Human evolution, radically reappraised
March 26, 2007
Special to World Science
Updated March 27
Human evolution has been speeding up tremendously, a new study contends—so much, that the latest evolutionary changes seem to largely eclipse earlier ones that accompanied modern man’s “origin.”
|
|
| Hominid skulls. Top: Homo erectus dated to 1.75 million years ago; Middle: an early “modern” Homo sapiens dated to 160,000 years ago; Bottom: a contemporary human. (Credits: top, Science magazine; middle, Tim White; bottom, NIH).
|
The study, alongside other recent research on which it builds, amounts to a sweeping reappraisal of traditional views, which tended to assume that humans have reached an evolutionary endpoint.
The findings suggest that not only is our evolution continuing: in a sense our very “origin” can be seen as ongoing, a geneticist not involved in the study said.
Gregory Cochran of the University of Utah in Salt Lake City, Utah, a co-author of the study, said the research may force a radical rethinking of the story of modern human evolution. “It turns it upside-down, pretty much,” he said. But skeptics question some aspects of the work.
The traditional picture of humans as a finished product began to erode in recent years, scientists said, with a crop of studies suggesting our evolution indeed goes on. But the newest investigation goes further. It claims the process has actually accelerated.
It also downplays the importance of a much-scrutinized era around 200,000 years ago, when humans considered “anatomically modern” first appear in the fossil record. In the study, this epoch emerges as just part of a vast arc of accelerating change.
“The origin of modern humans was a minor event compared to more recent evolutionary changes,” wrote the authors of the research, in a presentation slated for Friday in Philadelphia at the annual meeting of the American Association of Physical Anthropologists.
The authors are Cochran and anthropologist John Hawks of the University of Wisconsin-Madison. The findings will also be submitted to one or more scientific journals, Cochran said.
The proposal is “truly fascinating,” wrote University of Chicago geneticist Bruce Lahn in an email. He wasn’t involved in the work, though he did conduct earlier research finding that evolution may still be ongoing in the brain.
Even before the Hawks-Cochran study and its immediate forerunners, Lahn wrote, scientists had already noted a trend of accelerating change in the evolutionary lineage leading to modern humans from ape-like ancestors. But that phenomenon seemed to have occurred over time spans measured in millions of years; it was far from clear that it has continued in the recent past or today, he added.
Hawks and Cochran, by contrast, argue that the trend “is visible even in the last tens of thousands of years,” Lahn wrote. It “runs counter to the feeling in some quarters that the evolution of the human phenotype [form] has slowed down or even stopped in our recent past.”
Defining an origin
If the study is correct, it raises new questions about how to define the “origin” of modern humans—a rather arbitrary decision in any case, Lahn remarked.
The origin is “defined probably more as a matter of convenience rather than reflecting any actual watershed evolutionary event,” he wrote. That is, it’s “useful to say that any past creatures that are within certain levels of similarities to us today should be considered as ‘the same’ as us.”
But if the changes that accompanied this event are only a trifling part of a wider trend, he added, it becomes reasonable to ask whether that further deflates the rationale for calling it an origin.
“In a sense,” he wrote, one could say “the origin is still ongoing.”
Evolution occurs when an individual acquires a beneficial genetic mutation, and it spreads throughout the population because those with it thrive and reproduce more. Ceaseless repetitions of this can change species, or produce new ones. As beneficial genes spread, harmful ones are weeded out; the whole process, called natural selection, propels evolution.
Hawks and Cochran analyzed measurements of skulls from Europe, Jordan, Nubia, South Africa, and China in the past 10,000 years, a period known as the Holocene era. They also studied European and West Asian skulls from the end of the Pleistocene era, which lasted from two million years ago until the Holocene.
“A constellation of features” changed across the board, Hawks and Cochran wrote in their presentation. “Holocene changes were similar in pattern and… faster than those at the archaic-modern transition,” the time when so-called modern humans appeared. But these changes “themselves were rapid compared to earlier hominid evolution.” Hominids are a family of primates that includes humans and their extinct, more ape-like though upright-walking ancestors and relatives.
