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paleoanthropology, genetics and evolution

Photo Credit: Okladnikov Cave. John Hawks CC-BY-NC-ND

Third molar agenesis: a puzzling case of recent human evolution

One of the most obvious cases of recent human evolution is the increasing frequency with which individuals don’t develop third molars, what is called “M3 agenesis”. This condition is when the third molars, or wisdom teeth, don’t form at all – the individual never developed them.

I could be wrong, and I’m sure that a reader will remind me if I am, but I cannot think of an instance of M3 agenesis in hominins outside of modern humans. It was entirely typical for most hominins throughout our evolutionary history to develop and erupt third molars into normal occlusion.

Agenesis of the third molars is different from cases where the M3 fails to erupt normally or remains impacted within the jaw. It is also different from the dental or surgical removal of third molars, which is now carried out on a very high proportion of people in the United States and many other countries. The number of students in my courses at the University of Wisconsin that still have their third molars is pretty small, generally less than 20 percent and often less than 10 percent. The majority of third molar removal is for orthodontic purposes. When third molars erupt, they exert forces on the rest of the teeth that can cause crowding and malocclusion, and more rarely, severe pain. One of the easiest ways to maintain straight teeth is to pull the ones in the back.

The problems with crowding of the anterior teeth points to the reason that anthropologists have traditionally given for the change toward a higher incidence of agenesis. Our Pleistocene ancestors very rarely had malocclusions due to dental crowding. The development of our jaws and teeth evolved within a context where people lived hunter-gatherer lifestyles and ate fairly tough foods, even as young children. By contrast, crowding of the dentition and resulting malocclusion have been very common in many Holocene populations, mostly those with agricultural subsistence. Agriculturalists don’t eat as many tough foods; they eat a large fraction of cooked grains and easy-to-chew cooked plants, milk, and meats. The idea is that the development of the jaw is plastic, and a reduction of the forces exerted on the dentition during early childhood can alter the developmental trajectory of the mandible and maxilla. If the jaws don’t develop as to as large an adult size, the teeth will tend to be crowded, and the last teeth to initiate development, the third molars, may not form at all.

This is an elegant story in some ways, but in fact we don’t know whether it’s true.

We do know that the teeth develop from an embryonic tissue sheet, which is divided into segments by genes during early development. Those segments expand and wrinkle into the form of tooth germs in a characteristic pattern, which is retained across most mammal orders, although many of the orders have come to have different numbers of teeth, which result from evolution of this early developmental pattern. Within a species, if an individual ends up with a segment that is too small or doesn’t develop quite right, that tooth will not form. The most common change is to the extreme tooth, which is the third molar. Less commonly, individual teeth within the other types may fail to develop – P4 agenesis is fairly frequent, sometimes I2 agenesis. Some of these kinds of agenesis are syndromic, meaning that other biological traits covary strongly with them.

How jaw forces might influence the pattern of tooth development is not well understood. Nor is it well understood how heritable M3 agenesis may be – to the extent it seems to run in families, this might reflect similar environments or similar genetics.

We do know quite a bit about the incidence of M3 agenesis in different human populations. The trait is nearly universal – it occurs everywhere in large enough samples of people, even in hunter-gatherer peoples who have recently continued eating “wild” food diets. But it differs greatly in frequencies.

Carter and Worthington (2015) did a meta-analysis of studies that have estimated M3 agenesis frequencies in particular human populations. By looking at 92 studies in different regions of the world, they give a global picture of where humans are more or less likely to have agenesis of the third molars. They limited their analysis to include studies of living human populations with radiographic evidence, so the agenesis was clearly documented.

Here’s the picture summarizing the frequencies in different regions:

Studies showing M3 agenesis frequencies
Figure 3 from Carter and Worthington (2015). Original caption: "Bean plot of the proportion of individuals with agenesis of at least 1 third molar, partitioned by geographic provenience (92 studies, 100 effect sizes). Solid diamonds and error bars denote the means (inverse variance weighted) and 95% confidence intervals (CIs) of proportions for each region. Dark gray circles are individual studies, while light gray polygons are kernel density estimates of the distribution of proportions within each region. Analysis was performed on logit-transformed proportions, but means and 95% CIs are back-transformed for easier interpretation. LRT, likelihood ratio test."

