Did biologists really think that human evolution stopped?07 Mar 2009
Larry Moran has been writing a series of posts about quality science journalism. These have included descriptions of some well-written journalistic accounts of evolutionary science, and other that are in his opinion not so well-written.
In this latter category – what Moran considers to be poor examples of journalism – he puts a recent article about my work, by writer Kathleen McAuliffe, which appeared in the March 2009 issue of Discover.
Naturally I disagree. After speaking with McAuliffe several times and showing her around my lab here in Madison, I believe she has done an excellent job of describing our research, as well as putting it in the context of recent studies of human variation and evolution.
I think that Moran’s criticism can be split into three points:
The article opposes our work against the straw-man view that “human evolution stopped.”
The article does not spend enough space describing the views of scientists who doubt that human evolution accelerated, or who doubt the amount of acceleration.
The article includes some speculative “just so stories” for the causes of selection on some genes.
Those three points are too many for a single blog post, so I will focus on the first. Moran agrees with me and many others that human evolution has been continuing in recent times. He does not specify what he means by recent, but he does mention a few examples of genes, like lactase and sickle cell, that have been strongly selected within the last few thousand years. He also mentions a few examples – like mitochondrial Eve, which date back into the Middle Pleistocene. I do not tend to call these recent, but in the context of the 6 million years of hominid evolution, they are also comparatively new.
Given this well-known evidence for recent human evolution, Moran questions the article’s introductory sentence:
For decades the consensus viewamong the public as well as the worlds preeminent biologistshas been that human evolution is over.
He also questions a direct quote from me that appears in the article:
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.
If I were being snarky, I would simply point you to the long post that Moran wrote in 2007, which began this way:
We frequently hear claims that humans have stopped evolving. Most of these claims have to do with medical advances that are now allowing people to survive who might have died in earlier times. The idea is that natural selection is no longer working so we have stopped evolving.
I am left to wonder where we “frequently hear” this idea, if no “leading biologists” actually believe it. Or why we would give this idea any credence or attention?
McAuliffe’s article helps to fill in this blank. For example, it includes a direct quotation from Stephen Jay Gould:
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 weve called culture and civilization weve built with the same body and brain.
Moran can’t be bothered to look up the source of that quote, and intimates that it may not be accurate. I do silly things like finding sources of quotes. The source is
Gould, SJ. 2000. "The Spice of Life: An Interview with Stephen Jay Gould" Leader to Leader. 15 (Winter):14-19. (online).
McAuliffe provides another quote along the same lines from Leda Cosmides and John Tooby. I don’t imagine that most readers of Discover would like a long list of direct quotes in support of the first sentence of an article, but I can cite a number of others.
For example, in his book, Children of Prometheus, Christopher Wills gives us a quote from an obscure text:
To be sure, there may have been an improvement of the brain without an enlargement of cranial capacity [over the last 100,000 years] but there is no real evidence of this. Something must have happened to weaken the selective pressure drastically. We cannot escape the conclusion that man's evolution towards manness suddenly came to a halt.... The social structure of contemporary society no longer awards superiority with reproductive success.
That’s from Ernst Mayr’s 1963 book, Animal Species and Evolution. Of course, that’s decades ago. Here’s another, from Ashley Montagu, which accompanied the UNESCO Statements on Race, last in 1972:
It is only during the last 15,000 years of his history that some populations developed agriculture and some went on to develop an urban way of life.... In the course of man's evolution the selective pressures acted not toward the development of any particular ability, but toward the generalized ability of adaptability. Hence, there would have been no development of genetically based special abilities in one population differing from those developed in other groups. Since there was no particular premium placed upon the development of such abilities, there would have been no selection for them in any group.
Montagu is very important not because of his prominence as a biologist – although he had studied with Karl Pearson as an undergraduate, his training was primarily in the Boas school of cultural anthropology. He is important because he worked so hard to establish in biology and the public the idea of genetic equality of human races. As McAuliffe’s article points out, this is a major reason why biologists have resisted the idea of recent human evolution. It is a false idea – as Dobzhansky pointed out, genetic identity is irrelevant to equality. But it is an entrenched idea within human biology research.
