Why did our brains shrink 3000 years ago?

You have a big brain. I have a big brain. We, as a species, have pretty big brains. But this wasn't always the case. Way back when, our brains were much smaller; then they went through a bit of growth spurt, one that lasted for a couple million years. This steady ballooning of brain size is one of the key themes of the human story. But then there's a late-breaking twist in that story—a kind of unexpected epilogue. You see, after our brains grew, they shrank. But when this shrinkage happened and—of course, why—have remained mysterious. My guest today is Jeremy DeSilva, a paleoanthropologist at Dartmouth College. He's an expert on the evolution of the foot and ankle. But, it turns out the body is all connected, so he also thinks about brains and heads. In a recent paper, Jerry and his colleagues took up the mystery of human brain shrinkage. They first set out to establish more precisely when in our past this occurred. Using a large database of crania, spanning few million years, Jerry's team was able to establish that this shrinkage event happened much more recently than previously thought—a mere 3000 years ago. Naturally, the next question was why? What happened around that time that could have possibly caused our brains to deflate? To answer this, Jerry and his collaborators turned to an unexpected source of insight: Ants. That's right, ants. They argue that these ultrasocial critters may offer clues to why we might have suddenly dispensed with a chunk of brain about the size of a lemon. This is a really juicy paper and a super fun conversation, so we should just get to it. But I did want to mention: Jerry has a recent book from 2021 called First Steps that I whole-heartedly recommend. It's about origins of upright walking in humans—which it turns out, is bound up with all kinds of other important aspects of being human. So definitely check that out!

Thanks folks—on to my chat with Dr. Jerry DeSilva. Enjoy!

The paper we discuss is available here. A transcript of this episode is available here.

Notes and links

3:00 ­– A podcast episode from the Leakey Foundation about the so-called "obstetrical dilemma."

5:40 – A refresher for those who have trouble keeping their 'cenes' straight: the Pleistocene refers to the period from 2.58 million years ago to 11,700 years ago; immediately after that came the Holocene, which we are still in today.

7:00 – An article discussing the issue of unethical collections of human remains.

10:30 – The key figure form Dr. DeSilva's paper—showing the changing "slopes" of brain size over time—is available here.

19:30 – The original article by Leslie Aiello and Peter Wheeler on the "expensive tissue hypothesis." A more recent popular article on the hypothesis.

20:45 – An article by a major proponent of the social intelligence hypothesis, Dr. Robin Dunbar. A more critical review of the social intelligence hypothesis.

23:00 – A recent paper by Jeff Stibel and an older preprint by John Hawks evaluating the "body size" explanation of recent brain shrinkage.

24:00 – See our earlier episode on human self-domestication with Brian Hare.

29:00 – One of Dr. DeSilva's collaborators on this research is Dr. James Traniello, who specializes in ants.

34:45 – An overview of the earliest history of writing.

37:20 – Dr. DeSilva's book, First Steps, came out in 2021.

39:00 – A recent paper discussing the evolution of rotational birth in humans.

Dr. DeSilva recommends:

Kindred, by Rebecca Wragg Sykes (featured in an earlier episode!)

Origin, by Jennifer Raff

You can find Dr. DeSilva on Twitter.

Many Minds is a project of the Diverse Intelligences Summer Institute (DISI) (https://disi.org), which is made possible by a generous grant from the Templeton World Charity Foundation to UCLA. It is hosted and produced by Kensy Cooperrider, with help from assistant producer Cecilia Padilla. Creative support is provided by DISI Directors Erica Cartmill and Jacob Foster. Our artwork is by Ben Oldroyd (https://www.mayhilldesigns.co.uk/). Our transcripts are created by Sarah Dopierala (https://sarahdopierala.wordpress.com/).

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