The incidence of M3 agenesis is lowest in African populations today. This chart represents only two studies in Africans, but the observation of a very low frequency of M3 agenesis is in accord with studies of skeletal samples in my experience. Other populations have a higher frequency, although the study does not include any populations indigenous to Australia, New Guinea or the nearby areas of Melanesia, which in my experience also have low frequencies of M3 agenesis. Some of the highest population frequencies are observed in Asia; these are sampled broadly, including one sample from South India, several studies of Japanese, Turkish, Israeli and Iraqi. So it’s not specifically East Asian, but more broadly some population samples across the continent.

The most common M3 agenesis is a single missing third molar, and that is what is illustrated in the chart. It is less common to be missing two molars, and very uncommon to be missing three or all four of them. Women are slightly more likely to be missing an M3 than men (Carter and Worthington found a 14 percent greater likelihood in women).

I know that many anthropologists lecture about M3 agenesis as a recent evolutionary change, but it is a complicated one. Few look deeply into the pattern, which is strongly parallel across many human populations, not regionalized. In a broad sense, the frequencies of M3 agenesis across populations covary with molar sizes, and it may simply be that the evolution of smaller tooth size has M3 agenesis as a frequent side effect. The greater incidence of agenesis in women also might be expected as a correlate of smaller molar sizes.

There is no evidence that M3 agenesis is itself an adaptation that has been favored by natural or sexual selection. The possibility of sexual selection presents itself in this context because of the possible effects of a crowded dentition on mating preferences in past populations. But if M3 agenesis owes its recent high frequency to selection, it is likely as a side effect of selection for smaller teeth. However, even that is hardly so simple, as the pattern of size reduction in teeth was not uniform in Holocene populations, and the frequencies of M3 agenesis fluctuate substantially among studies.

And we do not know how much of M3 agenesis may be explained by plasticity of development within current environments. Some studies show a difference within a geographic sample between people who have originated from different immigrant populations (for example, in Singapore between people of Chinese, Malay, and Indian ethnicity), but none have really evaluated the frequencies in second-generation and third-generation immigrants. The genetics of the trait, even heritability, is basically an open question. Of course, if it were entirely explained by environment, M3 agenesis would not be an example of evolutionary change at all.

So M3 agenesis is a fascinating example of recent biological change in human populations, and we know very little about how and why it has changed.

Reference

Carter, K., & Worthington, S. (2015). Morphologic and Demographic Predictors of Third Molar Agenesis A Systematic Review and Meta-analysis. Journal of dental research, 94(7), 886-894. doi:10.1177/0022034515581644


Just a note that ducks provide many great examples of hybridization dynamics, particularly invasive ducks. This recent paper on geese by Jente Ottenburghs and colleagues (“Hybridization in geese: a review”) shows that they are much the same. Lots of geese species, lots of hybridization, including cross-generic hybridization.

Most hybrid geese are fertile; only in crosses between distantly related species do female hybrids become sterile. This fertility pattern, which is in line with Haldane’s Rule, may facilitate interspecific gene flow between closely related species. The knowledge on hybrid geese should be used, in combination with the information available on hybridization in ducks, to study the process of avian speciation.

Travis Pickering and colleagues have a paper presenting new teeth from Swartkrans, which they attribute to Paranthropus robustus: “New early Pleistocene hominin teeth from the Swartkrans Formation, South Africa”. I’m pointing to the paper because it has a long and clear review of the complex history of site formation in this ancient cave.

Neandertal anti-defamation files, 22

This, from ScienceAlert:

We caught modern genital warts because our ancestors were banging Neanderthals

Nothing like this story as been such a race to the bottom for headlines for a long time.

I will note that Annalee Newitz has a much better version of the story with this winning quote:

[A]lways practice safer sex, even if you're getting down with a Denisovan.

There is, by the way, the interesting question of whether Neandertal immune variants might influence susceptibility to the strain in question, which has made little inroad into sub-Saharan Africa.

