January/February 2019^ DISCOVER^41
ecological challenges (inding food,
avoiding predators) and cultural
challenges (being able to share
acquired knowledge and skills).
Most researchers believed that
social challenges drove greater
intelligence, but the models were
largely untestable.
In Nature in May, González-
Forero and his colleague Andy
Gardner debuted a computational
tool to understand what drove
human brains to get bigger. Their
approach, says González-Forero, is
more complicated than others but
also grounded in actual number
crunching. The goal is to look not
just at why our brains got bigger, but
why they got as big as they did.
“It’s based on empirical
evidence, with actual metabolic
costs, so it’s also more testable,”
says González -Forero.
The pair looked at the energy
costs of different kinds of tissues,
as well as body size. Growth of
any one component requires
increased energy, but it’s a trade-
off: Boosting brain size also boosts
skill, but a larger, energy-hungry
brain reduces the energy available
for the rest of the body, such as the
reproductive tissues.
The new model generated
multiple scenarios, each with
a different combination of
challenges, that could explain
various brain-to-body-size ratios.
González-Forero expected that
the need for social intelligence
— the leading hypothesis —
would come out on top, but the
opposite was true. What he calls
the “arms race” of ever-brainier
individuals competing with eachother backired.
“It moves you to exaggerated
brain size, but they get too expensive
[metabolically],” he says.
Instead, the study found the most
likely scenario to explain our brain-
to-body size ratio was 60 percent
ecological challenges, 30 percent
collaboration and 10 percent
between-group competition.
Social intelligence theorists
remain skeptical of the results, but
González-Forero notes: “We’re not
wedded to any particular hypothesis.
We’re just offering a tool.”
And he’s already planning to
reine that tool. The initial model
did not take into account the
cultural intelligence hypothesis,
which he hopes to incorporate.In a distant ancestor 8 million to
14 million years ago, a copying error
resulted in an “extra hunk of DNA.”