Published: 7 hours ago, 17:16 EST, December 10, 2007
<http://archive.physorg.com/10/12/2007>
Are humans evolving faster? Findings suggest we are becoming more different,
not alike
Researchers discovered genetic evidence that human evolution is
speeding up and has not halted or proceeded at a constant rate, as had
been thought indicating that humans on different continents are becoming
increasingly different.
We used a new genomic technology to show that humans are evolving rapidly,
and that the pace of change has accelerated a lot in the last 40,000 years,
especially since the end of the Ice Age roughly 10,000 years ago, says
research team leader Henry Harpending, a distinguished professor of
anthropology at the University of Utah.
document.write(""); Harpending says there are provocative implications from
the study, published online Monday, Dec. 10 in the journal Proceedings of
the National Academy of Sciences:
-- We arent the same as people even 1,000 or 2,000 years ago, he says,
which may explain, for example, part of the difference between Viking
invaders and their peaceful Swedish descendants. The dogma has been these
are cultural fluctuations, but almost any temperament trait you look at is
under strong genetic influence.
-- Human races are evolving away from each other, Harpending says. Genes
are evolving fast in Europe, Asia and Africa, but almost all of these are
unique to their continent of origin. We are getting less alike, not merging
into a single, mixed humanity. He says that is happening because humans
dispersed from Africa to other regions 40,000 years ago, and there has not
been much flow of genes between the regions since then.
Our study denies the widely held assumption or belief that modern humans
[those who widely adopted advanced tools and art] appeared 40,000 years ago,
have not changed since and that we are all pretty much the same. We show
that humans are changing relatively rapidly on a scale of centuries to
millennia, and that these changes are different in different continental
groups.
The increase in human population from millions to billions in the last
10,000 years accelerated the rate of evolution because we were in new
environments to which we needed to adapt, Harpending adds. And with a
larger population, more mutations occurred.
Study co-author Gregory M. Cochran says: History looks more and more like a
science fiction novel in which mutants repeatedly arose and displaced normal
humans sometimes quietly, by surviving starvation and disease better,
sometimes as a conquering horde. And we are those mutants.
Harpending conducted the study with Cochran, a New Mexico physicist,
self-taught evolutionary biologist and adjunct professor of anthropology at
the University of Utah; anthropologist John Hawks, a former Utah
postdoctoral researcher now at the University of Wisconsin, Madison;
geneticist Eric Wang of Affymetrix, Inc. in Santa Clara, Calif.; and
biochemist Robert Moyzis of the University of California, Irvine.
No Justification for Discrimination
The new study comes from two of the same University of Utah scientists
Harpending and Cochran who created a stir in 2005 when they published a
study arguing that above-average intelligence in Ashkenazi Jews those of
northern European heritage resulted from natural selection in medieval
Europe, where they were pressured into jobs as financiers, traders, managers
and tax collectors. Those who were smarter succeeded, grew wealthy and had
bigger families to pass on their genes. Yet that intelligence also is linked
to genetic diseases such as Tay-Sachs and Gaucher in Jews.
That study and others dealing with genetic differences among humans whose
DNA is more than 99 percent identical generated fears such research will
undermine the principle of human equality and justify racism and
discrimination. Other critics question the quality of the science and argue
culture plays a bigger role than genetics.
Harpending says genetic differences among different human populations
cannot be used to justify discrimination. Rights in the Constitution arent
predicated on utter equality. People have rights and should have
opportunities whatever their group.
Analyzing SNPs of Evolutionary Acceleration
The study looked for genetic evidence of natural selection the evolution
of favorable gene mutations during the past 80,000 years by analyzing DNA
from 270 individuals in the International HapMap Project, an effort to
identify variations in human genes that cause disease and can serve as
targets for new medicines.
The new study looked specifically at genetic variations called single
nucleotide polymorphisms, or SNPs (pronounced snips) which are
single-point mutations in chromosomes that are spreading through a
significant proportion of the population.