Race and the medical student

Ike Swetlitz in Stat has an article about the ways that some medical educators are trying to build a more anthropological knowledge of race and health in their students: “Teaching medical students to challenge ‘unscientific’ racial categories”. Unfortunately both the headline and article get a bit snarled in their use of language. Maybe that’s inevitable.

“You see ‘African American,’ automatically just circle ‘sickle cell,’” said Nermine Abdelwahab, a first-year student at the University of Minnesota Medical School, recounting tips she’s heard from older classmates describing the “sad reality” of the tests.
Medical school curricula traditionally leave little room for nuanced discussions about the impact of race and racism on health, physicians and sociologists say. Instead, students learn to see race as a diagnostic shortcut, as lectures, textbooks, and scientific journal articles divide patients by racial categories, reinforcing the idea that race is biological. That mind-set can lead to misdiagnoses, such as treating sickle cell anemia as a largely “black” disease.

In an episode of M*A*S*H from the early 1980s, Corporal Klinger starts suffering from a rare side effect of the anti-malarial drug primaquine. The doctors know that the drug has the potential of negative side effects in blacks, but issue it to everyone else. Hawkeye and the other doctors assume Klinger is just goldbricking. But another soldier, Private Goldman, starts to exhibit the same symptoms. The doctors determine that both Klinger and Goldman are suffering anemia, and take them off the primaquine. At the end of the program, it is revealed that people of Levantine origin (like Klinger) and Ashkenazi Jews (like Goldman) also may have the same susceptibility to primaquine side effects owing to their ancestry.

The side effect in question is a breakdown of blood cells and consequent anemia in people with G6PD deficiency, which is indeed very common in sub-Saharan and North Africans, and less common but still notable in people of broader Mediterranean descent.

I like the program quite a lot, and I remember it from the first time it was broadcast. It is a well-scripted way illustration of how a physician can make erroneous assumptions about ancestry and genetics that lead to bad treatment. But it also goes to show that there’s very little new in today’s attempts to improve medical school training with respect to race and medicine. These are all ideas that were well-known more than forty years ago and have been staples of anthropology.

Of course today we can know from anyone’s genotype data whether they have a susceptibility to some adverse drug reactions, and that includes many that do not have much higher frequencies in one population or another. Whatever there is to be said for genotyping, it beats census categories if you are looking to diagnose most common traits influenced by Mendelian genes. If we are training medical students for the world of five or ten years from now, allowing them to make effective use of this information should be the priority.

Will virtual reality compete with museums or help them?

Linking to a provocative piece that “VR Will Break Museums”.

The article discusses many issues with museums both large and small. The best museums add context to the objects that they display, putting them into a story that builds knowledge in the museum-goer. But some concepts are incredibly difficult to communicate in that fashion, and others rely so much on place that removing objects to a museum does not convey their context accurately.

And the crowds suck. Nothing is better than a huge museum on a very empty day, and those don’t happen very often.

The problem is, museums don’t scale well. The British Museum sees almost 7 million visitors a year. What would it take to accommodate double that number? Ten times that number? It simply cannot be done, not for any reasonable amount of money.
The Internet, on the other hand, is built for scale. The marginal cost of an extra YouTube viewer or app download is practically zero. That’s how a video about the history of Japan can be made for free, distributed for free, and enjoyed for free by more people in a single month than who walk through the doors of the Louvre in an entire year.

Museums are a very important part of human evolution research, both by serving as repositories for the objects we study and for helping the public to understand the importance of our science. I’ve consulted with many museums over the years and have visited a large fraction of the major museums of natural history in the world.

Human genetics is more and more important to how we understand human evolution. Yet this is one of the most difficult parts of science to illustrate in a museum setting. Museums excel at visual material and unique objects. While it is possible to do video or virtual content for genetics, whenever I encounter videos at a museum, I groan. They’re always a chore and rarely hit the mark as well as simple text accompanying an object.

Honestly, I think that museums face much the same problem as movies. Studios invest tremendous sums of money in movies that have bad scripts. There are many reasons for this – sometimes the director has too much power and keeps shifting the script, sometimes the original idea relied upon visuals that cannot be realized, sometimes studio executives ruin a cohesive script by committee. Whatever is the case, the ultimate reason why this situation happens so often is the same: Audience demand for certain kinds of movies is just not very responsive to script quality.

