the difference in the rate of change of “culture level” is sufficiently large, Neandertals are doomed.

In principle, the model allows the authors to investigate the way that a second parameter, “culture level”, might constitute an advantage if Neandertals had a lower rate of increase. But the paper does not actually deploy this model in a way that would inform us about the importance of this second parameter. As a result, the conclusion is boring. If we assume that Neandertals were stupid, we don’t need a differential equation to tell us what happened to them.

Neandertal in a suit

Is “culture level” relevant?

The problem is not that we lack models to show how culture may have helped modern humans beat the Neandertals. The problem is that the archaeological data suggest that culture alone may be a poor explanation.

For one thing, the Neandertals persisted in Europe and central Asia long beyond the entry of modern humans into Asia. Initial modern humans in Asia exhibited no obvious cultural superiority over other Middle Paleolithic people, who were presumably archaic humans. “No cultural superiority” is maybe an understatement: Archaeologists have trouble finding any consistent material culture differences between people in West Asia before 50,000 years ago.

Tens of thousands of years later, when modern humans did start to enter Europe, they seem to have mixed with Neandertals more extensively. The later Neandertals were making symbolic artifacts, using pigments, feathers and other ornaments. The people who made the earliest Aurignacian, often assumed to be the earliest modern humans in Western Europe, did not have the intensity of symbolic artifacts of later Aurignacian and Gravettian people. Instead they seem to have been sparse and little different in most cultural practices from Neandertals.

In other words, at the critical time when modern humans entered Europe and their population apparently grew, there was little cultural difference between them. There is even less evidence that there was any cultural advantage to modern humans who spread across southern Asia prior to 50,000 years ago.

What gives? If we assume that “culture level” was a continuous variable, and that “modern humans” had a higher rate of increase than Neandertals, we get a very simple pattern. The data are not a simple pattern. So the “culture level” model seems like a bad model to account for the complexity of what actually happened.

Vital forces

“Culture level” is an archaeological version of “vital force”. Plenty of archaeologists think there is something special about “modern human behavior”, and believe that a “spark” entered the human population. After this vital spark entered the modern population, they were able to grow their population, spread around the world, and conquer the earth with their cultural adaptability. Some have written that this “spark” was a key mutation, some believe it was fully human grammar, some believe that it was a special demographic or ecological setting.

They are not thinking like biologists. The evolution of human cognition was not magic, and it was not caused by a “spark”.

I don’t object to the idea that Neandertals may have been cognitively different than modern humans—in fact, I think this is likely. The idea that Neandertals were fixed for stupid and modern humans fixed for smart is biologically incredible. Instead, we need to consider that if many Neandertals had challenges learning to work with some cultural innovations, many modern humans should have had such challenges as well. Key innovations, if rare, must have been stochastic.

To understand human cognitive evolution, we must consider how specific behaviors may have contributed to reproductive success. Useful cultural innovations tend to be transferred readily across groups, and so make unlikely vehicles for a continuous growth of one population at the expense of another. Was the ability to learn some cultural behaviors heritable? If so, it is unlikely that the ability to learn two behaviors was equally heritable; some must have been more influenced by genes than others. To the extent that behaviors are learned by exposure to skilled individuals, this exposure causes the selection in favor of the ability to learn to weaken as the trait becomes more commonly expressed.

Cultural selection to enforce cultural uniformity can be very effective in fixing cultural traits in a population, but is not especially likely to enhance the traits that really matter for adaptation to new environments. Indeed, cultural selection in many recent human groups has been depressingly conservative, preventing innovation and reducing population growth by imposing various handicaps.

In the real world, some archaic people—including the Neandertals—really were more successful than most early modern human groups. Neandertals as a population contributed more DNA to people around the world than their “conquerors”, the Upper Paleolithic people of Europe. Some modern human populations have massively grown during the last 50,000 years at the expense of others, often for cultural reasons. When we look at the diversity of those situations, we can see that culture is not easily broken down into a linear variable.

