Digging up the past (single species hypothesis revisited): Part II

In yesterday's post, we offered a brief rundown of the competing arguments re. the history of human expansion from Africa.  Briefly, the rival multiregional (MR) continuity, and OutOfAfrica (OOA) replacement ideas became iconified as two competing (food-fighting) schools of thought, and so it was for decades.  Either Homo erectus forms expanded across and also out of Africa, their descendants dispersing over many generations gradually to colonize the rest of the Old World in the Pleistocene 2 million years ago, and evolved gradually into modern humans, or modern humans evolved in East Africa and expanded into the rest of Africa and the Old World, into contact with other humans already living wherever they wandered into, and somehow driving them to extinction.  The idea that the two may have interbred was prurient and juicy, but seemed dismissed by the fossil DNA data.  People, and certainly anthropologists, like to polarize into rival camps and are not particularly good at nuance.  So we had the dispute, fruit and vegetables flew back and forth, but the kiddie cafeteria seemed to settle down when the OOA hypothesis felt in a position to declare victory.

But the issues are heating up again, triggered this time by a typically melodramatic series of articles in a typically melodramatic May 3d issue of Nature. Further Neanderthal genome sequence, from a few specimens, compared to sequence from some contemporary 'anatomically modern' fossils, all roughly 30-40,000 years old, and then more recent data from a Siberian fossil group known as Denisovans, have muddied the waters.  They show, in various technical ways, that there seemingly was interbreeding among these groups: basically, there are many nucleotide variants found in humans that differ from chimpanzees, and seem to represent new mutations having arisen in our lineage.  But other variants, found in the Neanderthal and Denisovan sequences but not in chimpanzees, are also found in humans.  They seem to represent ancient variants later introduced (by interbreeding) into the our modern human lineage--you and me!

The new interpretation, which of course could be only as reliable as last year's newspaper, is that a small percent, currently estimated at about 2.5%, of our DNA is due to admixture between the Neanderthal/Denisovan/modern denizens of the Eurasian past. 

Now there are the expected crow-calls being heard on the blogosphere, claiming vindication if not victory for the Michigan, single-species or MR school (at least, that if there had been many early in human evolution, there was only one by the time of Neanderthals)!   Get your lunch boxes ready, folks, as it is time yet again to hurl what you don't like in your lunch at somebody else.

Please hold your fire!
But let's try to sort credible evidence from religion here.  In truth, if the evidence is being reliably interpreted, there does seem to have been some mixing and mingling (and mating) among lineages of our ancestors who had been long-separated since their common ancestry in Africa.  This clearly shows that a strict multiple species view was wrong.  But much hinges not on incisive insight but on definition--of what a 'species' is and the criteria by which species are declared.

We've posted before on the whole species question, which is well-known to be a vague area with various definitions.  The idea, in a nutshell, is that organisms diverge from common ancestry to an extent that, eventually, they cannot interbreed.   The presumption is that this is for genetic reasons.  But others would define species as populations that do not interbreed.  Be they on different continents or in different levels of a forest, if they don't mate they're different species, and we need not worry particularly about whether technically they could interbreed.  If they don't do it, then eventually the assumption (which was Darwin's as well) is that they will lose that ability even had they the lingering desire.

Aha!
In this case, assuming the evidence and interpretation that's current, clearly the Neanderthals and modern OOAers diverged in morphology and this probably occurred in geographically distant areas, which is what one would expect of adaptative as well as chance genetic change, because isolated individuals don't mate and hence blend their various characteristics.

In the morphological sense, there were differences and the OOA view is correct.  Even if we agree that the differences were major enough for species designation, by the usual definition these were different species at the time ancestors expanded out of Africa 100,000 years ago.  So much for the single species view.

Well, not really!
And yet, the fact of inter-mating, again if interpretations today are accurate, shows that in a technical sense the groups were not different species by the mating-incompatibility definition.  So, as my alma mater goes, Hail, Hail to Michigan, Champions of the West!

We must note that some anthropologists still argue that they see comparable trends of modernization in Asian fossils that resemble those in Africa, in support of their MR view, and that they didn't need the DNA evidence to make this point. And many would argue that in any practical sense, such as related to cognitive abilities and so on, that some of these inter-breeders like the Neanderthals, were not inferior to 'modern' humans--they were all 'human', just not the same as each other.

Splitting hairs
We should stop all of this, which largely amounts to hair-splitting, and try to be accurate scientists instead of tribal warriors.  Regional divergence arose, just as there is regional divergence among humans today: In many obvious ways Africans don't look like Asians, and this is manifestly clear in terms of geographic genetic variation, too.  Except in the narrow technical definition, there were different species.

But the OOAers have no reason to cheer.  That's because interbreeding shows that genetic species differences of the classical sort hadn't arisen.  In that sense, the MR view is correct. But there is no place for crowing, because in the do-not-mix definitional sense, there were separate species.

Since there is no single answer to the amount or nature of genetic difference required to prevent fertile breeding,  and since what we do know is that that can vary hugely from example to example, and even the species definition itself is somewhat arbitrary, the dispute is largely a fake one, and the news-splash it regularly receives is either ignorance,  marketing, or just sport.

What remains interesting is the question I raised last time: the way in which the more modern creatures expanded out of Africa,  and met and interacted with their less modern-looking brethren.   And why the modern-lookers did, the evidence still seems to suggest, drive the others out of existence.  That is the really important question, and it's far more interesting than the DNA-based food fight.

(Thanks to Holly for helping me fix some of the phrasing of this and the previous post.)

Digging up the past (single species hypothesis revisited): Part I

This and tomorrow's post are stimulated by the appearance of a series of papers on human evolution that have appeared, with the usual hullaballoo, in the May 3 issue of Nature.   They are good papers, summarizing current views and recent data, and while they don't present much if any new information, they package recent results into a show-piece issue that looks good on newsstands.  And they have the blogosphere buzzing, so we thought we'd comment on the issues which, as usual, are being somewhat overblown.