Imagine walking along two chromosomes the same chromosome from two
different people. Chromosomes are made of DNA, a twisting, ladder-like
structure in which each rung is made of a base pair of amino acids, either
G-C or A-T. Harpending says that about every 1,000 base pairs, there will be
a difference between the two chromosomes. That is known as a SNP.
Data examined in the study included 3.9 million SNPs from the 270 people in
four populations: Han Chinese, Japanese, Africas Yoruba tribe and northern
Europeans, represented largely by data from Utah Mormons, says Harpending.
Over time, chromosomes randomly break and recombine to create new versions
or variants of the chromosome. If a favorable mutation appears, then the
number of copies of that chromosome will increase rapidly in the population
because people with the mutation are more likely to survive and reproduce,
Harpending says.
And if it increases rapidly, it becomes common in the population in a short
time, he adds.
The researchers took advantage of that to determine if genes on chromosomes
had evolved recently. Humans have 23 pairs of chromosomes, with each parent
providing one copy of each of the 23. If the same chromosome from numerous
people has a segment with an identical pattern of SNPs, that indicates that
segment of the chromosome has not broken up and recombined recently.
That means a gene on that segment of chromosome must have evolved recently
and fast; if it had evolved long ago, the chromosome would have broken and
recombined.
Harpending and colleagues used a computer to scan the data for chromosome
segments that had identical SNP patterns and thus had not broken and
recombined, meaning they evolved recently. They also calculated how recently
the genes evolved.
A key finding: 7 percent of human genes are undergoing rapid, recent
evolution.
The researchers built a case that human evolution has accelerated by
comparing genetic data with what the data should look like if human
evolution had been constant:
-- The study found much more genetic diversity in the SNPs than would be
expected if human evolution had remained constant.
-- If the rate at which new genes evolve in Africans was extrapolated back
to 6 million years ago when humans and chimpanzees diverged, the genetic
difference between modern chimps and humans would be 160 times greater than
it really is. So the evolution rate of Africans represents a recent speedup
in evolution.
-- If evolution had been fast and constant for a long time, there should be
many recently evolved genes that have spread to everyone. Yet, the study
revealed many genes still becoming more frequent in the population,
indicating a recent evolutionary speedup.
Next, the researchers examined the history of human population size on each
continent. They found that mutation patterns seen in the genome data were
consistent with the hypothesis that evolution is faster in larger
populations.
Evolutionary Change and Human History: Got Milk?
Rapid population growth has been coupled with vast changes in cultures and
ecology, creating new opportunities for adaptation, the study says. The
past 10,000 years have seen rapid skeletal and dental evolution in human
populations, as well as the appearance of many new genetic responses to diet
and disease.
The researchers note that human migrations into new Eurasian environments
created selective pressures favoring less skin pigmentation (so more
sunlight could be absorbed by skin to make vitamin D), adaptation to cold
weather and dietary changes.
Because human population grew from several million at the end of the Ice Age
to 6 billion now, more favored new genes have emerged and evolution has
speeded up, both globally and among continental groups of people, Harpending
says.
"We have to understand genetic change in order to understand history, he
adds.
For example, in China and most of Africa, few people can digest fresh milk
into adulthood. Yet in Sweden and Denmark, the gene that makes the
milk-digesting enzyme lactase remains active, so almost everyone can drink
fresh milk, explaining why dairying is more common in Europe than in the
Mediterranean and Africa, Harpending says.
He now is studying if the mutation that allowed lactose tolerance spurred
some of historys great population expansions, including when speakers of
Indo-European languages settled all the way from northwest India and central
Asia through Persia and across Europe 4,000 to 5,000 years ago. He suspects
milk drinking gave lactose-tolerant Indo-European speakers more energy,
allowing them to conquer a large area.
But Harpending believes the speedup in human evolution is a temporary state
of affairs because of our new environments since the dispersal of modern
humans 40,000 years ago and especially since the invention of agriculture
12,000 years ago. That changed our diet and changed our social systems. If
you suddenly take hunter-gatherers and give them a diet of corn, they
frequently get diabetes. Were still adapting to that. Several new genes we
see spreading through the population are involved with helping us prosper
with high-carbohydrate diet.
Source: University of Utah
