One of the big questions surrounding the research is whether or not the climate data has the kind of resolution that is necessary to support the hypothesis:
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).
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.It is an intriguing idea and bears more investigation but I suspect it may be one of many variables driving brain size expansion at the time.
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