Schools of thought schools
When I was a graduate student (long, long ago), there were major divisions among prominent physical anthropologists.  Some -- probably the bulk of the profession -- saw different fossils that seemed to be contemporary as being samples of representatives of different species.

Others applied population thinking and held the view that human ancestors were a single variable species, fossils were never exact contemporaries or had been living in different places in Africa (or, for later fossils, in Eurasia).  To them, variation included sexual dimorphism and ordinary geographic variation such as we see among humans today.  In this view, there had always been only one species of humans or our direct ancestors.

The two views represent widespread differences between 'lumpers' and 'splitters' in the field of systematics--naming and classifying organisms. Lumpers see fewer species and focus on continuity and variation, while splitters tend to favor more categories when they see differences among specimens. We won't get into that bigger difference of view, because it's not in our expertise.

Multiregional hypothesis

The single-species hypothesis saw the attempt by paleoanthropologists to assign species names to every specimen they found as being a kind of naming-credit gambit, or else a subtle reluctance to accept anything brute-like as being in our noble ancestry.  They all represented extinct side branches: they just weren't human enough to be our relatives!

A more serious argument for the single-species view was that possession of culture was our defining trait and signs of it could be seen even in ancient sites.  It was our 'ecological niche', and by the 'competitive exclusion principle' of population ecology at the time, only one species could occupy a given niche: other interlopers would be driven out, and to extinction. The single-species view could be called the Michigan school of paleoanthropology, because it was the view at the University of Michigan (where I was studying) by major protagonists Loring Brace and Milford Wolpoff.  The view was also influenced by the sophisticated population thinking of geneticist Frank Livingstone, and that was how I was trained.

Making the story plausible

Migration out of Africa

But there was a problem!  Human-ancestor-like fossils were found, from about a million years ago in Africa and in Eurasia.  But modern-looking human fossils we can generically call Homo sapiens were found in Africa around 200,000 years ago, and only in Eurasia around 100,000 or fewer years ago.  This satisfied the multiple-species people, whose Out of Africa (OOA) replacement hypothesis held that the modern guys dispersed from Africa in an expanding gradually, generation by generation in a front of local hunter-gathering groups, that somehow exterminated the dolts already living in Eurasia. How they managed to achieve this displacement was the subject of much speculation.

The single-species response became known as the regional-continuity or multi-regional (MR) view.  It held that via gene flow of people choosing mates from neighboring populations, as primates and humans do, and/or similar selective forces worldwide, our ancestors developed modern-looking anatomy (and brains) all over our area of habitation--Africa and Eurasia:  we had always been, and remained, one species.

Earlier fossils may more clearly show species differences, but by the time of the Neanderthals and other fossil specimens, such distinctions were less clear, and more debatable, but that didn't answer the overall continuity argument.

But then along came genetic data from living populations in the mid 1970s, in the form of frequencies of a modest number of gene variants sampled in Africa, Europe, Asia.  If after settlement these continental populations had been totally isolated, one could estimate how long it would take for the observed differences to accumulate, and the answer was only around 100,000 years.  That seemed clearly to show the replacement hypothesis to be true.  One possibility, that I myself (with population geneticist Takeo Maruyama) investigated was whether, if there had not been complete isolation among continents, the mating between adjacent populations that must have happened, would have gradually spread advantageous 'modern' genes across the whole human-occupied range, keeping us a single species: the genetic time estimates could be wrong if the complete-isolation assumption were wrong because there had been gene flow (which, again, we know always occurs), the true ancestral time could be closer to 1,000,000 but the estimated time only 100,000.  So single species ideas might be true. 

Then along came the ability to extract DNA sequences from fossils of up to around 40,000 years old.  Those data, too, seemed to suggest the recent split-time, and more sophisticated modern genomic data show that current Eurasian variation represents a subset of current African variation.  That means that all our ancestors must have arisen in Africa, and some subgroup of them were the people who expanded gradually out of Africa.  The fossil DNA raised the question of 'admixture', or mating between Neanderthals and more advanced modern-looking hominids, but the data initially provided little if any evidence for that.

The  MR hypothesis seemed as dead as those unfortunate descendants. Textbooks changed:  only OOA was a legitimate hypothesis.  Michigan school:  Bah, humbug!

The more interesting question raised by OOA
But if the replacement hypothesis triumphed, it left by far the most interesting question: how could some hunter-gatherers, small dispersed bands, have expanded mile by mile, over many generations, out of arid, tropical East Africa (which is what the genetic evidence seems to indicate), and killed off Homo erectus?  The latter had occupied all sorts of ecosystems very different from Africa for hundreds of thousands of years, from hot to humid to temperate or even frigid....and whose skeletal anatomy seemed to many (especially to Michiganders) to have been modernizing in ways similar to what was going on in Africa.  And how did one isolated region of Africa evolve our modern form, leaving the rest of the continent behind, because OOA also implied that the moderns expanded across Africa and did in the Homo erectus that were there as well?

One could invoke (or more accurately, invent) various ecological and behavioral explanations for the extermination, but while superficial and rather untrammeled speculation was rife, the question was never substantially answered, or in many ways, not even seriously addressed. Indeed, the Michiganders themselves seemed to have retreated, tactically, saying that no, they never really said only one species!  Like Emily Litella of the old SNL shows: "Never mind."

But something interesting has happened, that we'll talk about tomorrow.  We will have some new humbug to discuss!

I Am Evolution

Last updated June 24, 2020

I am Holly Dunsworth and I am a biological anthropologist at the University of Rhode Island. My research and teaching are primarily concerned with scientific narratives of human evolutionary history, how those narratives are formed, interpreted, and employed, and how they impact culture and society. I help to refute narratives of, and myths about, human evolution that support sexist and racist views of human nature...so that everyone can claim human origins and evolution for themselves and so that no one can rely on it to oppress others. Here are some of the topics I have worked on... 

