The findings may lead to a very broad rethinking of human evolution, especially in the view that modern culture has essentially
relaxed the need for physical genetic changes in humans to improve
A team led by University of Wisconsin-Madison anthropologist John Hawks
estimates that positive selection just in the past 5,000 years alone
-dating back to 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
"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."
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.
In the hunt for recent genetic variation in the genome map the project has cataloged 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. 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
"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 tribute 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," Hawks continued, "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."
The recent changes are especially striking.
years is such a small sliver of time - it's 100 to 200 generations
ago. 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
Posted by Casey Kazan with Josh Hill. Adapted from a University of Wisconsin release.
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