Non-linear culture

What makes culture more than a simple system of accumulating knowledge?

There are conditions under which a cultural system may be hard to transfer across groups. A cultural system may have co-evolved with some genetic variants, like dairying and lactase persistence. The cultural traits in such a system, even if learned, might not have had the advantage for Neandertals as for the modern humans that developed it. A cultural system may rely upon some elaborate codification of social behavior, like religious rules, that are not readily adopted by new cultural groups. Again, the behaviors that seem tied to reproductive advantage in such a system may not be as advantageous to people who lack the essential cultural background.

What we lack is some empirical demonstration of what cultural factors among Late Pleistocene people actually led to higher reproductive success. Archaeologists have proposed several, but have tested few. Most, like evidence for symbolic behavior, have subsequently been found in the Neandertals themselves, making them poor explanations for Neandertal extinction.

Later modern humans exhibited greater material culture diversity and more symbolic expression than earlier modern humans, thousands of years after the Neandertals were gone. This is true not only in Europe and central Asia, it is also true in other places long after the first modern humans appeared there: in southern Africa, west Asia, and southeast Asia. In every part of the world, the evidence for elaborate symbolic culture occurs long after the earliest evidence of modern humans. And in most of these areas, the first evidence for symbolic culture occurs substantially before the earliest evidence of modern humans.

To me, this means it was not just “culture level” that made a difference. I can imagine that there may have been some specific aspects of culture, which may not have been archaeologically visible, that made a key difference. Archaeologically visible material culture may reflect demographic growth, but not necessarily the key aspects that mattered to the initial dispersal of populations.

But I can also imagine that non-cultural factors were more important. For example, disease has been a key factor underlying the survival or replacement of populations during the past 10,000 years. It is not a stretch to imagine that disease influenced the Neandertals and other archaic peoples differently from each other and from modern humans. The evidence for selection on genes related to immunity from these archaic humans is now strong, and some of these may reflect pathogens or parasites that were important at the time of population contacts among these people.

Exploration needed

This is why we need more data, more exploration, more archaeology. I don’t mind if people continue to think of mathematical models, as they may help us to understand which factors are important. Listing all the possible factors doesn’t necessarily get us closer to a test of which of those factors were crucial to our evolution, and including every factor in a model will make it untestable.

But we are past the point where a simple model is going to tell us something we don’t already know. Neandertals are gone. Their cultures did not persist. Yet they are among our ancestors. What is necessary is to test models against the timeline of modern human dispersal as we currently understand it, and to take note of those predictions that we have not yet observed. It is the novel predictions of a model that make it valuable to the future.


Gilpin W, Feldman MW, Aoki K. 2016. An ecocultural model predicts Neanderthal extinction through competition with modern humans. Proc Nat Acad Sci USA (online) doi:10.1073/pnas.1524861113

Aleš Hrdlička, in the concluding paragraphs of The Most Ancient Skeletal Remains of Man, his 1914 review of the fossil evidence of human evolution:

The gradually accumulating finds which throw light on the physical past of man, have naturally stimulated further exploration in the same lines; and the various failures and uncertainties connected with some of the finds in the past have impressed all investigators in the field with the necessity of the most careful and properly controlled procedure. Besides men of science, the educated public, engineers controlling public works, and even many among the workmen in Europe have been impressed by these remarkable discoveries, and in hundreds of instances are doubtless watching for new treasures. Under these conditions we are justified in hoping that from time to time we shall receive additions to the precious material already in our hands; that these additions will fill the existing vacua, and gradually extend farther back to the more strictly intermediary forms between man and his ancestral stock, and perhaps eventually even to the source of these link-forms themselves, to the peculiar morphologically unstable family of the anthropogenous primates.
While the anthropologist is thus painfully and slowly reconstructing the past physical history of man, he is also with every new fact adding another imperishable block to the foundation upon which will stand not only the knowledge of the future in regard to man himself, but also the laws of his further physical development, and radically even those of his beliefs and his moral behavior. This is a part of the service of anthropology to humanity.

