One species is Enhyrina schistosa, known as the Beaked Sea Snake, or the Hook-Nosed Sea Snake. It lives in the waters near Indonesia and Australia. This is known to be the most venomous of all of the sea snakes, and a certain number of people are bitten by them. In fact, most people who die of sea snake bites were bitten by Enhyrina schistosa. How many people get bitten by them? Hard to say. In Australia, between 1942 and 1950, 56 people died from sea snake bites. What is the meaning of that number? Hard to say; it is just one of those esoteric bits of information one finds in Wikipedia. These snakes probably don’t bite very many people, but when they do, you have a problem.

The Beaked Sea Snake feeds mainly on spiny catfish and blow fish, and as such benefits from have a large gape. Selection for the large gape has altered the morphology of this snake in a way that probably contributes to it’s beaked nose and a couple of other features that are used to distinguish it from other sea snakes and thus identify it to species. The problem is, this selection pressure seems to have caused two distinctly different groups of snakes (actually, three … see below) to converge on a single morphology. So, what we have been calling Enhyrina schistosa, the beaked sea snake, is clearly two distinct species that look enough alike to have been confused as one. This is destine to be a classic example of evolutionary convergence.

This is all being reported in a paper due out soon in Molecular Phylogenetics & Evolution by Kanishka D.B. Ukuwela, Anslem de Silva, Mumpuni, Bryan G. Fry, Michael S.Y. Lee and Kate L. Sanders. Caroline Bird of the University of Queensland Communications Office provided some background and the great snake picture.

From the abstract of the paper:

We present a striking case of phenotypic convergence within the speciose and taxonomically unstable Hydrophis group of viviparous sea snakes. Enhydrina schistosa, the ‘beaked sea snake’, is abundant in coastal and inshore habitats throughout the Asian and Australian regions … Analyses of five independent mitochondrial and nuclear loci for populations spanning Australia, Indonesia and Sri Lanka indicate that this ‘species’ actually consists of two distinct lineages in Asia and Australia that are not closest relatives. As a result, Australian ‘‘E. schistosa’’ are elevated to species status and provisionally referred to Enhydrina zweifeli. … Our findings have important implications for snake bite management in light of the medical importance of beaked sea snakes and the fact that the only sea snake anti-venom available is raised against Malaysian E. schistosa.

Have a look at this diagram:

You can see the two populations, from Australia (top) and Southeast Asia (bottom), separated by numerous other species that look very different. And, if you look at just the Southeast Asian group, they cluster into two subgroups as well. Apparently this genetic divergence and grouping was not noticed by prior researcher dividing the snakes up into species and genera on the basis of morphology. Having said that, it is also true that the sea snakes are a bit dicy in their overall phylogeny, and are understudied. This, apparently, is being rectified.

There are other potential explanations for this pattern that should be considered, involving the genetics. It is possible to come up with a genetic tree that inaccurately represents the actual phylogeny of the species at hand. This study, however, used multiple methods and multiple DNA sites, involving both mitochondrial and nucleic DNA, so the species tree you see here is probably reasonably close to accurate, and the conclusion that Enhydrina schistosa consists of two groups that are not monophyletic is strong.

“This mixup could have been medically catastrophic, since the CSL sea snake antivenom is made using the venom from the Asian snake based on the assumption that it was the same species,” noted Bryan Fry, one of the study’s authors. “Luckily, the antivenom is not only very effective against the Australian new species but actually against all sea snakes since they all share a very stream-lined fish-specific venom.”

Wear a wet suit!

Ukuwela, K., de Silva, A., Mumpuni, ., Fry, B., Lee, M., & Sanders, K. (2012). Molecular evidence that the deadliest sea snake Enhydrina schistosa (Elapidae: Hydrophiinae) consists of two convergent species Molecular Phylogenetics and Evolution DOI: 10.1016/j.ympev.2012.09.031

Get the latest news about Sungudogo, the science fiction adventure story set in the Congo, which serves as a new Origin Story for the modern Skeptics Movement, HERE.

That would be the way it would happen if two chimps walked into a bar. Or imagine two chimps, and each finds a nice juicy bit of fruit out in the forest. And instead of eating the fruit, because they are not hungry, they carry it around for a while (this would never happen, but pretend) and then accidentally run into each other. What would happen? Same thing. Event though neither chimp actually needed the fruit and each chimp had its own fruit, the dominant chimp (between the two) would end up with both pieces of fruit.

