Recall that domestication of our key grains and other foods involved
the successful application of polyploidy which entailed simply
doubling up the chromosome content of the cell. So though this is
clearly not the same, it is still involved expansion and supported
domestication.
This does finally provide us with the pathway that led so quickly to
the modern human brain. Once that was in place, the rest of the task
evolved around actually supporting such a valuable tool and that also
took evolutionary time.
It is also worth noting that the changes were unusual and the
structural effects surprising. What more do we not know and what
other changes could be safely contemplated once we understand all out
potentialities?
We are also learning a lot more about the nature of 'mutation' that
we can apply elsewheere.
Extra gene drove
instant leap in human brain evolution
by Staff Writers
London, UK (SPX) May 14, 2012
The
researchers don't think SRGAP2 is solely responsible for that brain
expansion, but the genetic interference does have potential benefits.
A
partial, duplicate copy of a gene appears to be responsible for the
critical features of the human brain that distinguish us from our
closest primate kin. The momentous gene duplication event occurred
about two or three million years ago, at a critical transition in
the evolution of the human lineage, according to a pair of studies
published early online in the journal Cell, a Cell Press publication,
on May 3rd.
The
studies are the first to explore the evolutionary history and
function of any uniquely human gene duplicate. These "extra"
genes are of special interest as they provide likely sources of raw
material for adaptive evolutionary change. Until now, studying them
has been a technical challenge because they are nearly
indistinguishable from each other.
"There
are approximately 30 genes that were selectively duplicated in
humans," said Franck Polleux, an expert in brain development at
The Scripps Research Institute. "These are some of our most
recent genomic innovations."
Intriguingly, many of
these genes appear to play some role in the developing brain. Polleux
and Evan Eichler, a genome scientist at the University of Washington,
focused their expertise and attention on one of the genes known as
SRGAP2. This gene has, in fact, been duplicated at least twice during
the course of human evolution, first about 3.5 million years ago and
then again about 2.5 million years ago.
The
new work shows that the second and relatively recent duplication
event produced only a partial copy of the gene. This copy acts at
exactly the same time and place as the original, allowing it to
interact with and block the ancestral gene's function.
"This innovation
couldn't have happened without that incomplete duplication,"
Eichler said. "Our data suggest a mechanism where incomplete
duplication of this gene created a novel function 'at birth'."
Interestingly, the
novel gene appears to have arisen just as the fossil record shows a
transition from human's extinct Australopithecus ancestors to the
genus Homo (as in Homo sapiens), which led to modern humans. That's
also when the brains of our ancestors began to expand and when
dramatic changes in cognitive abilities are likely to have emerged.
The researchers don't
think SRGAP2 is solely responsible for that brain expansion, but the
genetic interference does have potential benefits. Polleux and
colleagues mimic the function of the human-specific SRGAP2
duplication in mice. They show that loss of SRGAP2 function
accelerates neurons' migration in the developing brain, potentially
helping them reach their final destination more efficiently.
Moreover, neurons that
have decreased SRGAP2 function, due to expression of the
human-specific SRGAP2 display more knob-like extensions or spines on
their surfaces, making the neurons appear much more like those found
in the human brain. These spines enable connections between neurons
to form.
In addition to
providing insight into the origins of the modern human brain, the
findings offer clues to the neurodevelopmental disorders that humans
are so prone to developing, including autism, epilepsy and
schizophrenia, in which development of neuronal connections is
affected. The researchers point to known cases of humans with
structural brain defects and other symptoms that can be traced to
disruption of the ancestral SRGAP2. They now intend to search for
people carrying defects in the human-specific 'granddaughter' copy as
well.
If this gene
duplication did indeed produce an immediate effect during evolution
as Eichler and Polleux suspect, they expect there must have been a
fascinating period in human history characterized by "huge
variation" in human cognition and behavior.
SRGAP2
and other human-specific gene duplicates might also help to explain
the big differences between humans and other primates, despite few
apparent differences in our genome sequences.
"We may have been
looking at the wrong types of mutations to explain human and great
ape differences," Eichler says. "These episodic and
large duplication events could have allowed for radical - potentially
earth-shattering - changes in brain development and brain function."
Charrier et al.:
"Inhibition of SRGAP2 function by its human-specific paralogs
induces neoteny during spine maturation." and Dennis et al.:
"Human-specific evolution of novel SRGAP2 genes by incomplete
segmental duplication."
Related Links
No comments:
Post a Comment