I think I’m going to take that phrase, “another imperishable block”. Seems ripe for mischief-making…

Ed Yong has an article in Atlantic on the mysterious evolution of the human chin: “We’re the Only Animals With Chins, and No One Knows Why”. He builds on a recent review paper by James Pampush.

It’s a nice read about a trait that has occupied more than the usual amount of paleoanthropological mindshare over the last hundred years.

For example, during human evolution, our faces shortened and our posture straightened. These changes made our mouths more cramped. To give our tongues and soft tissues more room, and to avoid constricting our airways, the lower jaw developed a forward slope, of which the chin was a side effect. The problem with this idea is that the chin's outer face doesn't follow the contours of its inner face, and has an exceptionally thick knob of bone. None of that screams “space-saving measure.”

There are some true “chin nerds” out there who will likely think the article doesn’t treat their favorite hypothesis fairly. The fact is, it’s sort of embarrassing that we have not yet come up with a persuasive test of any of these hypotheses.

Christopher Henshilwood has written a short article for The Conversation describing the archaeological importance of the finds from Blombos and elsewhere in southern Africa: “What excavated beads tell us about the when and where of human evolution”.

Modern human behaviour can be defined as behaviour that is brought about by socially constructed patterns of symbolic thinking, actions and communication. This allows for material and information exchange and cultural continuity between and across generations and contemporaneous communities. The capacity for symbolic thought is not the key defining factor for modern human behaviour. It is rather the use of symbolism to organise behaviour that defines us.
In other words, early humans were first behaviourally modern when symbols became an intrinsic part of their daily lives.

The tranformation of archaeology toward understanding the complexity of Middle Stone Age assemblages in southern Africa is impressive. I think it may be premature to write off the behavior of earlier people as non-symbolic. After all, the production of symbolic objects can only be functional within a society in which symbolic communication is near-universal. Language provides such a basis. Even simple forms of vocal communication in other primate species involve arbitrary learned relationships between sounds and concepts, which is the definition of symbolic communication. Auditory symbol use must have been present in the earliest forms of human language as well, and evidence from the vocal and auditory channels place the origin of vocal language much earlier than the Neandertals.

This is not modern human, I would say it is simply human.

I think we should take seriously the hypothesis that the differences between Middle Pleistocene populations were quantitative and not qualitative. Symbolic behavior did not emerge instantaneously; it evolved within a context of complex social interactions of earlier archaic human populations. The use of symbolic artifacts is an important clue to ancient social systems, evidencing one aspect of complexity. The artifacts give us one kind of evidence about the connections among ancient groups and the persistence of symbolic traditions.

I endorse this message from Holly Dunsworth:

Everyone’s likely heard it or seen it written on a protest sign: “I didn’t evolve from a monkey.” It’s a well-worn refrain of those who resist the evolutionary perspective. The pat response we often hear is, “You’re right! We didn’t evolve from monkeys. We share ancestors with them.” However, this talking point isn’t entirely honest.
Yes, we share ancestors with monkeys; we share ancestors with every living thing. But, also, to be clear: We did evolve from monkeys.

That’s from her new blog, “Origins”, from the new website, SAPIENS. It’s a new public communication portal sponsored by the Wenner-Gren Foundation for Anthropological Research, and I’ll look forward to seeing more of what they have in store.

How long does it take to publish new hominin species after discovery?

In an earlier post, I looked at descriptions of new hominin species during the last 25 years, to see how long they took from submission to acceptance in the journal where each was published (“Hominin species and time in peer review”). The data show that these papers have taken a median time of 70 days to review. But as I noted, the time in peer review probably doesn’t tell us very much about the quality of review. What stands out is that the duration of review has not changed appreciably across the 25-year span.

