40 DISCOVERMAGAZINE.COM
SKULL IMAGES: FIDDES ET AL./CELL. CHART: DAN BISHOP/DISCOVER AFTER FIDDES ET AL.HUMAN ORIGINS
Researchers gain insights into human brain evolution and ind evidence
of our ancestors in new places. BY GEMMA TARLACH
How to
Build a Brain
The human brain is a big deal. In
relation to body size, it’s nearly
six times bigger than that of other
placental mammals. Our brains’ size
and organizational complexity are
hallmarks of being human.
But what were the genetic
changes behind our big, complex
brains, and what drove their
evolution? Key new evidence, and a
new model, move us closer to inally
iguring it out.
In May, a pair of papers
published by separate teams in
the journal Cell focused on the
NOTCH family of genes, found
in all animals and critical to an
embryo’s development: They
produce the proteins that tell
stem cells what to turn into, such
as neurons in the brain. The
researchers looked at relatives of
the NOTCH2 gene that are present
today only in humans.
In a distant ancestor 8 million
to 14 million years ago, they
found, a copying error resulted
in an “extra hunk of DNA,” says
David Haussler of the University
of California, Santa Cruz, a senior
author of one of the new studies.
This non-functioning extra piece
of NOTCH2 code is still present
in chimps and gorillas, but not
in orangutans, which went off
on their own evolutionary path
14 million years ago.
About 3 million to 4 million years
ago, a few million years after our
own lineage split from other apes, a
second mutation activated the once
non-functional code. This human-speciic gene, called NOTCH2NL,
began producing proteins involved
in turning neural stem cells into
cortical neurons. NOTCH2NL
pumped up the number of
neurons in the neocortex, the seat
of advanced cognitive function.
Over time, this led to bigger, more
powerful brains.
“This is the irst human-speciic
gene in a fundamental pathway
known to control brain size,”
Haussler said.
The researchers also found
NOTCH2NL in the ancient
genomes of our closest evolutionary
kin: the Denisovans and the
Neanderthals, who had brain
volumes similar to our own.Haussler and colleagues are
now hunting the speciic molecular
mechanisms at work.
NOTCH2NL helped make
our ancestors’ brains bigger, but
brain tissue is expensive stuff,
metabolically speaking. “The
brain is 4 percent the weight of
the body but consumes 20 percent
of your energy,” says Mauricio
González-Forero, a mathematical
evolutionary biologist at Scotland’s
University of St. Andrews. “And
you cannot shut it off.”
Something must have driven
the evolution of bigger, costlier
brains. Three contenders have
emerged: social challenges (both
cooperative and competitive),