This is why chimps could not possibly cooperate in any effort to scour the forest for various edible items, bring them all back to a central place, share and then cooperatively process the food items, and ultimately produce a meal that is eaten by all of the chimps on an as needed basis. Humans do that but chimps can’t. Explain this and you explain one of the major features of human evolution…

Some of us think that about two million years ago, an ape-like hominid ancestral population for humans gave rise to individuals with the novel capacity to do the following:

1) Make and control fire;

2) Cook food on this fire; and

3) Cooperate enough that individuals could in fact bring food morsels to a central place for processing and sharing.

The consequences of this nexus of novelties would be significant. There would be much more energy in the environment available for consumption because cooking converts a lot of inedible biomass into edible biomass. This could supply the necessary nutrients for bodies to grow larger and be maintained at larger sizes, which might be useful in the predator-rich environment of Africa. Note that where we can determine cause of death for australopiths, or at least guess reasonably what it might have been, predators are typically involved. This seems to stop happening with the larger bodied Homo erectus following this transition.

Another consequence is the extra nutrition to support the growth and maintenance of a large, costly brain.

These early human ancestors would have to have a way of cooperating rather than (almost) always competing over things like food. This could result in behaviors supportive of more complex and sophisticated technologies being regularly used, as we in fact see in the archeological record. The novel food sources plus the additional technology together would support this species’ movement into additional habitats previously not occupied by hominids. We also see this happening just at this time in the archaeological record.

For various reasons I won’t go into here, this would also have surely changed the overall social organization among these hominids, and we suggest that this may have been the origins of something not entirely different from modern (more or less monogamous) marriage.

(Here is a copy of a paper that discusses this idea in some detail.)

… We hypothesize that these individuals are myxoedematous endemic (ME) cretins, part of an inland population of (mostly unaffected) Homo sapiens. ME cretins are born without a functioning thyroid; their congenital hypothyroidism leads to severe dwarfism and reduced brain size, but less severe mental retardation and motor disability than neurological endemic cretins. We show that the fossils display many signs of congenital hypothyroidism, including enlarged pituitary fossa, and that distinctive primitive features of LB1 such as the double rooted lower premolar and the primitive wrist morphology are consistent with the hypothesis. We find that the null hypothesis (that LB1 is not a cretin) is rejected by the pituitary fossa size of LB1, and by multivariate analyses of cranial measures. We show that critical environmental factors were potentially present on Flores, how remains of cretins but not of unaffected individuals could be preserved in caves, and that extant oral traditions may provide a record of cretinism.

There are a handful of reasons to criticize these results based on the results themselves, but there is also a broader overarching reason to coming to the point that pathology explains this fossil population only as a last resort.The authors claim that Myxoedematous endemic cretinism occurs in various places at rates approaching 5%, but his is an overstatement in two ways. First, enlarged goiter and a range of iodine deficiency outcomes occur in some populations in Central Africa and Indonesia, but full blown ME dwartism of the type they attribute to Flores is very very rare in these areas. In fact, there are only two descriptions of this conditions used in this study, attesting to the rarity of this condition.Many of the osteological traits the authors use are also primitive traits. Primitive traits can show up in descendant populations either because of early branching (phylogenetically) or genetic changes, and are thus not good sources for phylogeny. Placing the Flores Hobbits within rather than along side a human lineage is a phylogenetic conclusion. This is a problem that will be difficult to avoid in this sort of analysis.The authors use local stories of cretin-like people to bolster their case. These stories may or may not have relevance, but in many places in the world there are stories of not-exactly-human forms that live out in the wild. Leprechauns come to mind, but there are many other examples as well. The assertion that this is evidence in support of a particular form of pathology is very weak.There is a broader reason to distrust pathological explanations. This is using the assumption, as suggested in the paper (especially in regards to local mythology) that the cretinoids lived on their own in the forest, even if they were part of a larger population.What if I told you about a major corporation that was run entirely by people with Down Syndrome, severe Autism, with a board of directors entirely comprised of advanced Alzheimer patients. This corporation produced products that were packed on trucks at a loading dock staffed only by comatose or quadruplegic workers. Further, I told you that this company was quite successful.You would not believe me.People with various (aforementioned) disabilities can certainly live productive lives, and can be integrated into any broader system and do fine, but this requires integration and compensation. We would not assume that an accounting firm staffed entirely by people incapable of doing math or a special forces army unit staffed entirely by people with no leg or arms would function. Why, then, do we assume that a subsociety, living on their own in the forest, made up of people with severe mental and physical deficiencies would survive?To assume that this is possible is to seriously underestimate the required capacities for humans to live as foragers. This is a commonly made, Western-biased, post-Agricultural centered, and racist mistake. The deeper assumption plays from the hero-myth of human history and evolution. “Advanced” societies, such as those with agriculture, pastoralism, cities, complex kinship systems, space ships, widget factories, etc. are assumed to be made up of people who are intrinsically (genertically and/or culturally) advanced over their hunter-gatherer forbearer’s.Nothing could be farther from the truth. Where data are available, we tend to find that hunter-gatherer populations have measurably larger brains, notably smaller percentages of mentally disabled participants, and on direct ethnographic observations, are extra smart.It is possible that this condition of cretinism was endemic to some population living in this area, and the cretins were sent off the forest, perhaps fed and cared for minimally, and did not actually function as an independent foraging society. I might believe that. But such a subset living on their own with sufficient persistence to leave numerous fossils and a reasonably rich archaeological record is extraordinarily unlikely.The authors do address this idea to some extent.

