New Scientist: Did an ice age boost human brain size?
rmacfarl
Did an ice age boost human brain size?
IT IS one of the biggest mysteries in human evolution. Why did we
humans evolve such big brains, making us the unrivalled rulers of the
world?
Some 2.5 million years ago, our ancestors' brains expanded from a mere
600 cubic centimetres to about a litre. Two new studies suggest it is
no fluke that this brain boom coincided with the onset of an ice age.
Cooler heads, it seems, allowed ancient human brains to let off steam
and grow.
Cooler heads, it seems, allowed ancient human brains to let off steam
and grow
For all its advantages, the modern human brain is a huge energy
glutton, accounting for nearly half of our resting metabolic rate.
About a decade ago, biologists David Schwartzman and George Middendorf
of Howard University in Washington DC hypothesised that our modern
brain could not have evolved until the Quaternary ice age started,
about 2.5 million years ago. They reckoned such a large brain would
have generated heat faster than it could dissipate it in the warmer
climate of earlier times, but they lacked evidence to back their
hypothesis.
Now hints of that evidence are beginning to emerge. Climate researcher
Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena,
Germany, modelled present-day temperature, humidity and wind
conditions around the world using an Earth-systems computer model. He
used these factors to predict the maximum rate at which a modern human
brain can lose heat in different regions. He found that, even today,
the ability to dissipate heat should restrict the activity of people
in many tropical regions (Climatic Change, vol 95, p 405).
If keeping cool is a problem now, Kleidon says, it would have been
even more challenging - perhaps too challenging - 2 or 3 million years
ago when temperatures were a few degrees warmer than today and air-
conditioning units were harder to come by.
A new study by Schwartzman and Middendorf suggests that a small drop
in global temperatures may have made a big difference. The pair used
basic equations of heat loss to estimate how fast the small-brained
Homo habilis would have been able to cool off. Assuming overheating
limited the size of H. habilis's brain, they then calculated what drop
in air temperature would have been needed for Homo erectus to be able
to support its bigger brain (see diagram). They found that a drop in
air temperature of just 1.5 °C would have done the trick (Climatic
Change, vol 95, p 439).
Given the timescales involved, it may be near-impossible to match
definitively the onset of an ice age with speciation, but a 1.5 °C
drop is consistent with the cooling climate of the time, says
Middendorf.
"In principle, I'm receptive to the hypothesis," says Dean Falk, a
palaeoanthropologist at Florida State University in Tallahassee, "but
I need the data." She says that if measurements showed that people
living in tropical countries today have smaller brains relative to
their body size than people in temperate climates, this would go
against expectation and lend support to Kleidon's model.
Being able to cool bigger brains can only be part of the story,
however. It would have lifted the brakes on expansion, says
psychologist David Geary at the University of Missouri in Columbia,
but there has to be something driving the increase.
Over the years, researchers have come up with three broad reasons why
bigger brains might have been advantageous: to give their owners the
ability to cope with changing climates by exploiting technologies such
as shelter, fire and clothing; to deal with the cognitive demands of
hunting and gathering; or to help people outsmart their neighbours.
To help narrow this down, Geary collected data from 175 fossil hominin
skulls, from 1.9 million to 10,000 years old. Then he looked to see
whether brain size was best correlated with climatic variability - a
crude measure of biodiversity which could indicate the complexity of
hunting and gathering - or the human population size at the time,
which could reflect the complexity of social interactions.
Geary's analysis found that population size was the best predictor of
brain size, suggesting that our ancestors' need to outcompete their
neighbours in order to survive may have been the strongest driver of
brain growth (Human Nature, vol 20, p 67).
The case is far from closed - Geary's study does not demonstrate cause
and effect, for one thing - but the picture beginning to emerge
suggests that an ice age set the stage for a socially driven brain
boom. And from that time on, it was the brainiacs who stole the show.
Greenhouse brains
If global cooling allowed humans to evolve their big brains, will
today's global warming take them away again? "I'd hate to think that a
difference of 1.5 °C might mean the end of humans because our brains
cook," says George Middendorf of Howard University in Washington DC,
"but I guess it's a scenario that might play out."
It probably won't, though, thanks to what those big human brains made
possible: culture.
"When culture comes in, it layers itself on top of the biological
constraints," says Tyler Volk, an Earth-systems expert at New York
University. Thanks to culture and technology, we now have ways of
buffering ourselves against hot climates, not only with air
conditioning, but also with basic tools such as fans, thick-walled
buildings and reservoirs to ensure we have plenty of water.
Only one thing could destroy that buffer - a total breakdown of
society.
Marc Verhaegen
IMO they might be right that the Ice Ages boosted human brain size.
But, of course, not for the reasons they think.
The solution is much simpler: the diaspora of Homo to other continents along coasts & inland along lakes & rivers.
When Pleistocene sea levels dropped, the durophagous & tool-using Homo were the first on the vaste new territories to exploit the abundant shellfish that contained lots of DHA & other brain-specific nutrients that facilitated brain growth.
copy & paste http://users.ugent.be/%7Emvaneech/Verhaegen%20et%20al.%202007.%20Econiche%20of%20Homo.pdf
> http://www.newscientist.com/article/mg20327194.000-ice-age-precipitated-the-hu
Lee Olsen
> The authors seem to think that African people have very small brains?!
>
> IMO they might be right that the Ice Ages boosted human brain size.
> But, of course, not for the reasons they think.
>
> The solution is much simpler: the diaspora of Homo to other continents along
> coasts & inland along lakes & rivers.
Homo had already doubled the size of their brains before they left
Africa, so any
imaginary diaspora postulated by Verhaegin had nothing what-so-ever to
do
with the increase in brain size.