Why did our brains shrink?

Remember these are hypotheses, which means no one is saying they are 100% correct, just that these ideas answer some of the patterns we observe. We're also not sure if some areas of the our brains have shrunk or if it has declined overall with no particular emphasis on a particular region. Thanks for watching you tiny brained apes!

It seems most likely that human minds may be more efficient than they were. Since brains are a huge resource burden, if humans can remain intelligent while shrinking their brains, that's an evolutionary advantage. Our brains may simply work better, once for ounce, than they once did.

Crows have a higher neurodensity than many other birds, that has enabled them to make simple tools, recognize people, and inform other crows of people who were good or bad to them. Perhaps humans have increased their neurodensity over time. It is difficult to know as we have no ancient samples of brain tissue to compare to that of modern humans.

Considering that humans have very complicated births due to our large heads, it could also be that mothers of babies with smaller heads were more likely to survive, and thus smaller heads meant a higher survival rate over time.

My guess is we lost our improved hearing, sight and smell because they are no longer as neccesary in an agricultural society. The "If you dont use it you lose it" doctrine.

4th Theory proposed by Dr. Temple Grandin is that when humans and wolves came together both brains got smaller. For wolves, humans started doing much of the thinking work- finding the game and coordinating the hunt, deciding who breeds.... and their cerebrum got smaller; for humans the amygdala and brain regions associated with sensing the outer world got smaller as dogs were much better able to provide the vigilance needed for survival- sensing approaching threats and finding game with their superior abilities to hear and smell. Dogs provided an emotional comfort allowing our brains to relax and contemplate things not pertaining to our immediate survival and help us sleep longer and deeper without worry of attack while we're unconscious. The human and dog together become a super organism! as both the canine and primate brain is able to become more specialized at what they had a natural proclivity to. I guess this goes along with the self domestication theory, but it's also hypothesized that humans learned a lot about social cohesion from their early wolf companions who are social in ways uniquely different from primates.

Something occurred to me the third time I watched this, is it possible that there could be a mix of actual brain size reduction as well as a reduction in CSF volume? Maybe head trauma was common enough throughout our ancestry that having lots of CSF was selected for because it lessened the damage. As we evolved into safer lives it became less necessary and we began losing the extra fluid as a result. I think we still would’ve lost actual brain matter by virtue of the observed body mass reduction and evident self domestication, but I think neglecting the amount of fluid between the brain and skull during the evolutionary window in question is an oversight.

One thing we can't measure is the density of the past brains. I do feel like the efficiency is probably very likely . Tightly compact neuron dense. At the same time the female pelvis is getting narrower... Definitely can see all the reasons why

I think the most likely scenario is our development of written language, which freed us from having to memorize everything.

I spit out my drink when you said "greatest cultural achievements" and cut to the McDonald's sign, thats hilarious

But does smaller brains mean less skill or less intelligence? I've heard once, that the size of a brain isn't as important as it's complexity. Like folds in the gray matter or shrinking parts of the brain that are more primitive and not necessary anymore.

“Or maybe you do, I’m not here to kink shame anyone” LOL that was fantastic!

It’s really interesting reading everyone’s opinions and ideas on this. My first two thoughts were: increased density, and a change in diet. Becoming less reliant on meat.

So basically what you're saying is, humans have gotten as close as we can get to a hive mind without actually being telepathic. We've become so focused on specializing, so good at dividing tasks between us, that we are basically sharing between all of us the brain capacity needed to keep our society running.

I'm an academic linguist. When I first studied linguistics, decades ago, we were taught that language was likely to be around 60,000 years old. I strongly suspected it was much much older than that because of its phenomenal complexity. Later textbooks suggested it was up to 100,000 years old. Nope, I though, keep going. Some of the latest research I have seen suggests that language evolved with making tools, and that it was necessary to pass down some of the skills demanded for tool-making to future generations, and the new hypothesis is that language development (i.e. linguistic signs working in tandem with other linguistic signs to create a grammar - and thus complex meaning - further advanced with simple metaphors enabling the handling of abstract concepts both cognitively and for communication) had its most rapid development with homo erectus, probably beginning around 1.5 million years ago. That, to me, seems far more realistic.

Maybe we have lost cognitive ability in the last 3 thousand years but the collective knowledge we have is so great that we aren't affected by the decrease in cognitive ability in the way that we would expect.

Our ancestors needed to know and to do so much more than we do need nowadays in our consumer society.

Personally, I think the 'shrinking' of our brains is essentially how many times our grey and white matter FOLDED instead. I remember reading that completely unfolded, our brains are literally the size of a pillow, which wouldn't really fit inside our craniums! Plus, it takes us as much energy for cognitive thought, to turn on a lightbulb, than the energy is taken for a super computer to solve the same problem! I think our bodies changed to become better efficient.

There is an interesting article by Bednarik (2014, Doing with less: Hominin brain atrophy. Journal of Comparative Human Biology), which has a different time table of the shrinking process (starting earlier than 3000 years ago). But it makes an important point regarding the third explanation - why do we see this process even among hunter-gatherer groups, who do not have to respond to the "pressure" of living in a large society. It is a genuine mystery. Great video, as usual - thanks!

This video is amazing!!! What a great storyteller this guy is!