SPEEDY EXPANSION OF HUMAN, APE CEREBELLUM
A new study published
in the Cell Press journal Current Biology on October 2 could rewrite the story
of ape and human brain evolution. While the neocortex of the brain has been
called "the crowning achievement of evolution and the biological substrate
of human mental prowess," newly reported evolutionary rate comparisons
show that the cerebellum expanded up to six times faster than anticipated
throughout the evolution of apes, including humans.
The
findings suggest that technical intelligence was likely at least as important
as social intelligence in human cognitive evolution, the researchers say.
"Our
results highlight a previously unappreciated role of the cerebellum in ape and
human brain evolution that has the potential to refocus researchers' thinking
about how and why the brains in these species have become distinct and to shift
attention away from an almost exclusive focus on the neocortex as the seat of
our humanity," says Robert Barton of Durham University in the United
Kingdom.
The
cerebellum had been seen primarily as a brain region involved in movement
control, adds Chris Venditti of the University of Reading. But more recent
evidence has begun to suggest that the cerebellum has a broader range of
functions. The cerebellum also contains an intriguingly large number of densely
packed neurons.
"In
humans, the cerebellum contains about 70 billion neurons -- four times more
than in the neocortex," Barton says. "Nobody really knows what all these
neurons are for, but they must be doing something important."
The
neocortex had gotten most of the attention in part because it is such a large
structure to begin with. As a result, in looking at variation in the size of
various brain regions, the neocortex appeared to show the most expansion. But
much of that increase in size could be explained away by the size of the animal
as a whole. Sperm whales have a neocortex that is proportionally larger than
that of humans, for example.
By
using a comparative method that controlled for those differences in the way the
two brain structures correlate, Barton and Venditti uncovered a striking
pattern: both nonhuman apes and humans depart from the otherwise tight
correlation in size between the cerebellum and neocortex found across other
primates due to relatively rapid evolutionary expansion of the cerebellum.
Barton
and Venditti say that the cerebellum seems to be particularly involved in the
temporal organization of complex behavioral sequences, such as those involved
in making and using tools, for instance. Interestingly, evidence is now
emerging for a critical role of the cerebellum in language, too.
While
plenty of work remains, the new study establishes the cerebellum as "a new
frontier for investigations into the neural basis of advanced cognitive
abilities," the researchers say.
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