Are Humans Still Evolving?

This is a follow-up to my earlier posting about the latest issue of Discover magazine [Ascent of Darwinism]. I want to discuss another article in that issue: "Are We Still Evolving" by Kathleen McAuliffe. The title of the web version is: They Don't Make Homo Sapiens Like They Used To.

In a minute we'll look at the quality of science journalism in this article, but first a little background.

The point of the article is that human evolution may have accelerated enormously in the past 40,000 years. The idea is based almost entirely on a few papers by John Hawks and his colleagues. What they did was to look at various polymorphisms in the human genome. The most common variants are single nucleotide mutations (single nucleotide polymorphism = SNP = "Snips"). Some people will have one tpe of variant while other people will have another. Almost all of these variants are neutral—they have no visible or functional effect—but some of them will affect fitness.

Some SNPs can linked to variants (alleles) that are under selection. If you assay for enough SNPs, you'll find several that just happen to be located near variants that are being selected. If selection is rapid then the nearby SNPs will be swept up along with the actual variant that improves fitness. A block of SNPs that forms a cluster found in many individuals is called a "haplotype." The presence of these haplotypes is evidence of a selective sweep (rapid increase in frequency due to selection).

As time goes on these clusters are broken up by recombination so using this technique you only see examples of recent adaptations.

Hawks and his colleagues claim to have found more than 11,000 examples of genes that are being selected in the human population. (Humans have about 20,000 genes.) They conclude that human evolution has accelerated in the past 40,000 years because our hunter-gatherer ancestors settled down to become farmers and this created a new environment. We have been adapting to that new environment ever since.

The important paper is Hawks et al. (2007). For more information read: Is Evolution Linked to Environmental Change?, Accelerated Human Evolution, Are Humans Evolving Faster? and Human Evolution Has Accelerated

The work is controversial. Many people are skeptical of both the result and the explanation. The general consensus among evolutionary biologists is "wait and see." They treat this as a preliminary result because they are well aware of the technical problems and how easy it is to score false positives. The technology is not foolproof.

Evolutionary biologists are not surprised by the claim that humans are evolving. The textbooks are full of examples of recent human evolution by both natural selection and random genetic drift. Besides, the evidence is all around us—you only have to look at the different appearance of people in Africa, Asia, and Europe to see the obvious. We also have the well-studied examples of human migration out of Africa and of coalescence to identify Mitochondria Eve. This is more evidence of recent human evolution.

So, evolutionary biologists aren't the least bit surprised by evidence of human evolution but they're skeptical of this particular study because it claims recent accelerated human evolution. The paper isn't that exciting to most people who know about evolution.

The popular press had a fit, aided and abetted by the PR departments at several universities and, more recently, by a newly published book: The 10,000 Year Explosion: How Civilization Accelerated Human Evolution.

This brings us to the article in Discover. The author, Kathleen McAuliffe, is a freelance writer who specializes in science and medicine. She has an M.A. in natural science. She just won an Alicia Patterson Journalism Fellowship "to continue her research into human evolution from the Stone Age to the present."

Here's how the article begins ....

For decades the consensus view—among the public as well as the world’s preeminent biologists—has been that human evolution is over. Since modern Homo sapiens emerged 50,000 years ago, “natural selection has almost become irrelevant” to us, the influential Harvard paleontologist Stephen Jay Gould proclaimed. “There have been no biological changes. Everything we’ve called culture and civilization we’ve built with the same body and brain.” This view has become so entrenched that it is practically doctrine. Even the founders of evolutionary psychology, Leda Cosmides and John Tooby, signed on to the notion that our brains were mostly sculpted during the long period when we were hunter-gatherers and have changed little since. “Our modern skulls house a Stone Age mind,” they wrote in a background piece on the Center for Evolutionary Psychology at the University of California at Santa Barbara.

I think this is wrong. I do not believe that the consensus among the world's preeminent evolutionary biologists¹ is that human evolution is over.

I'm not familiar with the Gould quotation. It seems to have been uncovered by Cochran and Harpending, two of the authors on the Hawks et al. paper. They use it on the first page of their book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Even if it's accurate, it does not represent the mainstream view of most evolutionary biologists.

So to suggest that humans have undergone an evolutionary makeover from Stone Age times to the present is nothing short of blasphemous. Yet a team of researchers has done just that. They find an abundance of recent adaptive mutations etched in the human genome; even more shocking, these mutations seem to be piling up faster and ever faster, like an avalanche. Over the past 10,000 years, their data show, human evolution has occurred a hundred times more quickly than in any other period in our species’ history.

There are two things wrong with this. First, the hype about blasphemy is a serious distortion of the truth. The scientific literature is full of examples of recent human evolution. Haven't you heard of the sickle cell gene, lactose intolerance, and blood types?

Second, while the Hawks et al. paper is interesting, it is extremely misleading to imply that their evidence is unchallenged. That's not good science journalism. (Incidentally, the peer-reviewed paper says that human evolution accelerated in the past 40,000 years, not 10,000.)

It's not until you get to the sixth paragraph that you find any sort of balance.