1. Human races are not like dog breeds, and to claim that they are is to employ a racist strategy for perpetuating inequity.
2. Human babies are not born when they’re born because of limitations to gestation length and fetal development imposed by the bipedal pelvis (a,k.a. the ‘obstetrical dilemma’); gestation and fetal growth are limited by metabolism and energetics as they are across primates and mammals (EGG hypothesis). There is no ‘obstetrical dilemma.’
3. Human babies do need care but it’s not because they evolved to be born early or prematurely; they’re born relatively and absolutely larger than all primates, after a longer than expected pregnancy. Early birth/truncated gestation cannot be a “solution” to the so-called “obstetrical dilemma” because humans are not born early and gestation is not truncated. Without this solution, there is no dilemma to solve.
4. Women’s hips are not a compromise between bipedalism and childbirth; they are adapted to multiple functions. Childbirth interventions are not evolutionary imperatives due to the so-called ‘obstetrical dilemma’ which doesn’t exist.
5. Women’s hips are not genetically programmed to be more capacious than men’s; they develop differently because they contain the virile and active gonads and genitals that take up space inside the pelves of females and the soft tissues of the female pelvic floor are replete with estrogen receptors which likely affect pelvic bone development and remodeling throughout life (VAGGINA hypothesis).
6. Women are, on average, shorter than men because of menstruation’s effects on bone growth.
7. Men are not taller than women because their tall masculine male ancestors won the competition for mates and produced tall male offspring; continued male growth at puberty, past the point when females stop, is a by-product of estrogen’s effects on all human long bone growth and growth plate fusion.  Estrogen is biphasic, causing long bone growth until its levels increase enough to cause long bone fusion, which ends growth. Different levels of estrogen expression and exposure, causing sex differences in the timing of long bone growth cessation, are due to sex differences in evolved reproductive physiologies. In all human bodies ,fertility depends on a delicate balance of estrogen. Without ovaries pumping out the high levels of estrogen involved in monthly cycling,  bodies without ovaries have no choice but to continue growing past the point that bodies with ovaries stop.
8. All biology is evolution. To categories growth and development as merely "proximate" and to elevate behavior to "ultimate" is not merely an unnecessary convention, but it is contributing to the persistence of unscientific and harmful just-so stories about human nature.
9. Nobody on Earth but Homo sapiens knows that sex makes babies or has a concept of paternity, not even Koko, the famous “talking” gorilla. This makes us different from other animals in profound ways and must have impacted humanity for as long as we've made the connection between intercourse and procreation.

** 

Below is a list of some of my favorite Mermaid's Tale posts and otherwise. Many of them are resources for the points I've made above. I've organized these links according to the morphology of the folktale (via Landau via Propp) which is the structure of my current book project--a biographical human evolutionary tale called I Am Evolution. Thanks to Anne and Ken for bringing me on to the MT in 2009! Thanks to everybody for all the good times and here's to many more,
Holly

Initial Situation (evolutionary thinking)

• I am evolution (NPR)
• Evolution is the only natural explanation. And it's all we need.

Hero (introducing [your name here])

• Surprise! Semen is required (Sapiens)
• When did sex become fun? (Discover Mag)
• Kissing the lipless 
• Do animals know where babies come from? (Scientific American)
• later re-published in the e Book Amazing Animals
• Sex Makes Babies (Aeon)
• Holly and Anne Tell Us Where Babies Come From (Penn's Sunday School)

Change (life in utero)

• Relatedness is relative: How can I be 85% genetically similar to my mom, but only related to her by half?
• If mutations go viral, adaptationism is less pitiful (Evolution Institute)
• Marshawn Lynch's extra placenta feeds the curiosity
• How do we know when our ancestors lost their tails? (PBS)
• Expedition Rusinga: Uncovering our adaptive origins (AMNH)
• The Ape in the Trees

Departure (birth)

• There is no 'obstetrical dilemma': Towards a braver medicine with fewer childbirth interventions (Perspectives in Biology and Medicine)
• Why is Childbirth Painful? (BBC World Service; CrowdScience)
• Why is the human vagina so big?
• The 'obstetric dilemma' hypothesis unraveled
• Thank your intelligent mother for your big brain (PNAS)
• Why is no one interested in vagina size? (NYMag)
• Metabolic hypothesis for human altriciality (PNAS)
• The evolution of difficult childbirth and helpless hominin infants (Annual Review of Anthropology)
• How to apply an evolutionary... (Scientific American guest blog)
• When evolutionary-minded medicine gets it (possibly) wrong about childbirth interventions

Test (childhood)

• Forget bipedalism. What about babyism?  (3QD charm quark!)
• Just-So Babies
• Homo erectus: The Winnie-the-Pooh of fossil hominins (Sapiens)
• We need another explanation for our big brains like we need a hole in the head 
• lolhumans
• Oh Koko!
• A prehistory of throwing things (linked here: http://what-if.xkcd.com/44/)
• Hurling words and turds, an evolutionary link
• Can you throw with half a brain?
• Talking Trash!  (826 National's Don't Forget to Write for Elementary Grades)
• Book review: Children of Time by Anne Weaver (Reports of the NCSE)
• The Evolution of Buttfaces Explained
• Take Your Pick (New Scientist's book)
• Dog eats book: A domestic application of the scientific method 
• Now a teacher resource "with a chuckle" on Berkeley's Understanding Science site

Donor (puberty and young adulthood)

• Expanding the evolutionary explanations for sex differences in the human skeleton
• How Donald Trump Got Human Evolution Wrong (Washington Post)
• Area Doctor Shatters Area Girl's Dream of Being Taller Than She Is. (A personal genomics parable)
• Are we removing the wisdom along with the teeth?

