Have we found the gene for 'humaness'?
http://www.eurekalert.org/pub_releases/2004-01/hhmi-gmb011204.php
---
Gene may be key to evolution of larger human brain
Howard Hughes Medical Institute researchers have identified a gene
that appears to have played a role in the expansion of the human
brain's cerebral cortex -- a hallmark of the evolution of humans from
other primates.
By comparing the gene's sequence in a range of primates, including
humans, as well as non-primate mammals, the scientists found evidence
that the pressure of natural selection accelerated changes in the
gene, particularly in the primate lineage leading to humans.
The researchers, led by Howard Hughes Medical Institute (HHMI)
investigator Bruce Lahn at the University of Chicago, reported their
findings in an advance access article published on January 13, 2004,
in the journal Human Molecular Genetics. Patrick Evans and Jeffrey
Anderson in Lahn's laboratory were joint lead authors of the article.
"People have studied the evolution of the brain for a long time, but
they have traditionally focused on the comparative anatomy and
physiology of brain evolution," said Lahn. "I would venture, however,
that there really hasn't been any convincing evidence until now of any
gene whose changes might have contributed to the evolution of the
brain."
In this study, the researchers focused on a gene called the Abnormal
Spindle-Like Microcephaly Associated (ASPM) gene. Loss of function of
the ASPM gene is linked to human microcephaly a severe reduction in
the size of the cerebral cortex, the part of the brain responsible for
planning, abstract reasoning and other higher brain function. The
discovery of this association by HHMI investigator Christopher A.
Walsh and colleagues at Beth Israel Deaconess Medical Center is what
prompted Lahn to launch an evolutionary study of the gene.
Lahn and his colleagues compared the sequence of the human ASPM gene
to that from six other primate species shown genetically to represent
key positions in the evolutionary hierarchy leading to Homo sapiens.
Those species were chimpanzee, gorilla, orangutan, gibbon, macaque and
owl monkey.
"We chose these species because they were progressively more closely
related to humans," said Lahn. "For example, the closest relatives to
humans are chimpanzees, the next closest are gorillas, and the rest go
down the ladder to the most primitive."
For each species, the researchers identified changes in the ASPM gene
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
pressure, 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. Thus, the ratio of
the two types of changes gives a measure of the evolution of the gene
under the pressure of natural selection.
Lahn and his colleagues found that the ASPM gene showed clear evidence
of changes accelerated by evolutionary pressure in the lineage leading
to humans, and the acceleration is most prominent in recent human
evolution after humans parted way from chimpanzees.
"In our work, we have looked at evolution of a large number of genes,
and in the vast number of cases, we see only weak signatures of
adaptive changes," said Lahn. "So, I was quite surprised to see that
this one gene shows such strong and unambiguous signatures of adaptive
evolution -- more so than most other genes we've studied."
By contrast, the researchers' analyses of the ASPM gene in the more
primitive monkeys and in cows, sheep, cats, dogs, mice and rats,
showed no accelerated evolutionary change. "The fact that we see this
accelerated evolution of ASPM specifically in the primate lineage
leading to humans, and not in these other mammals, makes a good case
that the human lineage is special," said Lahn.
According to Lahn, among the next steps in his research will be to
understand how ASPM functions in the brain. Studies by Walsh and
others hint that the protein produced by the gene might regulate the
number of neurons produced by cell division in the cerebral cortex.
Lahn and his colleagues plan functional comparisons of the ASPM
protein among different species, to understand how this gene's
function or regulation changes with evolution.
---
--
John Hachmann, aa #1782
- Question authority. Now more than ever. -
.
|