Possibly this quote from Luca Cavalli-Sforza (writing with Francesco Cavalli-Sforza and Sarah Thorne) in The Great Human Diasporas, p. 246, is also relevant:
The forces of evolution have been altered radically by the developments of the last ten thousand years. The number of people living on the planet has increased over a thousandfold since agriculture began. As a result, the effects of genetic drift are now much more modest, and we could almost say shelved.
Some types of natural selection have also been shelved.... For natural selection to work, some have to die where others survive, and some have to die more easily than others. Plummeting infant mortality has almost eliminated the effects of natural selection due to differences in mortality.
These examples (and there are many, many others) are sufficient for me to wonder how “leading biologists” can think that human evolution came to a standstill 50,000 years ago. I think that Moran is right – they must have been ignoring the data.
Or perhaps those biologists really agreed with Larry, but claimed otherwise for some purpose. Maybe they were all exaggerating – human evolution hadn’t really stopped, but had slowed down substantially. Some of them may have been lumping together what they didn’t really know about the last 10,000 years with their thoughts about the last 100 years, when mortality rates in Western nations really have decreased. But it’s clear that most of them weren’t considering the actual data of the last 50,000 years of human evolution.
Still, I think Larry is overstating the extent that today’s biologists think that humans have been evolving. Maybe he thinks that others all share his reasonable opinion that sickle cell and lactase are strong evidence of recent human evolution. Nevertheless, I have spoken to many biologists who disagree. In particular some remain skeptical that lactase persistence could have given a survival advantage to ancient people. This view is not tenable today, but until a few years ago, many human biologists simply assumed that such variations dated to the very distant past – much longer than 50,000 years ago.
Even Larry throws “blood groups” in with his examples of recent evolution, when the most prominent blood group polymorphism, ABO, is millions of years old. The frequencies of this gene have evolved recently, but when Larry asks, “Haven’t they heard of … blood groups?” he should understand that many human biologists think of this as an example of very ancient evolution, not recent change.
That’s one reason why I accentuate the prehistoric record of human morphological changes. Skeletal evidence of reductions in brain size, reductions in dental dimensions, progressive loss of third molars, and changes in the cranial index have been known for well over a hundred years. So there’s really no excuse for midcentury and later evolutionary biologists to deny that human evolution has been rapid in the last few thousand years.
Yet despite the abundant evidence that human biologists have opposed the idea of recent human evolution, I still think that McAuliffe’s opening sentence does construct a “straw man” argument. Many prominent examples don’t prove that there has been a decades-long consensus that human evolution stopped. And our research is not about human evolution merely continuing – we think it actually accelerated. Evidence that some biologists thought that human evolution stopped is interesting. But the reality is that almost no one has thought that human evolution accelerated.
That’s curious, because the same theory that implies that human evolution must not have stopped also predicts that it should have sped up. There’s no new theory here – heck, the extent of our theoretical model is a linear equation! Larger populations make it more likely for adaptive mutations to happen. The only reasons that evolution wouldn’t accelerate in recent humans are if adaptive mutations are in principle impossible, or if they are so common that they happen in small populations anyway.
Like Larry, I think that biologists are mostly convinced not by theory – however simple – but by concrete examples. That’s precisely what McAuliffe describes at the end of her article:
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.
Her clear description of these nuances – scientists applying different analytical methods, possibly using different standards of evidence – is one of the reasons why this is an good piece of science journalism. It describes the reasons for skepticism about our work as well as the ways that non-biologists may misinterpret it – a part of the article that adds up to almost 1000 words.
Larry disagrees that this is enough to provide balance to the article, and suggests that this section actually contradicts the idea that most biologists accept a “static” view of human evolution. After all, if there is “broad consensus” that a few prominent evolutionary changes happened recently, that must mean that human evolution hasn’t stopped, right?
Well, all I can say is that if all human biologists had the same attitude toward natural selection as Larry Moran, I doubt that we would have needed to publish our ideas about acceleration. Because they would already have been widely accepted!