Transformation (adulthood, the joys)

• Evolution reduces the meaning of life to survival and reproduction... Is that bad?
• A case for replacing "having it all" with "having all the choices, within reason and means"
• 'Obstetric dilemma' skeptic has c-section and remains skeptical ... & ... Why my c-section was natural childbirth
• Ten scratches on two bone fragments distinguish vegetarians from carnivores 
• Does Mark Watney dream of red potatoes?
• Whooza good gurrrrrl? Whoozmai bayyyy-bee boy?
• Such a good boy, Elroy

Test Again (adulthood, the oys)

• Human races are not like dog breeds: Refuting a racist analogy
• A leash of hemp: Does our slow, overbearing consciousness mislead us about human nature? 
• We are not the boss of natural selection. It is unpwnable. (io9)
• Walk this way, talk this way, roll in the hay: Bringing life and humanity to extinct Neanderthals (The Winnower)
• Evolution's got a P.R. problem
• The f-words of evolution 
• Another f-word of evolution 
• The amazing story of Holly Dunsworth and her osteopathic doctor
• I'm still mad about the Google Memo 
• My sexed-up Jordan Peterson fantasy
• It is unethical to teach evolution, no matter the organism, without confronting racism and sexism

Triumph (aging, wisdom, reflection)

• Humans are master meaning generators
• Monkeys all the way down (Sapiens)
• The End of the Beginning (McSweeney's) 

**

Where I appear in others' productions...

• Childbirth, explained (Vox)
• Males Are the Taller Sex. Estrogen, Not Fights for Mates, May Be Why. (Quanta)
• How Do We Know When Our Ancestors Lost Their Tails? (PBS)
• Labor of love: Flipping the scientific thinking on our species’ “difficult childbirth” (Knowable)
• The Babyland Diaries (Topic)
• Is This Gorilla Mother Consciously Protecting Her Baby? (NPR)
• Women rate the strongest men as the most attractive, study finds (WaPo)
• Why Is Giving Birth So Hard? (The Atlantic)
• Of Evolution, Culture, and the Obstetrical Dilemma (Undark)
• How did humans figure out that sex makes babies? (Slate)
• Why the GOP should love duck penises (Mother Jones)
• Why does pregnancy last nine months? (NPR)
• Why pregnancy really lasts nine months (LiveScience)
• Why must childbirth be such hard labor? (The Guardian)
• The disturbing, shameful history of childbirth deaths (Slate)
• Why is human childbirth so painful? (American Scientist)
• Bill Nye's creationism debate not a total disaster, scientists say (LiveScience)
• Why Babies Cry At Night (NPR)
• Are Wide-Hipped Women Promiscuous? Study Births a Controversy (LiveScience)
• Finally paying attention to vagina size (NYMag)
• Can animals think abstractly? (NPR)
• Anthropomorphism: how much humans and animals share is still contested (The Guardian)
• Not All Critiques of Evolutionary Psychology Are Created Equal (NYMag)
• The GOP’s New Bill Would Seriously Disrupt Genetics Research (Atlantic)

More on Mendel

We blogged about Gregor Mendel's insights in a recent post, and we mentioned various questions about what he did and what he thought he did and wanted to do.  There is no question, however, that his work was used to produce one of the most powerful research methods in the history of science.  Combined with the methodology that resulted from Darwin's ideas (that is, the understanding that life has diverged from common ancestry, and the traits that have evolved had to be viable in their respective environments), biology made great progress in understanding development and evolution.

Mendel

The  so-called 'Modern Synthesis' of the 1930s and 40s united patterns of inheritance with understanding of how inherited variation accumulated over time in diverging species, making a consistent underlying framework for understanding much of the nature of life.  The result today can be called the 'Darwinian' method (we blogged about this earlier in a series of posts starting here).

However, Mendel's work also led to the often unstated expectation of clear-cut 'phenogenetic' connections--that is, between genes and traits.  More specifically, his work led to the idea that genes 'cause' traits and that natural selection only works on genetic effects.  That's because non-inherited effects, which certainly do exist, disappear with the individuals bearing them,while genetic effects are 'written' in genetic posterity until modified by further genetic change.  This is over-simplified, but still a powerful and valid way to view life.

But the truth is that phenogenetic connections are not all that precise.  When traits are controlled by many genes, each varying in a population, then each individual with the 'same' trait may have it for different genetic reasons.  The expectation of strong phenogenetic connections is the problem limiting GWAS and other genome-based attempts to explain, much less to predict,  disease or other traits. Overall, the problem can be seen in the common confusing of inheritance of genes, which does happen, with the inheritance of traits which doesn't (except for those present in a fertilized seed or egg).  Traits arise from genes, at least in part, and may resemble those in ancestors, but it is inaccurate if not misleading to think of traits as inherited directly.

Seen another way, when traits are the result of many different contributing genes that can each vary, as is so common, then even the same trait in an offspring and its parent (e.g., how tall they are), are affected by genes but are not due to the same genotype.  There are many traits in which variation really is largely due to single genes, but these are the luscious hooks that have addicted us to Mendelian principles when they don't really apply.

Furthermore, from an evolutionary viewpoint, genetic (reproductive) success is a relative concept among individuals in a population, and hence does not lie in genes alone, but in the environment in which they find themselves, and that is always changeable.  Even the future genetic environment is not entirely predictable, nor are the non-genetic aspects of environment (like our lifestyles).

Also, Mendel's rules are about the transmission of elements--genetic variants--from parent to offspring when the parents are dipoid, that is, when there is sexual reproduction and each of two parents carries two copies of each gene.  Bacteria and many other species (which means most species!) are haploid (only one copy of their genome) and don't transmit in a Mendelian way which is to choose between two copies.  This is what got Mendel into trouble when he began to experiment with hawkweed.

