Feb 03, 2008 04:30 AM
Think that we humans are a fait accompli, a done deal that hasn't changed over the eons?
Think again.
Evidence is accumulating that the species is still evolving, and doing so at an unprecedented rate.
A major new study says that in the past 5,000 years, natural selection – gene mutations that spread because they're beneficial – has occurred 100 times faster than at any other period in human history.
American researchers have found evidence of recent mutations on about 1,800 genes, or 7 per cent of the human genome; traits such as lighter skin and blue eyes in northern Europeans and partial resistance to certain diseases in areas of Africa.
"We are more different genetically from people living 5,000 years ago than they were different from Neanderthals," said one of the study's co-authors, anthropologist John Hawks, at a presentation recently.
Just because modern humans are able to manipulate their environment, says University of Toronto molecular anthropologist Esteban Parra, "doesn't mean biological evolution has stopped. It has increased."
The new evidence contradicts the long-held view that it takes 1,000 to 10,000 generations – or 20,000 to 200,000 years – for an advantageous mutation to crop up in an individual, then spread through a population. The study has compressed the time frame to only 100 to 200 generations, which in evolutionary terms is extremely short.
"That's how long it's been since some of these genes originated, and today they're in 30 or 40 per cent of people," said Hawks. "What we are catching is an exceptional time."
One they've been able to catch only because scientists can now tap into the human genome that was sequenced in 2003.
Researchers analyzed 3.9 million genetic markers in 270 people from four groups: Han Chinese, Japanese, Africa's Yoruba people, and northern Europeans. (The DNA was supplied by the International HapMap Project, which is analyzing genetic similarities and differences around the world. The findings were published in the Proceedings of the National Academy of Sciences.)
A little background: Mankind's earliest ancestors split from the forerunners of today's chimpanzees about 6 million years ago. Roughly 2 million years ago, the predecessors of modern humans began the long trek out of Africa and into the rest of the world.
About 150,000 years ago, we appeared, modern humans. Some 50,000 years later, our brains made a stunning leap forward, developing complex language and abstract symbols. We had begun the journey to civilization.
At that point, the evolutionary process, having sufficiently ensured humans' survival as a species, basically stopped, slowing to a glacial pace. Or so it was thought.
By the late evolutionary biologist Stephen Jay Gould, for instance. In an essay published in 2000, he wrote, "there's been no biological change in humans in 40,000 years or 50,000 years. Everything we call culture and civilization we've built with the same body and brain."
Noted British geneticist Steve Jones broadly agreed, but dated the evolutionary slowdown much later, with the rise of agriculture at the end of the Ice Age 10,000 to 12,000 years ago.
When humans made the transition from hunting-gathering to raising crops and domesticating animals, the move led to dietary changes and to settled habitats in specific regions. Combined, they ignited a surge in human numbers.
Far from slowing down, it appears that, when there were enough people to, in effect, work with, the process of evolution rapidly began to accelerate.
Even without modern-day knowledge of genes, Charles Darwin wrote in his revolutionary The Origin of Species that in animal breeding, herd size "is of the highest importance for success" because large populations have more genetic variation. The same turns out to be true for us.
Since the advent of agriculture, the human population has grown steadily from about 5 million 10,000 years ago to 200 million in 1 AD (it's 6.5 billion today). But as people migrated to different geographic regions, they had to adapt to a variety of conditions and pressures.
One example cited by the new study is lactase, the gene that helps humans digest milk but which, for most of the planet's population, switches off in adulthood. At some time in the past few thousand years, northern European dairy farmers – living with weaker sunlight therefore less vitamin D exposure – developed a mutation that lets them tolerate health-giving milk throughout their lives. (U of T's Parra says other variations have also shown up in dairy-farming regions in Africa, even though sun exposure isn't a problem.)
Where genetic fine-tuning has been busiest, however, is in disease resistance. When more of our ancestors started living together in set locales, outbreaks of epidemic diseases periodically culled their numbers, leaving behind genetically different and fitter survivors.
Michael Bisson, chair of anthropology at McGill University, cites native North Americans who were felled by various diseases when Europeans first arrived. "But they subsequently developed genetic immunities which they still possess," he says. "So yes, there's been significant evolution even in the last 1,000 years," he says.
Malaria is one of the clearest examples of ongoing evolution, the U.S. study found. It's now known that more than two dozen genetic adaptations have evolved to resist it, including an entirely new blood type, called the Duffy blood type.
Why then does malaria still persist in Africa? Because the mosquito that spreads it is also adapting, says Esteban Parra: Genetically, humans are "in a race with disease, a very dynamic race."
Another recently discovered gene, which originated about 4,000 years ago, now exists in about 10 per cent of Europe's population. It was discovered recently because it's giving some people resistance to HIV/AIDS, though its original function was likely to ward off smallpox.
But with more and better drugs and vaccines, clean water, sanitation and plentiful food (at least for most of the planet), why does the species still need to tinker biologically to survive? Stephen Jay Gould, who died in 2002, was among those who thought it no longer did; that "natural selection has almost become irrelevant."
They were wrong, say those who can now access the complex inner workings of Homo sapiens' 25,000 (or so) genes. They say adaptation appears to be built into our DNA to respond to changing environmental, even cultural, stresses.
That could mean extended fertility spans, says John Hawks: "Any kind of genetic variation that increases the success of later fertility will be selected for," he predicts.
Another area of adaptation is likely to be the brain, as it responds to the pressures of pervasive technology. Brain size grew slowly over a long period of time, but an analysis of skulls by Hawks in a earlier study showed that size started diminishing about 10,000 years ago. Today, the brain is about an eighth of the size it once was. Evolution, Hawks theorized, was making it more compact and efficient
In 2005, University of Chicago geneticist Bruce Lahn reported that two "new" gene variations involved in brain size and complexity are still a work in progress. One emerged about 37,000 years ago and is now present in 70 per cent of humans; the other, only 5,800 years old, has spread to 30 per cent.
"Our environment and the skills we need to survive in it are changing faster than we ever imagined," Lahn said then. "I would expect the human brain, which has done well by us so far, would continue to adapt to those changes."
Most researchers prefer not to speculate on where genetic adaptation will take us next. Esteban Parra will "predict" only that "evolution isn't going to stop."
With one caveat, that is: It won't stop unless and until we do first.
Lynda Hurst is hurtling along the evolutionary highway. She can be reached at: lhurst@thestar.ca.
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