Science-- there's something for everyone

Thursday, May 17, 2012

How were humans able to expand across the globe? Meat!

It was meat-eating that allowed us to advance across the Earth. This is the theory proposed by Elia Psouni from Lund and Kristianstad Universities, Axel Janke from Goethe University and Martin Garwicz from the Neuronano Research Center. According to them, increased meat consumption was the first domino that subsequently led to earlier weaning, increased fertility, more offspring, and finally to world domination. Okay, it’s probably not that simple, but the researchers did find a correlation between meat consumption and time of weaning.

Here’s a comparison with our closest relative, the chimpanzee. About 5% of a chimp’s diet is meat and they wean their babies at around five years old. In contrast, about 20 to 50% of the diet of a modern human hunter-gatherer’s diet is meat and they wean their children at just over two years. Clearly, humans are weaning at a much earlier age. But is this a function of diet or of some other factor?

The scientists compiled information on 67 species of mammal across 12 different orders. Data points included adult female body mass, adult brain mass, time to weaning postnatal and time to weaning post conception. The last attribute represents the total physical investment of the mother. They compared this with data from forty-six human ‘natural fertility societies’ that presumably mimic conditions of the pre-agricultural age.

Regardless of adult size, all species wean their young when the offspring achieve a critical level of brain development. Because of their more nutritious diet, carnivores as a group were able to wean their offspring earlier than herbivores or omnivores. When adjusting for factors such as brain mass, humans wean their children at the same age as other carnivores, rather than when their vegetarian primate cousins wean.

Needless to say, diet is most likely only one of many factors that led to the human migration across the globe. However, weaning does result in a return to fertility for female mammals, so females that wean earlier could have had more babies.  It's therefore possible that the shift to meat-eating would have had a profound effect on the number of hominins, and thus on their ability to spread outward.