Likewise, public visitation to certain museums doesn’t respond much to the quality of stories they can effectively tell. A museum exhibition with bad videos is regrettable, but most people skip the videos anyway. Especially if the first few seconds of the first one doesn’t connect.

The great thing about the idea of virtual experiences is that they may be laboratories for real innovation in storytelling. The stories that work should be translated into the museums that audiences already value. As someone who has done a lot of museum consulting, I can really imagine having a lot of fun helping make virtual experiences that educate and convey exciting science.

Incidental capuchin flake manufacture

This is a nice article by Ed Yong about Michael Haslam’s research documenting how capuchin monkeys incidentally make stone flakes as a side effect of their nut smashing: “Rock-Smashing Monkeys Unintentionally Make Sharp Stone Tools”.

The interesting thing is that many of the flakes are indistinguishable on technical grounds from Oldowan flakes. That raises the possibility that intermittent, possibly local traditions of nut-cracking among some forms of primates might create the appearance of localized flake assemblages.

He also thinks that archaeologists should spend more time looking for the works of ancient monkeys and apes. “[We should] consider what other non-human primates in Africa and elsewhere may have been up to for the past tens of millions of years,” he says. “There is no reason why stone flakes may not be littered throughout primate history, at unknown places and times.”

Actually, I think there is nothing materially different about what these capuchins are doing and what early hominins were doing when they made flakes. One may say that the hominins made flakes “intentionally”, or with a goal in mind to use the flake. But the difference here is not cognitive, it that the capuchins have not learned socially to use flakes for anything.

Primatologists already observe capuchin monkeys learning socially to lift rocks nearly half their body weight, thrust them down onto a platform with a fruit, and repeat this until yummy bits of nutmeat scatter everywhere. It would not take any great cognitive advance for them to learn how to use flakes, if there were something useful for them to do. The ability to generalize is simply the ability to emulate others.


Notable paper: William Hutchison et al. (2016) A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans. Nature Communications 7, 13192. doi:10.1038/ncomms13192

Synopsis: Many of today’s lakes and volcanic calderas of the central Ethiopian rift were the outcome of a cluster of volcanic activity between 320,000 and 170,000 years ago.

Interesting because: Early examples of Middle Stone Age (MSA) archaeological industries were developing at this time, and possibly the immediate ancestors of most of today’s gene pool were making some of them. Volcanoes might have affected the local environment.

KILL THIS QUOTE WITH FIRE: “current evidence overwhelming [sic] suggests that all major events in hominin evolution occurred in East Africa”. No. Just no.

Bottom line: It is very hard to test how local environmental shifts may have affected hominin populations; either prompting adaptation or creating population sinks. My guess would be that a region full of active volcanoes probably acted either as a sink or a barrier to gene flow. As an extreme, these volcanoes may have impeded migration from the Afar region further south along the African rift during the later Middle Pleistocene, making this fossil-rich region a relative cul-de-sac. However, in my opinion a 30-km zone of unpleasantness surrounding a volcanic caldera is not much of a barrier to mobile and interconnected hominin populations. Ancient people probably looked in wonder at the great forces within the earth, and watched their children played in the snowing ash.


Theodosius Dobzhansky, in his essay, “On species and races of living and fossil man” (1944):

The opinion is often expressed that species and races are arbitrary categories. This opinion is false. If given the opportunity to secure the necessary data, a biologist is able in a majority of cases to decide beyond a reasonable doubt whether the forms under study are distinct species or only distinct races. Lion, tiger, leopard, and domestic cat are species ; Angora cat and alley cat are surely not species but races. However, “borderline cases”, in which it is impossible to decide whether one is dealing with species or with races, do exist. Indeed, their existence was used by Darwin to demonstrate organic evolution. If species are the primordial units of creation, or else if they arise by sudden leaps (as thought by G. St. Hilaire and recently by Goldschmidt), then we should be able to find methods to decide whether any two forms are still races or already species. If, on the other hand, species evolve gradually from races, then the decision will be possible only in some, perhaps in a majority, of cases, but at least some instances must be found in which forms are too distinct to be races but not distinct enough to be species. Evolutionists have concentrated their efforts on proving that such borderline cases do exist; by indirection they conveyed to biologists in general the impression that there are no other but borderline cases.