ME endemic cretins escape the severe neurological deficits of neurological endemic cretins … having milder mental deficiency, greater self-reliance and a general lack of mobility deficits (Wang et al. 1982). In agricultural populations, ME cretins are well cared for, but in seasonally mobile hunter-gatherer populations, the limited mobility of cretins could lead to separation, particularly of adult cretins. Use of caves by adult cretins and lack of burial would explain the cretin remains at LB, while seasonal mobility, alternative shelters and systematic burial would explain the absence of the remains of normal individuals. A population (n=25-100) with 1% cretinism is calculated to produce 4-15 deaths of adult cretins per kiloyear, which is enough to explain the discovered remains at LB.

By the way, where Cretinism occurs in Central Africa, it occurs only among agricultural populations who have moved into the region where iodine is rare within several centuries time, and not among indigenous foragers.

This paper is discussed by Afarensis, HERE.Obendorf, P.J., Oxnard, C.E., Kefford, B.J. (2008). Are the small human-like fossils found on Flores human endemic cretins?. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1–1. DOI: 10.1098/rspb.2007.1488

LADEN, G., WRANGHAM, R. (2005). The rise of the hominids as an adaptive shift in fallback foods: Plant underground storage organs (USOs) and australopith origins. Journal of Human Evolution, 49(4), 482-498. DOI: 10.1016/j.jhevol.2005.05.007

Homo floresiensis more widely known as the “Hobbit,” may have had arms that were very different from those of modern humans.

A paper in the current issue of the Journal of Human Evolution explores the anatomy of H. floresiensis. To explore this we first have to understand the concept of “Humeral torsion.” Humeral torsion is the orientation of the humeral head relative to the mediolateral axis of the distal articular surface. Don’t bother reading that sentence again, I’ll explain it.

The humerus is the upper arm bone, that runs between your shoulder and your elbow. The humeral head is the round ball-like part that fits into the “shoulder” … if you have a dislocated shoulder this is the part that is not where it is supposed to be. It articulates with the clavicle and the scapula, but really, all three of these bones are held together with a mass of connective tissue. The mediolateral axis of the distal humerus is the plane defined by your wrist. If you balance a pencil on your wrist, the pencil is more or less resting in the plane of your humeral (arm bone) distal (far end of the arm bone) mediolateral axis.

So, if your arm is at rest, the humeral head is more or less pointing towards the middle of your body. If, when your arm in a nice relaxed state your wrist is parallel to the side of your body, then the difference between the orientation of the humeral head and the distal humeral bit is minimal. If, however, when you stand at wrest your hand tends to point backwards, like if your arms were extra long you’d be dragging your knuckles, then the difference between these two orientations is large, like maybe 90 degrees or so. (Some anatomists call this angle, the 90 degree torsion, zero degrees and measure deviance from that number. Others call it 90 degrees and measure from that number. It is all a matter of style.)