Au. sediba
Australopithecus sediba was 20 months from first discovery to publication. Photo credit: Lee Berger

A more interesting time interval is from discovery to publication. This time includes not only the peer review and other editorial processes, but also the primary scientific work. Technicians prepare and conserve the fossils, specialists in anatomy take systematic measurements and observations, and they carry out comparisons with other samples of fossils and skeletal collections.

Analysis can take a lot of time. Some fossils require considerable reconstruction. Today such reconstruction can be carried out virtually after 3D scanning of the material, which sounds like it should save time but somehow always seems to take longer. If a specimen preserves a rare piece of anatomy, the comparable anatomical areas may not have been well-reported in other fossil samples, and therefore one or more researchers may need to make special research trips to study fossils from other parts of the world.

Considering the diversity of fossil preservation across different field sites, you might expect the process of publishing diagnoses of new fossil species to be equally diverse in how long they take to prepare.

The data show the opposite: the duration of scientific work preceding the diagnosis of new hominin taxa has generally been between one and two years, and has remained pretty much the same over the last 25 years.

Discovery dates

When was a species found? This is not a simple question, because the evidence for a new species is rarely limited to a single specimen. A team may find fragments that suggest the existence of a new species in one field season, and later find additional evidence that enhances the case.

For example, the earliest specimen now attributed to Australopithecus anamensis to have been discovered is a humerus fragment from Kanapoi, KNM-KP 271, which was found by Bryan Patterson’s research team in 1966. This fragment was known to represent a very early hominin, but does not present anatomical features that would have enabled a clear diagnosis relative to other known hominin species. Only much later did Maeve Leakey and colleagues uncover more remains from Kanapoi and Allia Bay that enabled a diagnostic comparison with other species.

Every formal description of a new species designates a single specimen, known as the “holotype”, that will serve as an anatomical reference for future scholars. Under the rules of the International Code of Zoological Nomenclature, the holotype is forever tied to the formal species name; it can never be recycled to serve as the holotype for any other species name. For Au. anamensis, Leakey and colleagues designated the specimen KNM-KP 29281, discovered in 1994, as the holotype of the species. So although the first specimen now attributed to Au. anamensis was found in 1966, the holotype was found 28 years later.

Even the holotype discovery date is not really the time of “species discovery”. Generally, a team of scientists tests the hypothesis that a new fossil assemblage belongs to an existing species. If the evidence rejects this hypothesis, a team may move toward formally defining a new species. “Discovery” of this new species may happen more-or-less gradually during the analysis of fossil remains, as researchers develop evidence in comparison with other fossil samples. In some cases the team may recognize this distinctiveness very rapidly, in others more slowly, depending upon the quality of the evidence and the difficulty of making the comparisons. Sometimes additional field seasons may be necessary to add more fossil specimens and thereby broaden the scope of comparisons. In some cases, it is the holotype that is unearthed during a later field season, well after a field team has other specimens that make them think a new species exists.

Looking at the records available in formal taxonomic diagnoses, the only practical alternative is to consider the time of holotype discovery. Papers usually report this date and do not reliably list the discovery dates of paratype specimens. Even with holotypes, the reporting is uneven. Some papers have reported a single day of discovery, in which case it is simple to calculate the time from discovery to publication. In many cases, however, only a month of discovery (e.g., “March 2014”) is provided. There several reasons why a single date may not be available. A holotype may have taken several days to uncover during excavation, or it may have been reconstructed from fragments found over a range of dates. In extreme cases, parts of a holotype might have been found in successive field seasons.

In a few cases, the paper gives only the year of discovery (for example, the only date given for KNM-KP 29281 is “1994”). For my comparisons here, if the paper gives no indication of the timing of the field season, I have assigned a date of January 1 to these discoveries. The resulting timeline is necessarily longer than the real time taken by a research team to describe its discovery, in theory up to 12 months longer if the specimen was actually discovered in December.