Misuse of statistics

This essay should be required reading for graduate students: “The problem with p-values”. David Colquhoun writes extensively about science and statistics, and in this essay he brings out many of the biggest misconceptions that drive poor conclusions in scientific practice.

Even quite respectable sources will tell you that the p-value is the probability that your observations occurred by chance. And that is plain wrong.

Paleoanthropology is one field in which papers that point out poor use of statistics are publishable. The primary fossil data are very sparse, and we work hard to establish what little we can say with confidence. There are generally scientists willing to criticize statistically misleading attempts to answer the unanswerable.

Personally, I’ve spent a good amount of time looking into the basic statistical underpinnings of human evolution datasets. For example, my paper “How much can cladistics tell us about early hominid relationships?” showed that the datasets of most hominin species are simply not big enough to yield confident conclusions about how they are related to each other. A later paper, “No brain expansion in Australopithecus boisei, worked through statistical issues with time-series data on which paleoanthropologists have often based conclusions about trends in morphological features over time.

My all-time favorite paper outlining statistical problems in human evolution research is by Richard Smith, “Biology and body size in human evolution: statistical inference misapplied.” Smith shows a systematic problem with the most common comparisons of ancient human relatives. Now, twenty years after that paper was first published, most papers that consider body masses of fossil hominins still get this wrong.

That’s a well-worn tale in science. Pointing out statistical errors is sisyphean. Many recent papers in human evolution reflect poor statistical practice. And a good number of “classic” results are based on datasets that today would be statistically doubtful. Science is self-correcting, but it is going to take some hard work to get this stuff straight.

References

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

Hawks, J. (2011). No brain expansion in Australopithecus boisei. American journal of physical anthropology, 146(2), 155-160. doi:10.1002/ajpa.21420

Smith, R. J. (1996). Biology and body size in human evolution: statistical inference misapplied. Current Anthropology, 37(3), 451-481. JSTOR

Everything Neandertal is not bad

This is not a bad story about Neandertals by Melissa Hogenboom: “What Neanderthals’ healthy teeth tell us about their minds”. It’s an overview of what scientists learn from various kinds of dental studies.

But I wanted to comment on one thing:

"We realised nobody had directly compared Neanderthal [teeth loss] to modern humans, so we didn't realise Neanderthals had [slightly less] tooth loss," says Weaver.
This flies in the face of previous studies, which suggested that several Neanderthals lived long after losing all, or nearly all, their teeth.
But bizarrely, the finding that Neanderthals apparently had healthy teeth actually suggests something rather negative about them.

I don’t disagree that there is a slight difference in tooth loss. That does not contradict the real observation that some Neandertal individuals had extensive premortem tooth loss.

La Chapelle-aux-Saints mandible, photo by Milford Wolpoff
Mandible of the Neandertal with the most premortem tooth loss, La Chapelle-aux-Saints. Photo by Milford Wolpoff.

I agree that some people have made more cultural conclusions from the observation of tooth loss than the data warrant. I wrote about this back in 2005, when the subject of discussion was total premortem tooth loss in the Dmanisi skull D3444: “Caring for the edentulous”. My conclusions today don’t differ from then:

In the case of life history variation, I think that the survival of a small number of individuals under extraordinary circumstances says little about the habitual capabilities of a species.

A handful of Neandertals lived with fairly extensive loss of dental function, but that probably doesn’t tell us much about Neandertals that we do not already know about many primates.

What I really object to in the linked article is the way this topic is framed: This good thing about Neandertals “bizarrely suggests something negative about them.”

Often in human evolution, research is presented with a simple storyline that goes like this: “This bad thing everybody knows about, well, guess what—it’s actually good when you think about it from the evolutionary perspective.”

That’s the storyline of the sickle cell mutation. It causes disease, but it is an adaptation to malaria. It’s also the storyline of the “thrifty genotype” idea: Diabetes is bad, but its occurrence today may be a side effect of ancient adaptations to food scarcity.