This means that the Modern synthesis was about diploid evolution, not evolution per se.  The evolutionary principles don't require Mendelian 'laws'.  This shows, in perhaps another way, that what Mendel did was provide a method for understanding variable transmission, that was used to discover what genes are and where they are in cells.

These issues make practical differences in science.  Today, GWAS and direct genome sequence comparisons used to look for causal genetic variation (say, of diabetes or autism) compare individuals sampled not from families as was necessary before we could really identify specific gene sequences, but sampled by their trait (cases vs controls, for example), with the genes searched directly to find variation preferentially associated with some specific trait value.  We can see this in the fact that new mutations arise all the time, that may have causal effect but were not transmitted from the person's parents.

In a sense, the properly lauded Modern Synthesis arose by the rejection of Mendelian causation, in the sense that clear-cut trait-inheritance was not required either for biological traits or for their evolution, if genetic variants, no matter how small or even statistically undetectable their effects, was transmitted in Mendelian fashion.  Rejecting the idea that traits were largely inherited as distinct effects made things consistent with gradual evolution of complex traits, and that made the synthesis possible.

But mixing Mendelian genetic transmission principles with genetic causal principles has led, we think, to expecting too  much of specific genes.

We need another explanation for our big brains like we need a hole in the head

(source)

Something was definitely up, up top, once our bodies, down below, committed to walking and running upright.

It’s only after things got familiarly human in the locomotor anatomy--when we got long legs, non-grasping toes, and reconfigured butts--that brains started increasing beyond ape proportions.

For the first four or five million years of hominin evolution (from 7-2.5 million years ago) the story’s about bipedalism. For the last two and a half, it’s about encephalization. We've known this thanks to fossils for a while and genetic evidence is saying the same thing. It’s natural, then, for such a cerebral organism to wonder whether the two are connected.

As you hypothesize, you could go the technology route. Freed forelimbs, not necessary for locomotion, are free to be handy. O! the possibilities for hurling turds and building worlds! So that's one idea: Selection for a brainier hominin (both physically and cognitively) could occur only after the hands were habitually free to be freaky.

You could go the ecology route. Once our bodies committed to bipedalism our diet changed to include more meat, hominin body size increased, and geographic dispersal did too, no doubt aided by our more efficient bodies built for long distance travel. These characteristics, together, have been compared to those of scavenging and predatory carnivores. Regardless of how small or large a part meat played in our ancestors’ diets, there’s no denying that an ecological shift occurred in the early Pleistocene, with an increase in diet and habitat diversity, and that shift must have included new requirements of the brain.

Or, you could go the sociality route. As hominins relied more and more on cooperative foraging and parenting behaviors, etc, navigating social networks became key. Once complex speech and language arrived, then there would be new demands on the brain as well.

These pressures, requirements, demands, however you want to think of them, could be working in concert and at different times (e.g. technology plus socializing) over deep, geologic time and many many hominin generations. By "working," I mean contributing to the more-or-less sustained differential reproductive success of hominins with slightly larger brains. And because it’s the way that the fossil and archaeological records reveal behaviors over time, I tend to think of these three categories (technology, ecology, sociality) as describing the last 2.5 million years in the order I listed them. Technology was strongest earliest (starting with the Oldowan stone tools by 2.6 mya), and persisted. An ecological shift came along with that technological shift and then persisted. And of course social complexity came along with the technological and ecological shifts and then persisted.

These are some of the most mainstream hypotheses for encephalization (1) and they're implicitly or explicitly predicated on the prior evolution of bipedalism.

But now there's a new tie between big brains and walking upright--offered up in a  new paper just out in PNAS--and it's based in the human-, not ape-, like tendency to fuse the metopic suture later in life, to delay the close of what starts as a hole in the top of a baby's head known as its anterior fontanelle. The authors suggest that we need this hole in our head to exit our mother's bipedally-adapted birth canal safely and we also, as they suggest, need it to grow up to be an encephalized creature.

According to the authors, the "Taung child", an Australopithecus africanus kid (a member of a well-known bipedal hominin lineage) had an unfused metopic suture, left as an imprint on the fossil brain endocast.

Raymond Dart with the Taung child fossil.  http://en.wikipedia.org/wiki/File:Raymond_Dart_with_Taung.jpg 

Seven other australopith and Homo fossils are also described in the paper as having unfused metopic sutures. You might too! The odds are small, but since you started with a hole in your head as a baby, you could still be walking around with it unzipped.

To any chimps reading this, your metopic suture most likely closed just after birth and before your first deciduous molar erupted. But for 90% of humans (as reported in the paper), the suture closes later, after the eruption of the first deciduous molar. There’s a much slower fusion rate in humans than in chimpanzees.

However, to interpret the Taung child’s anatomy, things get a bit dicey, like things just love to get with hominin fossils. So often they can go either way: chimpy or humany.

Check out the figures below. A is for Pan troglodytes (common chimps) and B is for Homo sapiens. Those are frequencies of metopic suture fusion per dental age group. By listing them this way, instead of by chronological age, we're able to compare between two species that grow at different rates but share the same pattern of dental eruption. Chimpanzees grow up faster than us and experience earlier metopic suture fusion than us. Flipped around, humans grow up slower than chimps and experience delayed fusion of this suture compared to them. The Taung baby's dental age category is starred (*) at the "M1" stage in A and B. (The Taung child died at around 3.8 years of age, when its first permanent molar, M1, was erupted.)

Because it's more likely you'll find a human at that age (*) with an unfused metopic suture than a chimp, the researchers leaned toward calling the Taung child's state human-like, rather than ape-like. They backed that assertion up by listing seven other late Pliocene-early Pleiostocene hominins with unfused metopic sutures... it's a trend in the hominin lineage that begins with some australopiths, like the Taung child, they say.