Most mammals, and this is the presumed primitive condition in mammals, have a humeral head that faces towards the back of the body, with the humerus sticking down towards the ground (mammals, generally, are quadrupedal so the humerus is a leg bone, not an arm bone!). Apes, however, have evolved a humerus with its head pointing more toward the middle of the body. Humans are pretty much the same as all the other apes at a gross level. However, within the apes, modern humans are “more different” than the other apes, with a fairly high degree of torsion.

To put it simply but clearly, the ape humerus is twisted a certain amount, an the modern human humerus is twisted a slightly different amount. The differences are small, but fairly consistent. The following graph shows the pattern of humeral torsion across a number of human samples, Neanderthal, and some early hominids. Flores is LB1/50. There is clearly a lot of overlap, but by grouping all of the early hominids we see that there may be a pattern, and that Flores may fit into the early hominid pattern better than the modern human pattern

Fig. 4. Mean humeral torsion angles plus 95% confidence intervals for samples (CIs) from literature sources for different modern human and fossil hominin groups, and torsion measurements for individual fossils.

This vartiation also plays out in other aspects of the bones. The techniques used by the researchers to investigate this involves sclaing to body size by plotting the length of the humerus and the length of the clavicle, as shown here:

Fig. 6. Scatter plot of mean clavicular length against mean humeral length in nonhuman primates, a variety of modern human groups, samples of early modern Homo and Neanderthals, and LB1 (Flores) and KNM-WT15000 (a Homo erectus).

As the humerus gets longer, so does the clavicle. This relationship across primates (including prosimians and New World monkeys, as well as Old World Monkeys and apes) follows a pattern from which there are a few divergences. Ateles and baboons have short clavicles, while some of the apes and all humans have long clavicles. One problem with these data is that the range of variation increases as one gets to the larger bodied animals. Is this because the variation itself is scaled to body size? That is fairly likely. But it is also true that the nature of the locomotion and positional behavior is diverse in the upper range, with gorillas being ground-doweling quadrupeds who knuckle walk and orangs having a virtually unique arboreal form of locomotion owing to their body size. However, it is apparent that the modern humans and Neanderthals are way “off the line.” Look close, you’ll find LB1, the Flores sample, which is dead on the line for primates in general. If all I knew about Flores was that it was small, had an ape-size brain, was bipedal and lived in southeast Asia, then I saw this graph, I’d ask “So, you think Flores is a ground dwelling gibbon? Interesting, there are no extant forms of this kind of gibbon, you’ve got something very interesting there…”

This study looks at a number of other factors, but the same conclusion is arrived at again and again. The Flores humerus is more like a generalized ape humerus at the broad scale, but it is most like an early hominid, in particular, a Homo erectus humerus in details. Again and again, Flores keeps turning out to be a miniaturized Homo erectus, but with enough differences to ask if the similarities are convergences or shared shared ancestral traits.

The authors suggest:

The unexpected combination of primitive and derived characteristics of H. floresiensis and early H. erectus shoulder material highlights our ignorance regarding the course of transformation of the hominin pectoral girdle and shoulder from a more ape-like ancestral condition to the morphology of modern humans.

Several hypotheses are suggested to account for the Flores/erectus pattern, and the subsequent shift to the modern human form in one lineage (not Flores, presumably). The reorientation of the scapula and its relationship to the humerus could be an adaptation to facilitate stone tool manufacture. The reduction of overhead hanging and grasping in connection with arboreal locomotion could also be indicated in early hominids, and the further reorientation in modern humans could be related to throwing. In other words, Flores would not have been throwing spears or rocks, or even baseballs, like modern humans would. Personally, I thik the initial shifts we see could relate to using spear-shaped implements to dig, with the later shifts involving using similar implements to both dig and hunt. As usual, running is also thrown into the mix: The modern human shoulder is better adapted to efficient running than the Flores or erectus shoulder.

The authors conclude:

Debate continues regarding the proper interpretation of the Flores hominins … Although the controversy may continue until additional material, especially new skulls, are found, studies looking beyond brain size … have observed unexpected morphology that defies simple explanations. Whatever the ultimate taxonomic attribution of the Liang Bua hominins, their unique morphology suggests unforeseen diversity in the human family. In regard to the present study, while LB1 and the Nariokotome [Homo erectus] skeleton differ in many ways and are known from very different times and places, they are similarly distinct in displaying a relatively short clavicle and low humeral torsion. We believe these are not chance similarities, but part of a previously unrecognized functional complex that characterized early H. erectus and was retained in H. floresiensis.

