Text 1208, 181 rader
Skriven 2004-12-29 06:21:00 av Jim McGinn (1:278/230)
Ärende: Human Evolution is Catego
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As I myself (Jim McGinn) have been saying for quite some time now

http://tinyurl.com/5krqj

University of Chicago researchers now concur that human evolution is
categorically distinct:

http://www.eurekalert.org/pub_releases/2004-12/uocm-uoc122304.php

Public release date: 28-Dec-2004

Contact: Catherine Gianaro
catherine.gianaro@uchospitals.edu
773-702-6241
University of Chicago Medical Center

University of Chicago researchers discovered that humans are a
'privileged' evolutionary lineage

The genes that regulate brain development and function evolved much
more rapidly in humans than in nonhuman primates and other mammals
because of natural selection processes unique to the human lineage.
Researchers reported their findings in the cover article of the Dec.
29, 2004, issue of the journal Cell.
"Humans evolved their cognitive abilities not due to a few accidental
mutations, but rather, from an enormous number of mutations acquired
though exceptionally intense selection favoring more complex cognitive
abilities," said lead scientist Bruce Lahn, an assistant professor of
human genetics at the University of Chicago and an investigator at the
Howard Hughes Medical Institute.

"We tend to think of our own species as categorically different -
being on the top of the food chain," Lahn said. "There is some
justification for that."

>From a genetic point of view, some scientists thought that human
evolution might be a recapitulation of the typical molecular
evolutionary process, he said. For example, the evolution of the larger
brain might be due to the same processes that led to the evolution of a
larger antler or a longer tusk. It's just a particular feature that is
exaggerated in the human species.

"We've proven that there is a big distinction. Human evolution is, in
fact, a privileged process because it involves a large number of
mutations in a large number of genes," Lahn said. "To accomplish so
much in so little evolutionary time - a few tens of millions of years
- requires a selective process that is perhaps categorically
different from the typical processes of acquiring new biological
traits."

Generally speaking, the higher up the evolutionary tree, the bigger and
more complex the brain becomes (after scaling to body size). But this
moderate trend became a huge leap during human evolution. The human
brain is exceptionally larger and more complex than the brains of
nonhuman primates, including man's closest relative, the chimpanzee.

One way to study evolution at the molecular level is to examine changes
of when and where proteins are expressed in the body. "But there are
many challenges to study the evolution of protein expression. Instead,
we chose to track structural changes in proteins," said graduate
student Eric Vallender, lead author of the article along with former
graduate student Steve Dorus, both of Lahn's laboratory.

Researchers examined the DNA of 214 genes involved in brain development
and function in four species: humans, macaques (an Old World monkey),
rats and mice. (Primates split from rodents about 80 million years ago;
humans split from macaques 20 million to 25 million years ago; and rats
split from mice 16 million to 23 million years ago.)

For each of these brain-related genes, they identified changes that
altered the structure of the resulting protein, as well as those that
did not affect protein structure. Only those genetic changes that alter
protein structure are likely to be subject to evolutionary selection,
Lahn said. Changes in the gene that do not alter the protein indicate
the overall mutation rate - the background of random mutations from
which evolutionary changes arise, known as the gene's molecular clock.
The ratio of the two types of changes gives a measure of the pressure
of natural selection driving the evolution of the gene.

Researchers found that brain-related genes evolved much faster in
humans and macaques than in rats and mice. Additionally, the human
lineage has a higher rate of protein changes than the macaque lineage.
Similarly, the human lineage has a higher rate than the chimpanzee
lineage.

"For brain-related genes, the amount of evolution in the lineage
leading to humans is far greater than the other species we have
examined," Lahn said. "This is based on an extensive set of genes."

They argue that a significant fraction of genes in the human genome
were impacted by this selective process. The researchers estimate there
may have been thousands of mutations in thousands of genes that
contributed to the evolution of the human brain. This "staggering"
number of mutations suggests that the human lineage was driven by
intense selection process.

To further investigate the role of selection on brain development, the
researchers compared the evolutionary rate of brain-related genes
against a control group of 95 genes, which are involved in basic
functions necessary for each cell in the body to survive.

"If there is something inherently different about humans in the
evolution of their genes - not related to selection - the control
genes should reveal it too. These basic, conserved genes are the last
to change," Vallender said.

The control genes looked the same. The researchers did not find an
excess of changes in these genes during human evolution. This provides
a sharp contrast to the tremendous excess of changes in the
brain-related genes.

The study also revealed two dozen "outliers" - those genes with the
fastest evolutionary rates in the human lineage. Of these, 17 are
involved in controlling brain size and behavior, arguing that genes
that affect brain size and behavior are preferential targets of
selection during human evolution. Lahn and his colleagues now are
focusing on these outlier genes, which may reveal more about how the
human brain became bigger and better.

For two of these outliers, ASPM and Microcephalin, previous work from
Lahn's group already has implicated them in the evolutionary
enlargement of the human brain. Loss-of-function mutations in either
ASPM or Microcephalin cause microcephaly in humans - a severe
reduction in the size of the cerebral cortex, the part of the brain
responsible for planning, abstract reasoning and other higher cognitive
function.

The researchers found that both the ASPM and Microcephalin genes showed
clear evidence of accelerated changes due to intensified evolutionary
pressure in the lineage leading to humans. For ASPM, the acceleration
is particularly prominent in recent human evolution after humans parted
way from chimpanzees. By contrast, the researchers' analyses of ASPM
and Microcephalin in the more primitive monkeys and in cows, sheep,
cats, dogs, mice and rats, showed no evidence of accelerated
evolutionary changes.

Lahn also is considering the wider impact of this research. "Are the
genes involved in the evolution of the human brain more likely to be
linked to diseases of the human brain? What happens when something goes
wrong in these genes? Does it create neurological and psychiatric
problems such as mental retardation or addiction? Could these genes
contribute to IQ differences in humans? Do people with a particular
mutation in one of these genes study better?"

According to Lahn, data from the Cell paper secures humans' privileged
position in the evolutionary tree. "Human brain evolution required a
major overhaul of the genetic blueprint -- perhaps much more so than
the evolution of other biological traits," he said.

But how did human ancestors encounter an environment where selection
for better brains suddenly became such a prominent force? Lahn suggests
that because humans have become a progressively more social species,
greater cognitive abilities have become more of an advantage.

"As humans become more social, differences in intelligence will
translate into much greater differences in fitness," he said, "because
you can manipulate your social structure to your advantage.

"Even devoid of the social context, as humans become more intelligent,
it might create a situation where being a little smarter matters a lot.


"The making of the large human brain is not just the neurological
equivalent of making a large antler. Rather, it required a level of
selection that's unprecedented," Lahn said. "Our study offers the first
genetic evidence that humans occupy a unique position in the tree of
life. Simply put, evolution has been working very hard to produce us
humans." 


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