"The presence of a still patent fontanelle and of a partially fused [metopic suture] in the Taung child, and the incidence of unfused [metopic sutures] in five adult and two other younger Australopithecus/ early Homo specimens is thus taken as evidence that a human-like pattern of late [metopic suture] fusion was already present in mid-to-late Pliocene gracile hominins."

Okay. Intriguing! But now we must explain!

[This is the part where, if you listen very carefully, you can hear the collective curmudgeonly groans from within and beyond the walls of paleoanthropology.]

Enter the new hypothesis for encephalization based on the late fusion of metopic sutures. The authors nod to two papers that offer "adaptively neutral" explanations for late metopic suture fusion but argue that the fossil evidence combined with the differences observed in chimps and humans beg for an adaptive explanation. (This tack is unsurprising given how paleoanthropology generally operates.)

The authors offer us three adaptive hypotheses to explain late metopic suture fusion:

1. Reorganization and expansion of the frontal neocortex (explains late metopic suture fusion)
Something about the changing and enlarging frontal cortex required changes to the cranial bones, how they form, grow, and fuse.

2. The difficulty of giving birth to large-headed neonates through birth canals that were reconfigured for bipedalism, the “obstetrical dilemma” (explains late metopic suture fusion)

The squishy neonatal head, thanks to the fontanelle,  "probably occurred in conjunction with refining the ability to walk on two legs," Falk (the lead author) said to the media. "The ability to walk upright caused an obstetric dilemma. Childbirth became more difficult because the shape of the birth canal became constricted while the size of the brain increased. The persistent metopic suture contributes to an evolutionary solution to this dilemma."

The trouble with this hypothesis as applied here is, although we know modern humans have a tight fit at birth now, there's little evidence for a tight fit between neonate and birth canal during australopith times.

And you can't help but wonder whether a squishy head was, or still is, required for successful birth. Do children suffering from craniosynostosis require a c-section to be born? Also, since the metopic suture fuses after chimpanzee birth, are we certain they aren't squishing their brains as they exit their relatively roomy birth canals? These questions may sound silly, but they're illustrating the built-in assumptions of the paper (or my ignorance about squishiness of baby heads).

The squishy head may be helpful during childbirth, but if it's occuring as early as australopith times, an adaptive explanation as a "solution" to an obstetrical dilemma is hard to swallow. That is unless DeSilva's estimates cited by the authors-- that australopiths had large neonates and tight fits at birth--are correct.

3. High early postnatal brain growth rates (explains late metopic suture fusion)
We know that humans have high rates of postnatal brain growth and this is what a lot of the news media picked up on: Your baby's head has gaps between the bones so the brain can grow like crazy after it's born to the gargantuan size of an adult human brain. As established in hypothesis #2, the need for the fontanelles in the first place is the crunch at birth thanks to the obstetrical dilemma, implying that without the pressure to be born small enough to escape the bipedal birth canal, we'd grow larger fetal brains in the womb.

So with this new paper we're presented with something even more fundamental than the notion that bipedalism as a necessary precursor for technological, ecological and social selection pressures for encephalization (as covered above): The tight fit at birth, caused by antagonistic selection for bipedal pelvic anatomy and large neonatal brains, created the selection pressure for a squishy neonatal head (which is facilitated by the fontanelles) and because of that roomy cranium, postnatal growth rates were able to ramp up in selective environments that favored encephalization.

So I'm left wondering, Do we need a hole in our head to be born successfully? Do we need a hole in our head to be encephalized? If the answer to both of those is yes, then what is a hole in the head doing in a hominin genus that may not have had much difficulty with childbirth and was hardly (if at all) encephalized? And, given the overlapping chimp and human fusion patterns, how can we be sure this feature on Taung is humany and not chimpy?

And, further, you can't (or at least I can't) help but wonder if there's a biomechanical/functional explanation for late metopic suture fusion, given how feeding behaviors and masticatory muscles put stress on the cranium. The skulls of australopiths and other hominins experienced stresses differently than chimpanzees. These differences may have begun as early as the nursing stage. Could this have anything to do with delayed fusion of the sutures? (here's just one study I found that addresses these kinds of questions)

And finally, it's hard not to link Falk (the lead author) to her research on Homo floresiensis. The hobbit (LB 1) looks like it has a fontanelle, something the disease-hypothesis folks point out is consistent with their perspective, and that’s one reason why I assumed these authors are onto this topic.

But LB 1 is conspicuously absent from the laundry list of hominin fossils in the supplementary section. Either they're saving what they've got on metopic suture and fontanelle anatomy in H. floresiensis for an upcoming paper or they just didn't think it was worthwhile to include this specimen. After all,  the latest paper on hobbit anatomy claims that the hobbit's "fontanelle" isn't real. Peter Brown writes, "direct examination of the asymmetrical hole in the posterior frontal of LB1, supported by CT scans, clearly indicates that this is the result of post-mortem excavation damage and is definitely not an unfused anterior fontanelle." (2)

Good thing, because if the hole in LB 1's cranium is of biological and not of taphonomic origin, then who knows how anybody'd explain its adaptive significance in such a tiny-brained hominin.

But, going way back to Taung and the australopiths: They were, after all, bipedal and the big brain train had to pull out at some point!

And, stay tuned. I got a tip from one of the authors about a paper coming out soon that demonstrates how weak the obstetrical dilemma hypothesis is, for explaining fetal size and growth, given the current evidence and given what we know about maternal metabolism.

Notes
(1) Of course, these hypotheses don’t represent all of paleoanthropology. I just intended to cover the major bases. And you need to consider what many paleoanthropologists assume which is that brain tissue is expensive so something extraordinary must have kept up selection on its increasing size for the last 2.5 million years. The assumption is that if brains were cheap, everyone would have big ones, but I don’t buy that. I think it's clear that other species aren’t encephalized because they don’t have to be. They do just fine without big brains. We have a rather warped perspective on selection for encephalization, thanks to our presentism and our big brains.
(2) Thanks to K. Baab for the tip.