And I’ll let them have the last word except on one point: Increasingly, it becomes possible to consider time and space in sussing out these evolutionary patterns (two things that are found very far from each other are less likely to be closely related than two things found near each other … in time or space). Fine. But using this logic too freely may result in misunderstandings. On one hand, one takes the risk of, essentially, asking the question: “If humans evolved from a chimp, why are there still chimps..” and o the other hand, one ignores the potentially dramatic events that can happen in biogeography. The zebra is more closely related to some Asian horse than it is to the springbok it is standing next to in the savanna.

Larson2007.    Larson, S.G., Jungers, W.L., Morwood, M.J., Sutikna, T., Jatmiko, Saptomo, E.W. et al.. Homo floresiensis and the evolution of the hominin shoulder. J Hum Evol 53, 718-31(2007).

Every few years a paper comes out “explaining” short stature in one or more Pygmy groups. Most of the time the new work ads new information and new ideas but fails to be convincing. This is the case with the recent PNAS paper by Migliano et al.

From the abstract:

Every few years a paper comes out “explaining” short stature in one or more Pygmy groups. Most of the time the new work ads new information and new ideas but fails to be convincing. This is the case with the recent PNAS paper by Migliano et al.

From the abstract:

Explanations for the evolution of human pygmies continue to be a matter of controversy, recently fueled by the disagreements surrounding the interpretation of the fossil hominin Homo floresiensis. Traditional hypotheses assume that the small body size of human pygmies is an adaptation to special challenges, such as thermoregulation, locomotion in dense forests, or endurance against starvation. Here, we present an analysis of stature, growth, and individual fitness for a large population of Aeta and a smaller one of Batak from the Philippines and compare it with data on other pygmy groups accumulated by anthropologists for a century. The results challenge traditional explanations of human pygmy body size. We argue that human pygmy populations and adaptations evolved independently as the result of a life history tradeoff between the fertility benefits of larger body size against the costs of late growth cessation, under circumstances of significant young and adult mortality. Human pygmies do not appear to have evolved through positive selection for small stature–this was a by-product of selection for early onset of reproduction.

I would like to add right in the beginning that the evolution of stature is probably, in my opinion, widely misunderstood. There are two problems. The first, which is not too related to the present discussion and that I’d like to dispense with right away, is that change over time in human stature is not easily attributed to genetic change. Secular change in body size seems in itself to be a capacity of certain mammals and may be a long or medium term response to ecological change. Increase in stature in human groups documented over the last century or more of time is not genetic change. We have no evidence that the short people … the people carrying hypothetical “shortness” alleles … were killed off differentially or failed to reproduce. Rather, increased nutrition (of some sort) seems to produce somewhat taller offspring in each generation. Perhaps there is a limit on growth that is determined by the mother. If nutritional load is increased beyond some limit, a given population produces somewhat larger (but not maximally larger) offspring, but over several generations, the full potential stature is realized.

But that is not the issue with African Pygmies, who are genetically short-statued.

The second issue is the idea that short stature is a derived adaptation in relation to “normal” or “typical” taller stature. The authors of this paper correctly point out that there is not a good single explanation for short stature. There are at least two possible reasons for this. First, we have not thought of an explanation yet, or among the list of possible explanations is the actual explanation, but we have not identified it or demonstrated it to a sufficient degree. Second, some subset of previously proposed explanations (possibly all of them?) is at work, but to different degrees in different situations. In other words, stature is an epiphenomenon resulting from multiple causes.

I would like to suggest that we begin to think of stature in a somewhat different ways. I propose that the following two things are true:

1. 1)For certain reasons, increased stature in humans is always good. Male-male competition, increased ranging behavior, and anti-predator tactics come to mind as selective forces for increased stature.
2. 2)For certain reasons, smaller stature in humans is always good. In the absence of selection for tallness, shortness has a significant energetic advantage, for instance.

The tradeoff between good reasons to be tall and good reasons to be short are always in play, so the optimal stature for a given population may shift even so dynamically that stature changes through either secular variation or allelic changes are only barely able to track the optimum.