The results

Number of months from holotype discovery to publication for hominin species since 1990
Asterisks (**) indicate species for which the discovery year was supplied but not the date; the point in this graph assumes discovery on January 1 of that year, making this the maximum possible time from discovery to publication. In reality they may be as much as 12 months less than indicated here.

In this chart, H. cepranensis is an outlier, as discussed below. The remainder of the data show no reduction or increase in the time from discovery to publication over the last 25 years. Out of the fifteen formally named taxa here, ten were published within two years after the discovery of the holotype specimen. Only two of those appeared within one year after discovery. The median time from discovery to description is 20 months.

The x-axis of the chart is the discovery date, not the publication date. There may be taxa that have been discovered in the past few years that have not yet been published, and obviously any such species would not be represented in the chart. H. gautengensis, published in 2010 but based on a holotype discovered in 1976, is not included in the chart.

This period of two years or less includes the time spent by the species in peer review and revision, which I discussed in the previous post. In the case of Homo floresiensis, for example, the time from submission to acceptance of the manuscript was more than 6 months, and the publication of the paper was still only 14 months after the discovery. The paper describing Australopithecus ramidus was slightly more than two months from submission to acceptance; the total timeline from discovery of the holotype to publication was only 9 months.

In light of discussion about the publication pace of Homo naledi, these data may surprise people. H. naledi may be remarkable for the quantity of anatomical evidence, but not the time from discovery to publication. It took substantially longer to move from discovery to publication for H. naledi than for Au. ramidus (9 months), S. tchadensis (12 months), or O. tugenensis (3 months), and even longer than H. floresiensis (14 months) and Au. garhi (17 months).

What explains the consistency in time to publication?

A varied array of research teams around the world have composed effective and highly-cited diagnoses on a varied array of fossil assemblages, with all necessary research and editorial handling and peer review within two years or less. Setting aside diagnoses of new taxa based on previously-published fossil assemblages, the timeline of the procedure has been remarkably consistent.

I think this consistency of timeline can in large part be attributed to the consistency of content.

  • Formal diagnoses follow a common recipe, with a stereotypical block giving essential diagnostic information, and a discussion that places the new taxon into a phylogenetic and adaptive context.

  • The addition of a new taxon generally must reiterate the key information of the next most similar taxon.

  • Most diagnoses of hominin taxa are relatively short, with a median of 7 text pages. Only Homo naledi and Homo gautengensis were diagnosed in papers longer than 11 pages.

  • Although scientific papers have undergone a major trend during the past 15 years to add supplementary information in addition to the main text, this has affected very few of the hominin taxa included here. Only Au. deyiremeda, Au. sediba, and H. floresiensis were accompanied by supplementary information of substantial length.

Some of these species are now represented by fossil samples that were difficult to reconstruct and analyze. But in nearly all cases, the reconstruction and analysis was a second phase of work that followed the formal diagnosis of the new taxon. Therefore most of the difficult work of reconstruction could follow formal diagnosis. This was perhaps most famously the situation with Au. ramidus, where the most famous specimen is not the holotype. But to one degree or another, the process of later, deeper description has been a routine part of the study of most of these species.

The exceptions

What about the exceptions, the species that took much longer than two years to diagnose?

Five taxa required more than two years from discovery to publication. The diagnosis of one of those taxa, H. cepranensis, was based on a holotype specimen that had been described in a peer-reviewed publication several years earlier. To this we can add several similar cases of new formal diagnoses of taxa published in the last 25 years based on previously published fossil material.

  • Curnoe (2010) based the diagnosis of H. gautengensis on StW 53, which was found in 1976, with a description published by Hughes and Tobias (1977).

  • White et al. (1995) based their diagnosis of the genus Ardipithecus on the species diagnosis of Au. ramidus published in 1994.

  • Haile-Selassie et al. (2004) based their diagnosis of the species Ar. kadabba upon the diagnosis of the subspecies Ar. ramidus kadabba that Haile-Selassie had previously published in 2001.