That framing doesn’t erase the bad aspects of such biological traits, but it does give people a different way of thinking about why bad things happen. “Everything bad is actually good” is a fairly useful frame for teaching human evolution.

The opposite storyline is also pretty common. “That good thing that everybody knows about? Well, guess what—it’s actually bad when you think about it from the evolutionary perspective.”

This is how the invention of agriculture gets portrayed nowadays. Jared Diamond famously called it, “The worst mistake in the history of the human race.” The idea is that once upon a time, humans were adapted to a hunter-gatherer existence, and agricultural subsistence caused scores of bad unforeseen effects.

This “everything good is actually bad” storyline is especially common when it comes to studying Neandertals. For example, Neandertals seem to have eaten lots of meat. Does that mean they were successful hunters optimizing resources in a harsh environment? No, it means they failed to build knowledge of plant foods, putting them at extreme risk of extinction when times got tough.

Another example: Some Neandertal sites seem to have different assemblages of tools that may be suited to different functional tasks, for example some toolkits include many scrapers for preparing hides, while others lack such a dominance of scrapers. Is this evidence of clever and flexible Neandertals? No, to some archaeologists it was evidence that Neandertals must have behaved like herd animals, with women and children in some camps, and small groups of bachelor males in others.

My favorite essay on such stereotypes is by the archaeologist John Speth: “News Flash: Negative Evidence Convicts Neanderthals of Gross Mental Incompetence”. He humorously reviews a series of extravagant claims about what Neandertals couldn’t do, mostly tests that would fail when applied to some groups of modern humans.

Every so often, new evidence convincingly debunks one of these Neandertal stereotypes. For example, over the past ten years, a series of papers describing starches and phytoliths in Neandertal dental calculus have documented their use of plant resources, including cooking of some grains and the possible use of medicinal plants. In this case and many others, the press has reported the “surprising” conclusion that Neandertals were very much like modern human subsistence foragers.

Just once, I would like to see a journalist report such results as unsurprising evidence that past archaeologists were incompetent.

Now I don’t want to go overboard in the opposite direction. There probably really were some strange things about some Neandertals. Culture did evolve, and the evolutionary history of Neandertals probably yielded cultural abilities that humans lack, just as modern humans may have abilities that they lacked. In other words, I do not assume that they were merely modern humans with browridges.

But these are among the hardest ideas to test with archaeological evidence. At the same time, ideas about Neandertal cognitive difference align with persistent stereotypes about Neandertals. For that reason, I maintain an attitude of skepticism.

We know a good amount. Some of the cultural behaviors of recent and living modern humans have never been noted in Neandertal sites. But their tools, the traces of animals and plants that they ate, and their use of space show that Neandertal subsistence behavior had a lot in common with modern human subsistence foragers.

Many modern human subsistence foraging groups have left little or no evidence of “symbolic” artifacts, “complex site structure”, musical instruments, projectile weapons or similar trappings. We now know that Neandertals used pigments, engraved objects and rock surfaces, wore ornaments, made and used many kinds of bone tools, used shellfish, birds and small mammals—basically all things that past stereotypes held they didn’t do.

The point is, the difference between Neandertal and modern human behavior is clearly not a yawning chasm. They overlapped.

Studying Neandertal biological traits in combination with archaeology has a lot to offer to understanding their behavior. Dental pathology is a great avenue to understand how their health relates to their subsistence behavior. For example, Neandertals were once believed to have a much higher incidence of developmental dental pathologies than modern humans, traits like linear enamel hypoplasias that result from stress on the developing teeth from nutritional shortfalls or disease. It turns out that many modern human groups have just as high an incidence of such dental traits as Neandertals, including children from many agricultural groups.

Both Neandertals and prehistoric modern human subsistence foragers have vastly lower incidence of dental pathologies like caries when compared to most agricultural peoples, so it’s interesting to see that tooth loss was actually less among Neandertals than in the prehistoric modern human groups.

Better teeth may reflect the basic fact that Neandertals died faster than the Upper Paleolithic modern humans that followed them. By our best estimates (provided by Rachel Caspari and Sang-Hee Lee), Neandertal mortality was greater across the adult life span. Most known Neandertal dental remains come from relatively young adults, less than thirty or so years old.

