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A UNIQUE twist in the development of the human brain may explain how we
evolved the capacity for complex abilities such as language and abstract
thought.

At some point in our evolution the brain areas that mediate these talents
began to expand. How did they do that? By sending an army of extra neurons along
a route uncharted in other mammals, say Kresimir Letinic and Pasko Rakic of Yale
University Medical School in Connecticut. This process allowed key brain regions
that normally develop separately to bulk up in synchrony.

This study is the first to show a difference in brain development between
humans and other mammals, says Yi Rao, a neurobiologist at Washington University
School of Medicine in St Louis. 鈥淭he general consensus has been that the vast
majority of developmental processes are conserved, from flies to humans,鈥� he
says.

Most uniquely human abilities arise in parts of the cortex. 鈥淭he cortex is
the integration place. Attributes like planning, language and high-order
thinking depend on it,鈥� says Katerina Semendeferi, a physical anthropologist at
the University of California at San Diego. But if the cortex expands, other
related areas need to grow too. One such area is the dorsal thalamus, a key
gateway for information destined for the cortex.

But these two areas of the brain develop independently. As the brain forms,
neurons migrate from specialised zones where they are produced to their final
destination. According to neuroscience dogma, there鈥檚 no cross
traffic鈥攏eurons in the thalamus start off in one zone, neurons for the
cortex in another. So how the dorsal thalamus and the cortex coordinated the
explosion in their growth is a puzzle.

Letinic and Rakic used a fluorescent dye to track migrating neurons in slices
of brain tissue from aborted mouse, macaque and human fetuses. They found that
the human cells were breaking the rules鈥攏eurons made in the ganglionic
eminence (GE), a zone that normally sends brain cells to the cortex, were
travelling to the thalamus too.

Rakic says this could explain how the different brain regions important for
language and cognition developed together. As the GE boosted its neuron
production to bulk up the cortex, some of the extra cells were diverted to the
dorsal thalamus.

A second set of experiments showed that the pathway doesn鈥檛 exist in mice.
The researchers found that neurons taken from the human GE migrated across a
culture dish towards a chunk of dorsal thalamus. But in mice, the dorsal
thalamus didn鈥檛 attract GE neurons. In fact, nearby structures actively repelled
them
(see Diagram).
Rakic says this simple difference means the brain expansion
in humans could have come from just a few鈥攐r even just one鈥攎utation
in the genes for the signalling molecules that attract or repel migrating
neurons.

But Semendeferi warns that it鈥檚 too early to say this migratory pathway is
the special preserve of humans. 鈥淭o suggest this is a uniquely human phenomenon,
you鈥檇 have to look at chimps,鈥� she says.