Paranthropus’ diet and their brain size

I’ve received an awful lot of email over the past few days and it’s making me feel good. I encourage you all to contact me if you want, so I can continue to bask in the warm feeling that comes as a result of being liked by the strangers on the internet.

One of these emails posed a question and I thought I’d take time out from my busy summer holidays schedule of video game playing to answer it. Vanessa asked

I’m trying to associate Paranthropus robustus new found diet of C4 plants to an expanding brain.. However I’m having a few issues finding any information on this point of view… I was wondering if you had any advice or journals that could lead me in the right direction. Any information you’re able to provide on this matter would be greatly appreciated.

Paranthropus and Australopithecus split at some point between 2.5 and 3.5 million years ago. Both lineages then continued on their own evolutionary paths. Australopithecus would travel through Lucy and similar species, ultimately arriving at us whilst Paranthropus took a very different route. They developed huge jaws; which earned them the nickname “nutcracker man” when they were first found in the 1950s and 60s.

Paranthropus (left), Australopithecus (centre) and early Homo (right)

Paranthropus (left), Australopithecus (centre) and early Homo (right)

However, as Vanessa points out they didn’t crack nuts. New isotope studies of the fossils reveals their diet actually consisted more of grasses and/or legumes (peas, beans and such). Their large jaws appear to have been so they could chew for hours and hours, which is what they’d need to do if they wanted to survive off low calorie food like grass. It would seem they were more like a human cow than nutcracker man.

Now on the human side of things, diet appears to have played an important role in allowing our brains to grow big. As we began to each meat and cook food we gained access to more calories. This meant that those with big, energetically expensive brains could survive. It opened up a new evolutionary door for us and we jumped right in.

So what of Paranthropus and their grass? Did that influence their brain size? It’s hard to say. What we do know is that their brains never increased in size like ours. They flourished for over a million years, splitting into a range of different species; yet their brains never became larger than 500cc. Meanwhile on the human branch, our brain was more than doubling in size from 400  to 1000cc.

Was the lack of brain growth in Paranthropus a result of their diet? It could be. Our large brains require 20% of our daily caloric intake, and an ape living off low calorie food might not have that energy to spare. On the other hand it might be that there was no need for them to develop large brains. After all, munching on grass isn’t known for its massive intellectual demands.

This mystery could be resolved if someone were to calculate how many calories Paranthropus could conceivably have obtained from their diet, and whether this would be enough to sustain a larger brain. If it could, then the fact it remains small suggests it was because there was no evolutionary drive for it to become bigger.

Unfortunately nobody has done such a calculation, and I lack the knowledge on 2 million year old African grasses to carry out the work myself. If a grass expert does happen to be reading this then please get in touch. Until then all we can say for sure is that Paranthropus never developed a large brain. This may be a result of their diet.

Or it might not.

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