Such high mortality probably does indicate something about Neandertal social relationships. Today’s human cultures owe much to the knowledge and experience of older adults, common in human societies. If those older adults were rarer, with some groups lacking older adults altogether, Neandertal cultures must have been poorer for it.

But adults in their twenties and thirties are not poor caregivers today. In fact, they are the primary caregivers toward both children and the most aged adults in our societies.

For that reason, I resist the framing that Neandertals good teeth may have meant they took less care of the sick. It may well have been true that sick Neandertals did not live as long, or have as good a chance of recovery. But I attribute that to the basic challenges of subsistence, not social incompetence.

References

Caspari, R., & Lee, S. H. (2004). Older age becomes common late in human evolution. Proceedings of the National Academy of Sciences of the United States of America, 101(30), 10895-10900. doi:10.1073/pnas.0402857101

Speth, J. (2004). News flash: negative evidence convicts Neanderthals of gross mental incompetence. World archaeology, 36(4), 519-526. doi:10.1080/0043824042000303692


National Geographic has a neat story by Michael Greshko about ancient footprints near Lake Natron, in Tanzania: “Treasure Trove of Ancient Human Footprints Found Near Volcano.

Local villager Kongo Sakkae found some of the footprints prior to 2006, but the site didn’t reach scientists’ attention until 2008, when Pennsylvania-based conservationist Jim Brett happened to be staying at the Lake Natron Tented Camp, just a few hundred yards from the footprints. Stunned by what he saw, Brett snapped as many pictures as he could and resolved to pass them along to a scientist he knew he could trust: Liutkus-Pierce, whom he had met when she was a postdoctoral researcher.
The trouble was, Brett picked the worst possible day to call.
“It was April Fool’s Day, I kid you not,” says Liutkus-Price. “He called me and said, ‘I think I have found some really cool hominid footprints.’ And I said, ‘Jim, can you call me tomorrow, so I know that this is not a joke?’”

The article gives a very nice account of the realization that these are fossil footprints, notes that they had been known to some local people earlier, discusses how collaborators were brought into the project, does not avoid mentioning a big problem with one of the researchers, and generally does a great job of showing how science is done.

Talking to the public is essential to the future of science

Susanna Martinez-Conde, Stephen Macknik and Devin Powell have a short article with advice for would-be science popularizers: “How Scientists Can Engage the Public without Risking Their Careers”.

Daniel Kahneman, who published Thinking, Fast and Slow nearly a decade after he won the Nobel Memorial Prize in Economics, went to almost heroic lengths to ensure that traditional academics would approve of his book. “It took me four years to write my book—four exceedingly miserable years because I was explicitly trying to do two things,” he says. “I wanted to write a popular book, without losing the respect of my colleagues. I paid a lot of money to have my book reviewed anonymously by young associate professors, four of them, I paid them two thousand dollars each, to tell me whether this would damage my reputation. And interestingly enough, it did damage my reputation among a subset of my colleagues.” Even so, Kahneman feels that the academic reaction to his book was by and large, very positive

I admire scientists like Kahneman who make the decision to reallocate their time toward public activities. By doing so, they advance the work of their colleagues and their field in general. His solution seems a little extreme, but I am glad to see how he values the referee work and the opinions of other scientific colleagues.

Interacting with the public is hard work. Being effective at public communication is a skill that scientists develop only through extensive practice, talking and writing honestly about the value of scientific work and its implications. Being an effective teacher is equally challenging, and while the skillsets do overlap, they are not identical.

I do a lot of public writing, blogging, documentary consulting, and filming. This work has made me a better scientist. I see the effects throughout my research. I make discoveries, and as much as possible, I get to take lots of other people with me. I don’t want to work any other way.

Most academics in my field have been totally supportive of the way I work. I do everything I can to promote the great work of friends and colleagues around the world. It’s always great to hear about connections made, and I’ve found so many friends in unexpected places.

I can’t deny that I’ve heard some criticism over the years. That kind of criticism is what stops many young scientists from even trying to engage outside their narrow subdiscipline. It’s entirely rational to keep your head down and stick to academic journal articles.