Getting in each other's jeans....or genes?

Life, evolution, is about reproduction, how effectively we get into each other's jeans, so to speak.  As individuals, we may be independent most of the time, but we can't make a new individual alone.  To do that we must conjugate our genome with someone else's, and getting into their jeans is how we do that.

Or is it?

Perhaps we've been making a big evolutionary mistake
Maybe what is as important is how we and other species get into each other's genes.  Organisms--all of us--are collections of large numbers of cells descended from a single starting cell (the fertilized egg).  We tend to view ourselves as having one genome, and in that sense being a unitary, biological whole.  Evolutionary theory is about how collections of such wholes, the fertilized eggs of a population in a generation, change over time.

But increasing evidence shows several important facts.  First, each time a cell divides into two 'daughter' cells, mutations occur, and these are then transmitted to the next generation of daughter cells.  And, at the molecular level, the division of one cell into two, even forgetting mutations, is never exactly equal.  One gets more of one protein than the other, the other gets more mRNAs and so on. You are a huge collection of cells with such a branching history of variation from a single fertilized egg cell.

But our life is not just what happens to that set of cells.  Instead, as evidence is now showing more clearly, we are colonized by countless other cells--bacteria and other micro-organisms--and they, too, divide on and in us, accumulating mutations along the way.  And, more importantly, we are unable to live without them, nor they without us.  The simple classical and perhaps clearest example is our intestinal flora, that is responsible for vital aspects of our digestion and hence our survival.

A burgeoning area of investigation called microbiomics (every field needs a sexy label, of course), the study of the many different microbes that inhabit various parts of our bodies, is off and running.  This work is finding a wealth of relationships between us and others, so much so that researchers are suggesting that our genome can't in fact be fully understood on its own, but rather must be thought of as just one part of a larger genome that also includes the genes of all our colonizers.  Or from the microbes' perspective, their own and their host's genomes.

Many different common chronic diseases that for several decades have been thought to be due to wear-and-tear of long life and modern lifestyles, are being shown to involve genes involved in aspects of our immune systems. So in both regard to disease and to the microbiome of normal variation, our traits may be more 'infectious' than we had thought.  The term 'infection' implies the old idea that we're normally bug-free unless we are sick.  Instead, perhaps we are sick if we are bug-free!

Going further: what, after all, is an organism?  What is evolution about?
Maybe our entire concept of organism and its traits is biologically badly mistaken.  Perhaps metazoans really are often, or largely, a colony of cells with one genome plus adherent colonies of cells with other genomes.  Neither can do without the other.  They've evolved jointly.  Aberration in either can cause cell death.  When it's of the 'organism' we give it different name from when it's just of one of the cells.

We might call this 'coevolution' and perhaps it could lead to substantial rethinking about what we are, or what species are, or their evolution.  Or how we use our notion of organism to dichotomize vis-à-vis the 'environment', when basically they are more unitary?  The ultimate in co-operation.

Again, as with sex, we may have been misled by Noah's ark, Linnaeus, and Darwin into life science based on organism, when that is only one aspect of how things are organized.  The idea that Nature is composed of a series of distinct species, a legacy of classical thinking up to the present, is actually a large subject with much that is interesting and perhaps truly profound to think about.  We'll discuss it in a subsequent post.

Meanwhile, the idea of meddling in, or the responsibility to keep out of,  or delve into, each others' genes is not just something for Levi Strauss to consider.

What do we know about Gregor Mendel and his work?

Gregor Mendel: public domain

What 'everyone knows' about Mendel
Ken and I were asked to compile an annotated bibliography on Gregor Mendel for Oxford University Press's new bibliography initiative, Oxford Bibliographies Online (OBO).  Our contribution will be in their new and growing Anthropology section.  The Press is investing a lot in this, and they hope it will be widely used.  Access is subscription-based, however, so presumably it's intended to be primarily for students and academics, whose university libraries subscribe.

Anyway, we agreed to do it, thinking it would be a good reason to explore more about Mendel.  And it was.  We thought we knew pretty much enough about him to do this, since we knew what everyone knows about Mendel -- he was the Father of Modern Genetics, he was a Moravian monk who experimented on peas, which led him to propose his Laws of Inheritance.

But, as it turns out only some of the things that "everyone knows" about Mendel are unassailable, while the rest of his legacy continues to be debated.  Why did he do his experiments?  Was he interested in understanding hybridization or heredity?  What did he think he discovered?  Did he actually propose his laws?  Did he fudge his results?  Was he brilliant and innovative, or did he do ordinary science, producing results that everyone already knew?  And, why was his work ignored for 34 years after it was published?

These questions have been written about extensively, to say the least.  Mostly, of course, authors laud Mendel, reinforcing his status as an icon of foundational brilliance in biology.  But while some scholars suggest that the answers are clear in his 1866 paper, this isn't enough for others.  Not long after Mendel's work was rediscovered in 1900, the statistician RA Fisher accused Mendel of cooking the books, since for statistical evidence his results were so close to what was expected by his theory.  In particular they were too close to a mistaken expectation based on his theory.  That debate has been ongoing, although most scholars believe Fisher misinterpreted Mendel's explanations of what he had done, and that Fisher's accusations were unwarranted.  (Ken wrote one of his first columns for Evolutionary Anthropology on this, in 2002, coming down firmly in favor of Mendel's integrity and the importance of his work.  Another fine contribution to this debate is a 2001 paper by Daniel Fairbanks and Bryce Rytting, in the American Journal of Botany, in which they address these issues.)

Mendel's life and work
Mendel was born in 1822, into a farming family in Austrian Silesia, a province of the Austrian Empire,  now the Czech Republic.  His parents sent him to school, but hoped, as their only son, that he would one day take over the farm.  He studied with a teacher interested in horticulture, who recognized him as a student with promise, and encouraged him to go to university.  Mendel was determined to further his education, but due to difficulties at home, including that his father had a serious accident which meant he couldn't farm, at least for some time, Mendel was unable to finance his way.  He seems to have been a sensitive soul, and describes this difficult time in a short autobiography he wrote when he was 28 as a part of his application for a teacher's examination.

