The evolutionary mystery of gigantic human brains

Rather there's a fix point: The brain is adapt at increasing its size. That's what everything does: Growing. The brain is just especially good at it.

The advantage also manifests in the "schema of childlike characteristics" (Kindchenschema) which includes a big head. So it's evidently also important for affection on a different level: child care and protection.

There are pretty bold statements in your comment, that may be countered with a lack of constraints on the growth. Intelligence affords fitness in myriads of ways, of course.

I don't think so. Curious detail about that tail: The females don't have it.

Herds are a less complicated form of socialization. By contrast humans need to keep track of dozens of interpersonal relationships as a matter of personal survival. If you aren't paying attention, even the biggest toughest strongest man can be undone by 3 or 4 of his rivals banding together and attacking him as one. I'm reminded of one instance I read about several low status chimps all pouncing on an alpha who'd gotten too complacent and castrating him with their teeth, thereby ensuring he was permanently out of the running for leadership. Under such conditions, being clever is not just a nice-to-have - it is a matter of survival.

The frugivore arguement might be totally valid too. As does the "natural" defense arguement - animals that have natural defenses like shells, quills, or other bodily forms of armor and defense tend to be less intelligent than creatures that lack such features. So it stands to reason that humans, as the species with the least equipped with natural defenses, would also be the cleverest of the bunch.

But ultimately we just don't know. All we have is raw speculation.

The frugivore argument gets more interesting when you consider how it came to be that fruit is colour coded in the first place. Why do fruits advertise that they are ripe to eat by changing colour?

Well, if you are a tree with plans for global domination than producing a few nuts for squirrels isn't going to cut it. What you really need are primates to carry your seed far and wide, to get it properly planted with some 'manure'.

So, given the world was once fully monochrome, how did it happen that, as the trees evolved colour coded fruit the primates evolved colour eyesight?

I blame the Blind Watchmaker. There is no god so therefore colour coded fruit could not have been specially created just for the primates.

I think that it helps to have vegetarian sympathies to reject the social brain hypothesis. The idea that bigger brains are needed just to spot what fruit to eat doesn't cut it if your food is beige coloured. Imaginably we needed those bigger brains to go on the hunt and to round up woolly mammoths with spears and stuff.

A bigger brain is very energetically demanding and fruits are a lot energetic than leaves. Maybe, only primates that eat fruit can afford big brains?

* fire is a very powerful weapon, at the same time it will weed out the ones unfit to control it

* energy from the outside means less calories need to be burned on the inside.

* cooking kills pathogens, I'm not sure that may be included in the 5x account

Still one has to wonder whether the discovery was dependent on language, and whether language and social competition was a bigger boost to brain size.

Agricultural year cycle of planting and harvest, with long periods of storing the produce is very recent phenomenon in evolutionary history of Homo sapiens, and not even universal at that — it is northern thing, because it is tied to grain based agriculture . African and South American peoples often practice agriculture when they simply leave tubers in the ground and harvest as needed. It makes the social dynamics much different too, because it makes it less profitable to raid and steal neighbors food supplies, as they will spoil relatively quickly when pulled out of the ground, unlike wheat, rye, millets or rice.