But honestly, most critical comments come from a small number of professionals who spend a lot of effort trying to become “academic celebrities” themselves. Some people assume that a certain degree of success in science will automatically cause people to listen to everything they say. The world doesn’t work that way, nor would anyone sensible want it to.

I’ve learned a lot from those interactions. I’m still finding out new ways to persuade people that our origins matter, to show them how our scientific work makes a difference.

Brief thoughts on Upper Paleolithic 'hybrids'

Science News has a long feature article by Bruce Bower that recounts the new wave of examining hybridization in human origins: “Animal hybrids may hold clues to Neandertal-human interbreeding”. He features the symposium at the AAPA meetings this spring organized by Rebecca Ackermann, which involved many scientists researching hybridization in primates and other mammals.

The big idea is that the biological consequences of hybridization are often recognizable in the skeleton. Fossil human populations may include individuals with ancestry from diverse populations, like Neandertals and Denisovans, much more genetically different than any living populations are. Paleontologists might use insights from hybridization in other mammals to understand which fossils show evidence of such population mixture. The big success story is the Oase 1 mandible, which has a high proportion of Neandertal ancestry:

Since the fossil’s discovery in 2002, paleoanthropologist Erik Trinkaus of Washington University in St. Louis has argued that it displays signs of Neandertal influence, including a wide jaw and large teeth that get bigger toward the back of the mouth. In other ways, such as a distinct chin and narrow, high-set nose, a skull later found in Oase Cave looks more like that of a late Stone Age human than a Neandertal.
Roughly 6 to 9 percent of DNA extracted from the Romanian jaw comes from Neandertals, the team found.
“That study gave me great happiness,” Ackermann says. Genetic evidence of hybridization finally appeared in a fossil that had already been proposed as an example of what happened when humans dallied with Neandertals.

Not only does the Oase mandible have a high fraction of Neandertal ancestry, roughly 3 times higher than any living people today, but also that Neandertal-like DNA occurs in large chunks, indicating a recent Neandertal ancestry. This individual may have had a Neandertal great-great grandparent.

Such a recent potential Neandertal ancestor means that the population mixture in this real-life case is not too far from what occurs in some natural and laboratory experiments on hybridization. The models from the present day have great relevance to this past case.

I will add, the Oase individual is not a singular case. Many early Upper Paleolithic skeletal remains from Europe have comparable evidence of anatomical features that are common in Neandertals but very rare or absent in other populations. David Frayer has documented many of these in detail, joined by other scientists, especially Milford Wolpoff.

It just happens that Oase 1 is the first specimen with ancient DNA evidence showing substantially elevated Neandertal ancestry beyond that observed in later populations. The ‘Ust-Ishim femur does not have more Neandertal ancestry than recent humans, and although the value for Kostenki 14 is high, it is not an outlier. Both those specimens have Neandertal DNA in relatively longer linkage tracts than today’s people, allowing us to see that mixture had preceded the birth of these individuals by only 10,000–15,000 years or so. But Oase 1 was nearly on the scene; where this individual lived, population mixture was probably still underway.

We know from ancient DNA evidence of later specimens. The earlier Upper Paleolithic populations had more genetic influence from Neandertals than populations from the Mesolithic and later. In part, this reduction of Neandertal ancestry reflects natural selection against functional parts of the Neandertal genome; in part it probably reflects repeated immigration into Europe of people from West Asia or other parts of the world. A pattern that appears in morphological traits, with Neandertal resemblance declining over time in European Upper Paleolithic and later populations, is paralleled by genetics.


Joe Simmons on psychology: “What I Want Our Field To Prioritize”.

I am writing all of this because it’s hard to resolve a conflict when you don’t know what the other side wants. I honestly don’t know what those who are resistant to change want, but at least now they know what I want. I want to be in a field that prioritizes replicability over everything else. Maybe those who are resistant to change believe this too, and their resistance is about the means (e.g., public criticism) rather than the ends. Or maybe they don’t believe this, and think that concerns about replicability should take a back seat to something else. It would be helpful for those who are resistant to change to articulate their position. What do you want our field to prioritize, and why?

Simmons gives an example of his past work being shown false by non-replication in a vast sample, and makes some important points. Probably most salient, description of methods should allow other researchers to build on results.