Why are our brains shrinking?

Cutting-edge (heh) technology 70,000 years ago was the stone blade as well as the earliest known primitive art.

An interesting guide whenever you're trying to put this kind of thing into context: https://en.wikipedia.org/wiki/Timeline_of_prehistory

> The reduced amount of white matter suggests that domestic rabbits have a compromised information processing possibly explaining why they are more slow reacting and phlegmatic than their wild counterparts.

1: https://phys.org/news/2018-06-differences-brain-morphology-w...

Except this requires extrapolating brain volume from the skull , which probably inaccurate compared to modern day MRIs. So i would take this claim that brains are shrinking with some skepticism.

Brains getting smaller doesn't mean any capacity is lost. Maybe the design was improved, as in metabolicaly less demanding, or developmentaly more reliable.

Who knows

I would therefore propose that some event occurred which pushed humans out of africa, which lead to those that remained having smaller brains, and ultimately lead those that left to the same position. Perhaps that explanation is simply population density. Populations tend to expand geometrically, such that the largest portion of population growth occurs in the latest generations. We also know that all modern humans are descended from a very small set of people, maybe as few as fifty (?).

So humans began at that population, slowly expanded across africa for a few tens of thousand of years, started running out of space and were forced to migrate away from africa, then shortly filled up the rest of the earth. Having no where else to escape to, we were forced to deal with increased population density. This lead to lower food availability to support large brains, as well as increased need to reduce aggressive behaviour, achieved via more juvenile brains. Thus, brain sizes reduced across the globe.

1. https://www.youtube.com/watch?v=YJF01_ztxwY

I like to think of a scenario comparing the first commercially available computers being the size of rooms, now we have exponentially more computing power in the size of the palm of my hand.

Is it possible that there is long term development of emergent traits in a species, through a process that is not evolution ?

I don't buy the domestication hypothesis, because I see no proof that aggression has been disincentivized over what we know about ancient human history. There might be unintended consequences, where those who don't go to war are more likely to live and therefore reproduce, leading to propagation of non warring = low-aggression genes.

Lastly:

> This may mean that smaller brains are being naturally selected for. Of course, this hypothesis relies heavily on premises that are both not confirmed and highly controversial.

When the only reason to reject a hypothesis, is that it is controversial, I am inclined to assign it a higher likelihood.

I’m surprised there was no speculation involving memes in the article (“meme” as in the original meaning coined by Dawkins).

We’re running a complicated and evolved software suite now. The idea that we have less raw computing to do than our bare-brained ancestors might be worth exploring.

When we think of brain and size, we naturally think of the connection to the intellect. But intellectual activities require patient and slow application of the brain, using external tools like making notes, drawing diagrams and checking one's work for mistakes.

Intellectual pursuits are not actually what the brain is good at. If we make an analogy to computing, it's almost like the intellect is a software layer that is bootstrapped on top of the brain in spite of its shortcomings, using some slow, inefficient scripting language. It's not native to the brain.

An example of what the brain is natively good at is real-time visual processing and control, like navigating in a complex environment full of obstacles and dangers. You don't have to concentrate your intellect for minutes at a time to tell apart a shape from the background; you just point your eyes at it and it pops out at you instantly and effortlessly, thanks to a power in the brain that you can't even control consciously. (Try as you might, you can't not see the shape.)

This is probably why domestication shrinks brains. Perhaps you don't need that much real-time processing in the relatively stable, calm, boring domestic environment.

Interesting that our brains are so resource-intensive. It doesn't seem that surprising that natural selection would favor smaller or more efficient brains in many cases.

Paleolithic life expectancy was something like 22 to 33, and much of that after very high infant/maternal death rates. With such life expectancies, females would likely be bearing children at ages we today would find positively shocking. Simply surviving childbirth would be a huge selection factor for both mother and child. And girls become fertile well before they are fully developed physically (yes?), and I suppose their ability to survive childbirth presumably increases a lot with each year after menarche until they are fully grown.

This might be noticed and communicated culturally. There could certainly be selection advantages to parents and families communicating this to their daughters. On the other hand, adolescents certainly want to have sex at early ages, and our ancestors wouldn't have the same hangups and cultural barriers humans have had for the past several millenia. (Indeed, part of the story here is that developing those hangups is, to some extent, adaptive.). And with shorter lifespans, there would have been some expectation of getting on with it. Sex and mating would have been socially "valid" choices at far earlier ages.

If so a girl (/family?) with the intelligence and self-control to 1) notice this and 2) act to postpone mating, even by a year or two, could do a lot to improve a girl's/woman's odds of surviving childbirth and so passing on her genes. If (a big if?) we presume that these characteristics are related to brain size, that might explain it. (If the ability to notice, and communicate, the advantage of waiting were in the _parents_, that would still be be selective for intelligence/control.)

As human lifespans extended, pressure to reproduce at an early age would diminish, and it would be easier to create and communicate the cultural norms protecting girls from maternity before they were fully grown. Indeed those norms would become adaptive, as having sex too early would be needlessly dangerous. At which point, _all_ girls would be protected from premature pregnancy, and the selection for intelligence/control would diminish.

(Apologies for any ignorance of how the ability to bear children changes after menarche, I have only the standard male knowledge of these things. And for "girl"/"woman" nomenclature issues, the ages in question here are very different than the ones we are today accustomed to.)

> We see an erosion of grey matter, not white matter.

Given the lack of an actual brain, I assume this is some sort of assumption based on which part of the brain has changed size?

There's a also a huge variation in human brain size. [1] suggests almost a factor 2. I don't know if variation was similar 70.000 years ago.

[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4262916/

Until the second half of the last century it was hard for the majority of people to find enough food.

I would expect that our brain will start to grow slowly if food won't be a problem for a long period. The same way our bodies started to grow once we had enough food.

It's probably the same thing happening in humans. It would be interesting to measure the brain volume of politicians and business leaders to see if they follow the same trend as the general population.

However, I only see attempts to explain that genetically. Aren't there environmental factors that can influence this measure?