This is why the life history approach taken by the research being discussed here is a good one. Life history theory is about tradeoffs. In the absence of a qualitative shift of some kind, an organism has a limited amount of energy that must be budgeted over short periods of time and over a lifetime. Classically, the three major “budget lines” for this energy are growth, reproduction, and maintenance (maintenance including immune system activity). Stature does not fit into one of these categories, but rather, should be responsive to all three. Stature determines a good portion of the energy partitioned into growth. Larger vs. smaller individuals have more vs. less daily maintenance demand, and producing larger vs. smaller offspring demands more or less energy.

This paper attempts to assertain if Pygmy stature is a function of nutritional variation by comparing data from Pygmies with other groups to examine the growth curve. This is largely a rehash of previous research, done some time ago, that established that African Pygmies are genetically short of stature. This is one of those rare cases where a genetic cause is not only evidenced by the appearance of the phenotype, but the gene involved is known.

The next thing they do is to estimate mortality and fertility patterns for Pygmies, and in this effort they find Pygmies to be more akin to Chimpanzees than to other humans. This result is fairly dicey, however. The ideal data for both mortality and fertility is simply not available for chimps or humans. Yes, there are data, but the sample sizes are inadqueate. In addition, their literature search indicates that Pygmy life expectancy is low, averaging 18 years and ranging from 15.6 to 24.2 years, compared to other groups including the !Kung, Ache and Turkana. The problem with this is that while !Kung and Ache life expectancy is indicated to be longer, they are not at all very much taller, and the !Kung live in a savanna environment. In other words, the comparison is being made between mostly rain forest populations with data collected from periods when disease from the outside may have a strong effect, with data from various time periods of people living in environments including those with much less disease. The authors write-off the importance of these effects but, well, they can’t really do that.

I would not be surprised if there was a link between stature and life span, but the data available at this time are not sufficient to make the case. Indeed, the relationship could easily be the opposite of what is asserted here. Life history theory would predict that growing big could produce, as a matter of trade offs, decreased life span, when comparing populations that are different in their life history parameters but otherwise similar (same species, same environment, same diet, etc.).

I still prefer the hypothesis of diet and thermoregulation as explanations for Pygmy stature and body form. The authors of this paper produce a red herring by grouping together Asian and African populations into one, ignoring what is known about the histories of the two groups. African Pygmies have been living in the rain forest for quite some time, as opposed to non-Pygmy groups who have moved into the region fairly recently. The history of Asian “Pygmy” populations is probably less well understood, but there is reason to expect a similar history. The authors cite Diamond’s conjecture that forest dwelling is not an explanation for pygmy stature because of the Asian cases, but Diamond is probably simply wrong on this. South Asian short stature groups may well have been living in rain forests much longer than other groups.

Bamberg Migliano, Andrea Vinicius, Lucio Lahr, Marta Mirazon. Life history trade-offs explain the evolution of human pygmies. PNAS Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0708024105, ().