Not surprisingly, the new findings have raised hackles. Some scientists are alarmed by claims of ethnic differences in temperament and intelligence, fearing that they will inflame racial sensitivities. Other researchers point to limitations in the data. Yet even skeptics now admit that some human traits, at least, are evolving rapidly, challenging yesterday’s hallowed beliefs.

What kind of balance is that? Where's the sober description of the consensus view; namely, that humans are always evolving and the evidence of Hawks and his colleagues isn't convincing?

John Hawks is featured prominently in this article.

These overriding trends are similar in many parts of the world, but other changes, especially over the past 10,000 years, are distinct to specific ethnic groups. “These variations are well known to forensic anthropologists,” Hawks says as he points them out: In Europeans, the cheekbones slant backward, the eye sockets are shaped like aviator glasses, and the nose bridge is high. Asians have cheekbones facing more forward, very round orbits, and a very low nose bridge. Australians have thicker skulls and the biggest teeth, on average, of any population today. “It beats me how leading biologists could look at the fossil record and conclude that human evolution came to a standstill 50,000 years ago,” Hawks says.

Beats me how John could possibly think that "leading biologists" have ignored the data.

McAuliffe also interviews Henry Harpending and Robert Moyzis, two other authors on the original paper. Not surprisingly, she gets a similar story from them about how their revolutionary ideas are overthrowing entrenched dogma.

McKauliffe is now on a role and she includes a number of just-so stories.

Paralleling the constant war against pathogens, human sperm may also be evolving at high speed, driven by the race to get to the egg before another man’s sperm. “It could be that cities create more sexual partners, which means fiercer competition among males,” Hawks says. Because sperm can fertilize an egg up to 24 hours after being ejaculated in the vagina, a woman who copulates with two or more partners in close succession is setting up the very conditions that pit one man’s sperm against another’s. Hawks infers that “sperm today is very different from sperm even 5,000 years ago.” Newly selected mutations in genes controlling sperm production show up in every ethnic group he and his team have studied; those genes may affect characteristics including abundance, motility, and viability. The selection for “super sperm,” Hawks says, provides further corroboration that our species is not particularly monogamous—a view widely shared by other anthropologists.

As agriculture became established and started creating a reliable food supply, Hawks says, more men and women would have begun living into their forties and beyond—jump-starting the selection pressure for increased life span. In support of that claim, Moyzis is currently performing a genetic analysis of men and women in their nineties who are of European ancestry. He has traced many early-onset forms of cancer, heart disease, and Alzheimer’s to older human gene variants. “The idea is that people with more modern variants tend to have greater resistance to these chronic illnesses of old age and should be overrepresented in the age 90-plus population,” Moyzis says.

Harpending and Cochran had previously—and controversially—marshaled similar evidence to explain why Ashkenazi Jews (those of northern European descent) are overrepresented among world chess masters, Nobel laureates, and those who score above 140 on IQ tests. In a 2005 article in the Journal of Biosocial Science, the scientists attributed Ashkenazis’ intellectual distinction to a religious and cultural environment that blocked them from working as farm laborers in central and northern Europe for almost a millennium, starting around A.D. 800. As a result, these Jews took jobs as moneylenders and financial administrators of estates. To make a profit, Harpending says, “they had to be good at evaluating properties and market risks, all the while dodging persecution.” Those who prospered in these mentally demanding and hostile environments, the researchers posit, would have left behind the most offspring.

The last one is accompanied by a passing reference to reality, "Critics note that the association between wealth and intelligence in this interpretation is circumstantial, however."

Six pages into the article we come to this ...

NOT SO FAST
Despite all these clues that human evolution has continued and accelerated into modern times, many evolutionary biologists remain deeply skeptical of the claims. Their resistance comes from several directions.

Some independent experts caution that the tools for studying the human genome remain in their infancy, and reliably detecting genomic regions that have been actively selected is a challenging problem. The hypothesis that human evolution is accelerating “all rests on being able to identify recent areas of the genome under natural selection fairly accurately,” says human geneticist Jonathan Pritchard of the University of Chicago. And that, he warns, is tricky, involving many different assumptions (about population sizes on different continents, for instance) in the poorly documented period before recorded history.

Given such uncertainties, researchers are more likely to be persuaded that a mutation has been recently selected if they understand its function and if its rise in prevalence meshes well with known human migratory routes. Genetic variants fitting that description include those coding for lighter skin coloring, resistance to diseases such as malaria, and metabolic changes related to the digestion of novel foods. There is broad consensus that these represent genuine examples of recent adaptations.

Hmmm ... there is "broad consensus" that humans have evolved recently.

How is that consistent with the outrageous claims in the opening paragraphs?

Do we blame science writer Kathleen McAuliffe or John Hawks and his colleagues for this misleading article?

Does the article contribute positively to educating the general public about human evolution or would we be better off if it had never been published?

---

1. I realize that McAuliffe said "biologist," not "evolutionary biologists" but surely the only biologists who count are the experts in the field? After all, you wouldn't ask ecologists their opinion about biochemistry, would you?

Hawks, J., Wang, J.T., Cochran, G., Harpending, H.C. and Moyzis, R.K. (2007) Recent acceleration of human adaptive evolution. Proc. Natl. Acad. Sci. (USA) 104:20753-20758 [doi: 10.1073/pnas.0707650104]

[Photo Credit: MSNBC]