Four years later, due to several successive disasters, his parents were completely unable to meet the expenses necessary to continue his studies, and it therefore happened that the respectfully undersigned, then only sixteen years old, was in the sad position of having to provide for himself entirely. For this reason, he attended the course for "School Candidates [applicants] and Private Teachers" at the district Teacher's Seminary in Troppau. Since, following his examination, he was highly recommended in the qualification report (enclosure B), he succeeded by private tutoring during the time of his humanities studies in earning a scanty livelihood. 

When he graduated from the Gymnasium in the year 1840, his first care was to secure for himself the necessary means for the continuation of his studies. Because of this, he made repeated attempts in Olmiitz, to offer his services as a private teacher, but all his efforts remained unsuccessful because of lack of friends and recommendations. The sorrow over these disappointed hopes and the anxious, sad outlook which the future offered him, affected him so powerfully at that time, that he fell sick and was compelled to spend a year with his parents to recover.

Pisum sativum: painting by Otto Wilhelm

The entire autobiography, only two pages long, is well worth reading, even if somewhat melodramatic.  Because his parents could not afford to send him to university, his best remaining option was to enter a monastery, which he did at age 21, joining the Augustinian Abbey in Brno.  Because he was so sensitive, he was unable to perform priestly duties such as visiting the ill, so the abbot, a scholarly man himself, with interest in horticulture, sent him to the University of Vienna instead, where he studied physics, meteorology, botany, and mathematics.  He left after three years and returned to the abbey where he resumed teaching.

And, he began his experiments with peas. In 1854 he acquired 34 varieties of pea seed, Pisum sativum, from local nurseries and spent the next 2 years growing them to determine which characters bred true. Of these he chose 7 traits to follow through multiple generations--plant height, blossom color, pea characteristics, and so on. He produced 7 crosses, each repeated twice, from a seed plant with a particular trait and the second time with that same trait from the pollen plant, thus demonstrating that the male and female contribute equally to the offspring.

The traits of interest to Mendel: public domain image

He carried on his experiments for 8 years, counting the occurrence of these traits in nearly 30,000 peas. Because he chose his traits well, intentionally avoiding traits that did not breed true, he was able to document what became known as recessive and dominant inheritance, as well as the random segregation of alleles in offspring, and the independent inheritance of genes for traits using a sample size large enough to produce robust results.

Mendel described his findings in two lectures, which he gave to the Brünn Society for Natural Science in 1865. It is said that no one in the audience understood the importance of what they heard that night. The lectures were subsequently published as Versuche über Pflanzenhybriden, or, in English, Experiments on Plant Hybridization in the Proceedings of the Natural History Society of Brünn, but his work met a muted reception and was long ignored.

Among others, Mendel sent his paper to renowned Swiss botanist, Karl Wilhelm von Nägeli, for review but even Nägeli seems not to have understood its importance, and recommended that Mendel verify his findings with further experiments, this time on Hieracium, or hawkweed. Unfortunately, these are plants with unusual modes of inheritance, and the experiments did not give Mendel the results he would have expected, given what he knew about peas.

Mendel died in 1884, either believing his work would be recognized one day, or discouraged, and believing he really hadn't found anything of importance, depending on who you read. Because he was not a person of note when he died, all of his personal papers, letters, notes from his experiments, were burned not long after his death. For this reason, it's difficult to reconstruct his experiments, his thoughts about his motivation and what he believed he found -- the aspects of his life and work that are still debated.

Among the few documents written by Mendel that survived his death were the letters he wrote to Nägeli. He also wrote a paper describing his work on hawkweed, which was published, after which he did no further work with plants. Whether this was because he was discouraged or because he was distracted by work, as he had become abbot by then, is not known. He had a lifelong interest in meteorology, as well as horticulture, and published a paper in 1871 suggesting the cause of tornadoes. His reasoning was largely correct, but this paper, too, was long ignored.

His legacy
These are the bare bones of Mendel's life and work. A handful of excellent biographies have been written, as well of hundreds of papers asking and addressing the questions of the legitimacy and motivation of his work. The most measured of these (a 1992 paper by Orel and Hartl is among the best) suggest that if people would simply read his paper, they'd see that he explained what he did and why, and what he thought he had discovered. Indeed, he wrote to Nägeli that he was sure his work would be understood in the fullness of time.

Mendel did not formulate what have come to be known as his Laws, although he did understand about independent assortment and segregation of whatever it was that caused his traits, he didn't discover genes, although he understood there were Anlage, or 'elements' that were inherited and caused his traits, he didn't discover dominant and recessive alleles, although he understood that pairs of elements were inherited, but only one trait was seen in the first generation, while two were seen in subsequent generations, nor were his experiments muddied by genetic linkage, two genes close together on a chromosome that were inherited together rather than independently.

Mendel did many things for science, regardless of how he felt about his work. He laid out the foundations for the understanding of genetics. But that was because of the methodological path his work opened. Genes became real, discoverable causal entities fundamental to life. His 'cheating' was totally minor as far as this goes.

But Mendel misled us in important ways, as well. By making his points with highly selected, simple traits, he provided an expectation of strong single-element causation as the basis of biology. That helped find genes when they were closely connected to traits, as in his peas. But most biological causation--relationship between genes and traits, is far from simple in that way. So he also misled us, in ways Ken tried to outline a few years ago. We'll note some of the implications of his work in a forthcoming post.

Meanwhile, Mendel is properly honored for laying the groundwork for the time when enough was known about genes and heredity that his results could be understood in context, and built upon. As such, he was indeed the Father of Modern Genetics.