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I have always found this theory questionable because the shrinkage happened somewhat uniformly across disparate human populations. There's no reason that every

I have always found this theory questionable because the shrinkage happened somewhat uniformly across disparate human populations. There's no reason that every significant human subpopulation would suddenly evolve more efficient brains at the same time.

klyrs 5y ago
There's evolutionary pressure to reduce brain size because big heads make childbirth more injurious/lethal to the mom. I find it quite plausible that it would happen everywhere along a similar timeline.
- BuyMyBitcoins 5y ago
  I see what you’re getting at, but wouldn’t the same pressure to reduce head size also inhibit the heads from getting big in the first place?
  - bpodgursky 5y ago
    Exactly. And clearly there was pressure to downsize heads! But the tradeoff was that we lost brains, and until 40k years ago, that wasn't worth it. But with the advent of proto-civilization, the calculus changed...
  - dredmorbius 5y ago
    The fitness target isn't a stationary one. Larger heads might have been facilitated by a larger pelvic girdle (this seems to be the case in Neadnderthal skeletons AFAIU), which is an adaptation which would enable large (and possibly inefficient) brains, but at a cost for bipedal mobility, speed, and/or efficiency. Supposing a later competitor emerged with 1) smaller and 2) more effcicient brains as well as 3) narrower hips, then the previous advantage would become a (comparative) liability. It's a bit like Clayton Chistensen's Innovator's Dilemma: evolving yourself (or your species) into a corner. Advantageous for a while, but ultimately maladaptive, and hard to back out of. As with my other comments here, all but entirely conjecture.
dredmorbius 5y ago
Disclaimers first: I'm not a geneticist or even remotely close. I'm mostly aware of this dynamic from h. sapies and h. neanderthalensis (or h. sapiens sapiens* and h. sapiens neanderthalensis* comparisons. In a nutshell, Neanderthals were in numerous ways the apparent heavy-duty / high-output model, Sapiens the more efficient / economy model. Neanderthals had larger muscles, shorter limbs (less speed but greater strength, simply on leverage), and larger cranial volumes. All of which would have, everything else being constant, increased overall energy demands. Mind: this is all my own conjecture, though I wouldn't be surprised to find some literature suggesting similar mechanisms. Where Neanderthals were competing against other mammalian species and each other, the differentiation was apparently favourable. Possibly also in an ice-age climage. With Sapiens on the scene ... Neanderthals largely disappeared from the record, though in all likelihood also interbred with Sapiens to some extent. For whatever the reasons, the Neanderthalic adaptations seemed less suitable than the Sapiens ones. The presumption is that this was a fitness selection, though it could well have been other factors (mating preferences, superior numbers for Sapiens, dumb luck and/or chance). But ... in general, while bigger can seem better and more appealing for various reasons, it often is not, and adaptations which require higher energy or have a higher minimum sustaining requirement ... could well prove to be less suitable especially where the environment itself is in transition, or where resources are highly marginal. There is a fair bit of research into the loss of vestigal function, particularly among sequestered cave populations such as blind fish and invertebrates. It turns out that visual processing has high energy demands, and in an environment in which sight conveys no advantage, lack of sight is actually advantageous, as it reduces neurlogical metabolic load (by as much as 1/5 to 1/4 if I'm recalling my sources correctly). In the case of "Dragon Man", we're looking at a new instance and have little data to go on. But other homo variants and general evolutionary trends suggest at the very least possible directions.

klyrs 5y ago

There's evolutionary pressure to reduce brain size because big heads make childbirth more injurious/lethal to the mom. I find it quite plausible that it would happen everywhere along a similar timeline.

BuyMyBitcoins 5y ago
I see what you’re getting at, but wouldn’t the same pressure to reduce head size also inhibit the heads from getting big in the first place?
- bpodgursky 5y ago
  Exactly. And clearly there was pressure to downsize heads! But the tradeoff was that we lost brains, and until 40k years ago, that wasn't worth it. But with the advent of proto-civilization, the calculus changed...
- dredmorbius 5y ago
  The fitness target isn't a stationary one. Larger heads might have been facilitated by a larger pelvic girdle (this seems to be the case in Neadnderthal skeletons AFAIU), which is an adaptation which would enable large (and possibly inefficient) brains, but at a cost for bipedal mobility, speed, and/or efficiency. Supposing a later competitor emerged with 1) smaller and 2) more effcicient brains as well as 3) narrower hips, then the previous advantage would become a (comparative) liability. It's a bit like Clayton Chistensen's Innovator's Dilemma: evolving yourself (or your species) into a corner. Advantageous for a while, but ultimately maladaptive, and hard to back out of. As with my other comments here, all but entirely conjecture.

dredmorbius 5y ago

Disclaimers first: I'm not a geneticist or even remotely close. I'm mostly aware of this dynamic from h. sapies and h. neanderthalensis (or h. sapiens sapiens* and h. sapiens neanderthalensis* comparisons. In a nutshell, Neanderthals were in numerous ways the apparent heavy-duty / high-output model, Sapiens the more efficient / economy model. Neanderthals had larger muscles, shorter limbs (less speed but greater strength, simply on leverage), and larger cranial volumes. All of which would have, everything else being constant, increased overall energy demands. Mind: this is all my own conjecture, though I wouldn't be surprised to find some literature suggesting similar mechanisms. Where Neanderthals were competing against other mammalian species and each other, the differentiation was apparently favourable. Possibly also in an ice-age climage. With Sapiens on the scene ... Neanderthals largely disappeared from the record, though in all likelihood also interbred with Sapiens to some extent. For whatever the reasons, the Neanderthalic adaptations seemed less suitable than the Sapiens ones. The presumption is that this was a fitness selection, though it could well have been other factors (mating preferences, superior numbers for Sapiens, dumb luck and/or chance). But ... in general, while bigger can seem better and more appealing for various reasons, it often is not, and adaptations which require higher energy or have a higher minimum sustaining requirement ... could well prove to be less suitable especially where the environment itself is in transition, or where resources are highly marginal. There is a fair bit of research into the loss of vestigal function, particularly among sequestered cave populations such as blind fish and invertebrates. It turns out that visual processing has high energy demands, and in an environment in which sight conveys no advantage, lack of sight is actually advantageous, as it reduces neurlogical metabolic load (by as much as 1/5 to 1/4 if I'm recalling my sources correctly). In the case of "Dragon Man", we're looking at a new instance and have little data to go on. But other homo variants and general evolutionary trends suggest at the very least possible directions.