For Father’s Day, here’s a write-up of some research I heard about at the Royal Society conference me and my dad went to earlier this week (I highly recommend them, they’re free and fantastic) – Happy Fathers’ Day, Dad!

Australopithecus afarensis and Homo erectus skulls - an increase in brain size of 80% (images from NSCCM via flickr cc)
Australopithecus afarensis and Homo erectus skulls – an increase in brain size of 80% (images from NCSSM under CC)

We humans know that we’re weird – there are no other bald, bipedal apes with disproportionally large brains. So why did we evolve to be the way we are? What pressures were acting on our ancestors?

Humans evolved in Africa. Most textbooks about our evolution have linked it to climate changes in the continent over the last ten million years. The traditional story (I’m paraphrasing here) is that the continent became drier and hotter, and the jungle gave way to savannah – at which point our ancestors learnt to walk upright, grew brainier, lost most of their hair, picked up some tools and went off to hunt some of the large plains-dwelling mammals that had appeared.

It turns out I’m behind the times and it may be a bit more complicated than that… Researchers have known for years that this warming of the African continent has not been a smooth, constant process. Newer research by Mark Maslin and others looks more closely at the local climate and tectonic changes along the Rift Valley in East Africa – right where we find most of the ancestral human fossils.

East Africa has changed a lot over the last ten million years. Originally flat and covered in a vast and unchanging tropical forest in which ancient early hominins such as Orrorin flourished, it’s now a generally hotter area with many different habitats. As the tectonic plates of the Rift Valley region moved, they created mountains and valleys, deserts and high-altitude forests. And the famous Rift Valley lakes.

Two environments in modern East Africa - Lake Nakuru and the Turkana basin (photos from Matthias Kihr and Desc/Em via flickr CC)
Two environments in modern East Africa – Lake Nakuru and the Turkana basin (photos from Matthias Kihr and Desc/Em via flickr CC)

Because of the local mountains and valleys, rainwater is funneled into these lakes – so they’re a sensitive indicator of local climate change. An increase in rainwater will lead to an expansion of the lakes (and the forests that surround them), and a dry spell causes them to shrink. Shultz and Maslin looked at geological evidence to assess the extent of the lakes at different points over the last five million years.

They found that the lakes grew and shrank at several points – 2.6, 1.9 and 1 million years ago – that match up with important periods of human evolution. When the lakes were at their largest, at around 1.9 million years ago, the fossil record shows many new hominin species appearing. It also corresponds with the biggest increase in hominin brain size, with the emergence of the Homo erectus species – a brain 40% larger than the earlier Homo habilis, and 80% larger than Australopithecus (the famous “Lucy“).

Periods of lake expansion correspond to increases in hominin brain size. From Shultz & Maslin 2013.
Periods of lake expansion correspond to increases in hominin brain size and the diversity of species. From Shultz & Maslin 2013.

Maslin, speaking at the Royal Society conference on Monday, said that we still don’t know exactly what it is about these periods of lake expansion that caused the evolution of new species and the pressure or opportunity to evolve large brains. Perhaps the high stress of alternating wet and dry conditions led to adaptable, intelligent species that could survive in different environments. Or maybe the large lakes and lush forests made it easier to find high-energy foods, fuelling large brains that were useful for other reasons.

This is only one group’s work , and there are plenty of other interesting human evolution/climate studies, including this one that looks at changes in Earth’s orbit. And aside from climate change, brain development was probably influenced by other interesting factors such as sexual selection and the importance of social intelligence in groups of early humans.

So did climate change make us smarter? It seems that the rapidly changing climate where our ancestors lived had a big influence on making my dad extremely brainy (not to mention bald) compared to an Australopithecus.

 

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