December 29, 2004
Human Brain Evolution Was a 'Special Event'
Genes that control the size and complexity of the brain have
undergone much more rapid evolution in humans than in non-human
primates or other mammals, according to a new study by Howard Hughes
Medical Institute researchers.
The accelerated evolution of these genes in the human lineage was
apparently driven by strong selection. In the ancestors of humans,
having bigger and more complex brains appears to have carried a
particularly large advantage, much more so than for other mammals.
These traits allowed individuals with “better brains” to
leave behind more descendants. As a result, genetic mutations that
produced bigger and more complex brains spread in the population very
quickly. This led ultimately to a dramatic “speeding up” of
evolution in genes controlling brain size and complexity.
“People in many fields, including evolutionary biology, anthropology and sociology, have long debated whether the evolution of the human brain was a special event. I believe that our study settles this question by showing that it was.”
Bruce T. Lahn
“People in many fields, including evolutionary biology,
anthropology and sociology, have long debated whether the evolution of
the human brain was a special event,” said senior author Bruce
Lahn of the Howard Hughes Medical Institute at the University of
Chicago. “I believe that our study settles this question by
showing that it was.”
Lahn and his colleagues reported their data in a research article
published in the December 29, 2004, issue of the journal
Cell.
The researchers focused their study on 214 brain-related genes, that
is, genes involved in controlling brain development and function. They
examined how the DNA sequences of these genes changed over evolutionary
time in four species: humans, macaque monkeys, rats, and mice. Humans
and macaques shared a common ancestor 20-25 million years ago, whereas
rats and mice are separated by 16-23 million years of evolution. All
four species shared a common ancestor about 80 million years ago.
Humans have extraordinarily large and complex brains, even when
compared with macaques and other non-human primates. The human brain is
several times larger than that of the macaque — even after correcting
for body size — and “it is far more complicated in terms of
structure,” said Lahn.
For each gene, Lahn and his colleagues counted the number of changes
in the DNA sequence that altered the protein produced by the gene. They
then obtained the rate of evolution for that gene by scaling the number
of DNA changes to the amount of evolutionary time taken to make those
changes.
By this measure, brain-related genes evolved much faster in humans
and macaques than in mice and rats. In addition, the rate of evolution
has been far greater in the lineage leading to humans than in the
lineage leading to macaques.
This accelerated rate of evolution is consistent with the presence
of selective forces in the human lineage that strongly favored larger
and more complex brains. “The human lineage appears to have been
subjected to very different selective regimes compared to most other
lineages,” said Lahn. “Selection for greater intelligence
and hence larger and more complex brains is far more intense during
human evolution than during the evolution of other mammals.”
To further examine the role of selection in the evolution of
brain-related genes, Lahn and his colleagues divided these genes into
two groups. One group contained genes involved in the development of
the brain during embryonic, fetal and infancy stages. The other group
consisted of genes involved in “housekeeping” functions of
the brain necessary for neural cells to live and function. If
intensified selection indeed drove the dramatic changes in the size and
organization of the brain, the developmental genes would be expected to
change faster than the housekeeping genes during human evolution. Sure
enough, Lahn's group found that the developmental genes showed much
higher rates of change than the housekeeping genes.
In addition to uncovering the overall trend that brain-related genes
— particularly those involved in brain development — evolved
significantly faster in the human lineage, the study also uncovered two
dozen “outlier” genes that might have made important
contributions to the evolution of the human brain. These outlier genes
were identified by virtue of the fact that their rate of change is
especially accelerated in the human lineage, far more so than the other
genes examined in the study. Strikingly, most of these outlier genes
are involved in controlling either the overall size or the behavioral
output of the brain — aspects of the brain that have changed the most
during human evolution.
According to graduate student Eric Vallender, a coauthor of the
article, it is entirely possible by chance that that two or three of
these outlier genes might be involved in controlling brain size or
behavior. “But we see a lot more than a couple — more like 17
out of the two dozen outliers,” he said. Thus, according to Lahn,
genes controlling the overall size and behavioral output of the brain
are perhaps places of the genome where nature has done the most amount
of tinkering in the process of creating the powerful brain that humans
possess today.
There is “no question” that Lahn's group has uncovered
evidence of selection, said Ajit Varki of the University of California,
San Diego. Furthermore, by choosing to look at specific genes, Lahn and
his colleagues have demonstrated “that the candidate gene
approach is alive and well,” said Varki. “They have found
lots of interesting things.”
One of the study's major surprises is the relatively large number of
genes that have contributed to human brain evolution. “For a long
time, people have debated about the genetic underpinning of human brain
evolution,” said Lahn. “Is it a few mutations in a few
genes, a lot of mutations in a few genes, or a lot of mutations in a
lot of genes? The answer appears to be a lot of mutations in a lot of
genes. We've done a rough calculation that the evolution of the human
brain probably involves hundreds if not thousands of mutations in
perhaps hundreds or thousands of genes — and even that is a
conservative estimate.”
It is nothing short of spectacular that so many mutations in so many
genes were acquired during the mere 20-25 million years of time in the
evolutionary lineage leading to humans, according to Lahn. This means
that selection has worked “extra-hard” during human
evolution to create the powerful brain that exists in humans.
Varki points out that several major events in recent human evolution
may reflect the action of strong selective forces, including the
appearance of the genus Homo about 2 million years ago, a major
expansion of the brain beginning about a half million years ago, and
the appearance of anatomically modern humans about 150,000 years ago.
"It's clear that human evolution did not occur in one fell swoop," he
said, "which makes sense, given that the brain is such a complex
organ."
Lahn further speculated that the strong selection for better brains
may still be ongoing in the present-day human populations. Why the
human lineage experienced such intensified selection for better brains
but not other species is an open question. Lahn believes that answers
to this important question will come not just from the biological
sciences but from the social sciences as well. It is perhaps the
complex social structures and cultural behaviors unique in human
ancestors that fueled the rapid evolution of the brain.
“This paper is going to open up lots of discussion,”
Lahn said. “We have to start thinking about how social structures
and cultural behaviors in the lineage leading to humans differed from
that in other lineages, and how such differences have powered human
evolution in a unique manner. To me, that is the most exciting part of
this paper.”
|
