Karin Isler, PD Dr.
The human brain is about three times as large as that of our closest relatives, the chimpanzees. To understand how this remarkable characteristic of our species evolved, I study brain size evolution in animals by comparing a broad array of mammal and birds species.
Having a large brain brings many obvious benefits, but it is also costly as brain tissue is energetically expensive. To evolve a larger brain than its ancestors, a species can either increase its total energy turnover, or allocate its energy budget differently. We found evidence for both pathways in mammals. However, rather than trading off brain size against the size of other expensive body organs such as the gut, there is a strong negative relationship between brain size and reproductive output. Although relatively large-brained species compensate their reduced annual fertility rate by a prolonged lifespan, their population growth rate under optimum conditions is still relatively low, resulting in an ultimate negative correlation between brain size and demographic viability of a species. Only if the mother receives help from conspecifics during breeding, the energy subsidies alleviate this constraint. We therefore conclude that the human lineage was only able to evolve an ever larger brain due to a change in lifestyle towards cooperative breeding.
At present, we are studying patterns of correlated evolution between environmental conditions such as seasonality or unpredictable periods of food shortage, adipose depots, and brain size.