Genome study places modern humans in the evolutionary fast laneDec. 10, 2007by Brian MattmillerCountering a common theory that human evolution has slowed to a crawl or even stopped in modern humans, a new study examining data from an international genomics project describes the past 40,000 years as a time of supercharged evolutionary change, driven by exponential population growth and cultural shifts.In a study published in the Dec. 10 issue of the Proceedings of the National Academy of Sciences (PNAS), a team led by UW-Madison anthropologist John Hawks estimates that positive selection just in the past 5,000 years alone — around the period of the Stone Age — has occurred at a rate roughly 100 times higher than any other period of human evolution. Many of the new genetic adjustments are occurring around changes in the human diet brought on by the advent of agriculture, and resistance to epidemic diseases that became major killers after the growth of human civilizations.”In evolutionary terms, cultures that grow slowly are at a disadvantage, but the massive growth of human populations has led to far more genetic mutations,” says Hawks. “And every mutation that is advantageous to people has a chance of being selected and driven toward fixation. What we are catching is an exceptional time.”The findings may lead to a very broad rethinking of human evolution, Hawks says, especially in the view that modern culture has essentially relaxed the need for physical genetic changes in humans to improve survival. Adds Hawks: “We are more different genetically from people living 5,000 years ago than they were different from Neanderthals.”While the correlation between population size and natural selection is nothing new — it was a core premise of Charles Darwin, Hawks says — the ability to bring quantifiable evidence to the table is a new and exciting outgrowth of the Human Genome Project.Along with co-author Gregory Cochran, an anthropologist at the University of Utah; and Eric Wang of Affymetrix, Inc., in Santa Clara, Cal.; Hawks analyzed data from the International HapMap Project, short for haplotype mapping. This project is working to catalog genetic similarities and differences in human beings by studying genes from distinct sample populations around the globe. While the HapMap will ultimately be used to identify genes that affect human health, it can also provide a road map of genetic variation from the ancestral human population.While more than 99 percent of the human genome is common across all humans, the HapMap project is cataloguing the individual differences in DNA called single nucleotide polymorphisms (SNPs). The project has mapped roughly 4 million of the estimated 10 million SNPs in the human genome. More importantly, it is identifying different regions of DNA, or haplotypes, that contain a large number of SNPs and are shared by multiple individuals.In the hunt for recent genetic variation in this map, Hawks’ research focuses on a phenomenon called linkage disequilibrium (LD). These are places on the genome where genetic variations are occurring more often than can be accounted for by chance, usually because these changes are affording some kind of selection advantage.The researchers identify recent genetic change by finding long blocks of DNA base pairs that are connected. Because human DNA is constantly being reshuffled through recombination, a long, uninterrupted segment of LD is usually evidence of positive selection. Linkage disequilibrium decays quickly as recombination occurs across many generations, so finding these uninterrupted segments is strong evidence of recent adaptation, Hawks says.Employing this test, the researchers found evidence of recent selection on approximately 1,800 genes, or 7 percent of all human genes.This finding runs counter to conventional wisdom in many ways, Hawks says. For example, there’s a strong record of skeletal changes that clearly show people became physically smaller, and their brains and teeth are also smaller. This is generally seen as a sign of relaxed selection — that size and strength are no longer key to survival.But other pathways for evolution have opened, Hawks says, and genetic changes are now being driven by major changes in human culture. One good example is lactase, the gene that helps people digest milk. This gene normally declines and stops activity about the time one becomes a teenager, Hawks says. But northern Europeans developed a variation of the gene that allowed them to drink milk their whole lives — a relatively new adaptation that is directly tied to the advance of domestic farming and use of milk as an agricultural product.The biggest new pathway for selection relates to disease resistance, Hawks says. As people starting living in much larger groups and settling in one place roughly 10,000 years ago, epidemic diseases such as malaria, smallpox and cholera began to dramatically shift mortality patterns in people. Malaria is one of the clearest examples, Hawks says, given that there are now more than two dozen identified genetic adaptations that relate to malaria resistance, including an entirely new blood type known as the Duffy blood type.Another recently discovered gene, CCR5, originated about 4,000 years ago and now exists in about 10 percent of the European population. It was discovered recently because it makes people resistant to HIV/AIDS. But its original value might have come from obstructing the pathway for smallpox.”There are many things under selection that are making it harder for pathogens to kill us,” Hawks says.Population growth is making all of this change occur much faster, Hawks says, giving a nod to Charles Darwin. When Darwin wrote in “Origin of the Species” about challenges in animal breeding, he always emphasized that herd size “is of the highest importance for success” because large populations have more genetic variation, Hawks says.The parallel to humans is obvious: The human population has grown from a few million people 10,000 years ago to about 200 million people at A.D. 0, to 600 million people in the year 1700, to more than 6.5 billion today. Prior to these times, the population was so small for so long that positive selection occurred at a glacial pace, Hawks says.”What’s really amazing about humans, that is not true with most other species, is that for a long time we were just a little ape species in one corner of Africa, and weren’t genetically sampling anything like the potential we have now,” he says.The recent changes are especially striking, he says. “Five thousand years is such a small sliver of time — it’s 100 to 200 generations ago,” he says. “That’s how long it’s been since some of these genes originated, and today they are in 30 or 40 percent of people because they’ve had such an advantage. It’s like ‘invasion of the body snatchers.'”[source]

Hawks, John Hawks, Eric T. Wang, Gregory M. Cochranâ?¡ Henry C. Harpending, and Robert K. Moyzis. (2007) Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences. Forthcoming. PNAS.