Hawks and Cochran also analyzed past genetic studies to estimate the rate of production of genes that undergo positive selection—that is, genes that spread because they are beneficial. “The rate of generation of positively selected genes has increased as much as a hundredfold during the past 40,000 years,” they wrote.
There are ways to detect positive selection in genome data. But Mark Thomas, a genetic anthropologist at University College London, was skeptical that these would be enough to make Hawks’ and Cochran’s case. “The issue is that the most powerful methods for detecting selection are ones that lose their sensitivity going more than 30,000 years back,” he said. Other techniques can’t “distinguish between selection and population growth.”
Thomas added that he understands the skeletal data to show something different from what Hawks and Cochran say, but that he would need a fuller account of their findings to make a judgment.
Worrisome findings?
Hawks and Cochran said some of the most notable physical changes in humans have been ones affecting the size of the brain case.
A “thing that should probably worry people is that brains have been getting smaller for 20,000 to 30,000 years,” said Cochran. But brain size and intelligence aren’t tightly linked, he added. Also, growth in more advanced brain areas might have made up for the shrinkage, Cochran said; he speculated that an almost breakneck evolution of higher foreheads in some peoples may reflect this. A study in the Jan. 14 British Dental Journal found such a trend visible in England in just the past millennium, he noted, a mere eyeblink in evolutionary time.
Research published in the Sept. 9, 2005 issue of the research journal Science by Lahn and colleagues found that two genes linked to brain size are rapidly evolving in humans.
Anthropologist Jeffrey McKee of Ohio State University said the new findings of accelerated evolution bear out predictions he made in a 2000 book The Riddled Chain. Based on computer models, he argued that evolution should speed up as a population grows. This is because population growth creates more opportunities for new mutations; also, the expanded population occupies new environmental niches, which would drive evolution in new directions.
Lahn said he’s not convinced that the accelerated physical evolution is tied to population growth. “It may be a long way before” anyone can test the truth of this, he wrote.
But other factors could also explain an acceleration, according to anthropologist John Kingston of Emory University in Atlanta, Ga. Evolution might speed up because we have changed our own environment, which in turn changes the evolutionary pressures. “We now control our own environment and ecology to some extent,” he said.
For instance, if you invent spears, you perhaps can afford to be slighter-framed because you can stand further away from wild animals, Cochran said. He argued that a powerful synergy between these sorts of changes and expanding population explains the “fantastically rapid” recent evolution.
“A very big change”
Overall, the findings could amount to “a very big change” in traditional thinking for two reasons, according to McKee. First, he said, many researchers had mistakenly assumed population growth would slow down evolution, because new mutations would take too long to spread through a large population.
Second, the findings deal a final blow to a lingering view among researchers of evolution as a process “with us as the be-all-end-all,” he said. That idea went out of fashion in the 1950s but still persists “in the backs of our minds,” he added.
Many of the changes found in the genome or fossil record reflect metabolic alterations to adjust to agricultural life, Cochran said. Other changes simply make us weaker.
In the June 2003 issue of the research journal Current Anthropology, Helen Leach of the University of Otago, New Zealand wrote that skeletons from some populations in the human lineage have undergone a progressive shrinkage and weakening, and reduction in tooth size, similar to changes seen in domesticated animals. Humans seem to have domesticated themselves, she argued, causing physical as well as mental changes.
Despite all the alterations, McKee said he believes the notion of an “origin” of modern humans around 200,000 years ago remains useful. “It’s just a threshold point” at which humans take on most of the physical features we recognize, he remarked, and as such, needn’t be discarded. Cochran said it can still be argued that the key change was language; but when this originated remains far from clear.
Whatever the implications of the recent findings, McKee added, they highlight a ubiquitous point about evolution: “every species is a transitional species.”
I read that selection pressures for those outside of Africa (at least based on a limited sample of Nigerians) is less than those in Europe and East Asia…suggesting adaptation to environment is happening at a more rapid rate in Asia and Europe.
I would like to know what the selection pressures are like in the African American population…America is obviously not West Africa, the environment is completely different I would think that and our history of oppression have put us under heavy selection pressures.