I don’t think these cases are directly comparable to the original first assessment of newly excavated material. Such secondary study is also a process of discovery, sometimes undertaken due to the recovery of additional material (as in the case of Ardipithecus), but the timeline of such research is extremely variable.

Two of the taxa that took longer than two years to diagnose were subspecies: Ar. ramidus kadabba and H. sapiens idaltu. Providing such formal diagnoses of subspecies is a relatively new innovation in hominin taxonomic practice. One motivation to name a subspecies is to preclude the use of a holotype specimen for other taxonomic diagnosis in the future. As the case of Ar. kadabba shows, future discoveries may force a reassessment of the variation presented by a sample, sometimes prompting the promotion of such a subspecies to a species-level taxon.

I can think of two hypotheses to explain why the diagnosis of these subspecies has occupied a longer period of time than species or genera. One possibility is that the relatively subtle anatomical variation that distinguished a subspecies may require more time to fully understand and characterize. A second possibility is that researchers may work on new species and genus-level diagnoses with greater intensity than a subspecies diagnosis. These are not mutually exclusive, and probably there are other possibilities as well. But again, it is not clear that researchers treat the formal diagnosis of a subspecies as the same kind of task as the diagnosis of a higher-level taxon.

This leaves only Au. deyiremeda and H. antecessor, which required approximately 4 years and 3 years from holotype discovery to description, respectively. Both of these are instances in which additional field seasons may have added more information from additional specimens, and as I move forward, I’ll consider whether the sample size and anatomical regions preserved in the holotype and paratype specimens help to explain the timeline of either of these taxa.

Is everyone “rushing” their research?

The standards by which I judge the quality of science are replicability and originality, not speed. The formal diagnosis of a taxon is one of the least creative exercises in paleoanthropology, with several highly standardized parts. These essential ingredients have emerged through history as a way of ensuring the replicability of the key observations that contribute to attributing other fossils in the future. Considering that many hominin fossils are practically inaccessible to independent scientists, scientists must insist that the formal description of such fossils will meet a high standard of replicability.

With that in mind, is there a correlation between replicability and speed of publication?

I don’t think so. Among the papers that are published within four years of holotype discovery, I just don’t see any obvious correlation between time to publication and replicability.

Keep in mind that this is a very small sample to try to find such a correlation. Certainly there are papers here that omit crucial data, there are papers that have given rise to years of controversy, and there are papers that have led to relatively little subsequent reassessment of the broader phylogenetic pattern of hominins. Any set of independent scientists would probably have a wide variety of “favorites” or choices as “best hominin description EVAH”.

But even if we take the strongest critics of new species, I don’t think we see any relationship between a person’s preferences about which diagnoses are replicable and the timeline of publication.

As an example, Tim White has been publicly critical of many of these species as examples of “taxonomic inflation”. Such “inflated” species include H. naledi, Au. deyiremeda, Au. sediba, K. platyops, O. tugenensis, S. tchadensis, Au. anamensis, and H. georgicus. I may be missing several. The median time from discovery to publication of these eight taxa is at most 20 months. The median time for the three taxa published by White himself is 17 months. Whatever the standard of quality and replicability applied by White, time is not an explanation.

Production of a taxonomic assessment for newly discovered hominin fossils is a basic responsibility of field research. Assessing whether a fossil or an assemblage belong to a previously-known taxon is relatively straightforward. That assessment may rely upon small anatomical details, but a review of the anatomy of a fossil will not likely miss those details if they are present. So there’s nothing about the procedure that in principle should take many years to accomplish.

In light of the evidence from the last 25 years of formal taxonomic diagnosis in hominins, it is clear that in most cases, this process is very efficient.


  1. As in the previous post, I am missing data for Australopithecus bahrelghazali.


References for this post are the same as listed in “Hominin species and time in peer review”.