Rereading wikis on occipital buns and such again. I’ve been piecing together some thoughts in response to an imaginary critic, who is asking “Why do you believe the four major brain lobes in homo sapiens are survivors of different human species, when this is not the case in other mammals?” That is, most mammals have separate occipital lobes, what is special about humans that means they come from neanderthals?
Well, part of the answer is “not everybody has the super special kind of occipital lobe”. Pure, unmixed homo sapiens has an occipital lobe all its own (if such a thing makes sense at all, given that Tex says they’re genetically engineered from parts off the shelf), distinguished by its cranial development process.
Now, compare this with a thal skull:
We still see this latter shape in some modern people (particularly among Basque people and other populations with high neanderthal admixture):
I’ve made the general observation that the skull must grow to accommodate the brain, or else the brain gets all squished up in a too-small compartment. I’m pretty ignorant of genetic specifics, but logically I can only see the following options: the instructions for the occipital bone and the occipital bun (Edit: <-should have said "lobe" here, not bun) are either on the same gene, or they aren't. If they are on the same gene, we'd expect that occipital buns and intuitive skill in visual packing problems are 100% comorbid. In the specific example of occipital buns this actually seems to be the case, but I want to generalize the idea (to turricephaly and such) so I'm going to assume it's not.
That means we're assuming the instructions to form the occipital bone a particular way are separate from the instructions to form the occipital lobe in a particular way. Analytically, it doesn't matter that they are heavily correlated. That means we'd occasionally find people with occipital buns who can't parallel park, and plano-occipital people who are preternaturally skilled architects.
Aside from the pathological problems that may arise from trying to grow a brain in cramped quarters, I can think of a couple of good reasons that brains tend to fit their containers. Big skulls raise infant mortality, which was a big deal back before C-sections. If a small or average IQ accompanied the big skull, it would be maladaptive. But humans have high encephalic indices because high IQs tend to be adaptive (except maybe in modern conditions, as I've pointed out). Big brains are also energy intensive, so low IQs (and small brains) are preferred unless high IQs are generally adaptive.
Okay, now that all of the groundwork laid, let's observe the major difference between neanderthal skulls and homo sapiens. Neanderthal skulls don't have the sutures that characterize skulls with multiple bones that grow together in adulthood. See? The thal cranium is just one big bone.
The sapiens cranium is made up of several bones growing together. Here’s a quick snippet about how that works:
About the fourth year the squama and the two lateral portions unite, and about the sixth year the bone consists of a single piece. Between the 18th and 25th years the occipital and sphenoid become united, forming a single bone.
But modern sapiens, mixed with thal, sometimes recreates the occipital bun highlighted in the Basque guy above. In this case, we see the genetic instruction to “build the neanderthal-style occipital bone”, which includes the bun and probably houses the neanderthal-style occipital lobe.
Now, recall that this style of occipital lobe presupposes a more anterior foramen magnum, as seen on Ettore Whatsisface:
But the Basque dude has a normal posture. What gives? It seems that you can hack the two cranium styles together in the same human, given different genetic instructions from two different ancestor species.
So there’s that.