Brain and tooth size didn't co-evolve in humans

Washington: Contradicting a prevalent perception, a new study says that our brain enlargement and dental reduction did not happen in lockstep.

The findings suggest that evolution of brain and tooth size in humans were likely influenced by different ecological and behavioural factors.

"Once something becomes conventional wisdom, in no time at all it becomes dogma," said study co-author Bernard Wood, Professor at George Washington University, US.

"The co-evolution of brains and teeth was on a fast-track to dogma status, but we caught it in the nick of time," Wood noted.

This research challenges the common view that reduction of tooth size in hominins is linked with having a larger brain. 

The reasoning is that larger brains allowed hominins to start making stone tools and that the use of these tools reduced the need to have such large chewing teeth. 

But recent studies by other authors found that hominins had larger brains before chewing teeth became smaller, and they made and used stone tools when brains were still quite small, which challenges this relationship.

The new study -- published in the journal Proceedings of the National Academy of Sciences -- evaluated this issue by measuring and comparing the rates at which teeth and brains have evolved along the different branches of the human evolutionary tree.

"The findings of the study indicate that simple causal relationships between the evolution of brain size, tool use and tooth size are unlikely to hold true when considering the complex scenarios of hominin evolution and the extended time periods during which evolutionary change has occurred," lead author Aida Gomez-Robles from George Washington University noted.

For the study, the researchers analysed eight different hominin species. 

They identified fast-evolving species by comparing differences between groups with those obtained when simulating evolution at a constant rate across all lineages, and they found clear differences between tooth evolution and brain evolution.