Vertebrate embryonic development is a highly conserved process, particularly in the earliest phases. (note: you may find this September 2020 post on regulation of bee brain size to be interesting.)
A wide variety of vertebrate animals including humans start out with embryonic body plans that are similar. However, somewhere along the way, the ratio of the mass of the human brain/total body mass becomes very unusual, in fact completely unique.
What is the mechanism for the in some sense abnormally large human brain?
On a simple level there are really only two options. Either human brains have (A) more cells or (B) bigger cells. Or some combination of both of those factors, which I think is most likely to be the case in human brains although surprisingly no one is really sure!
The super-sized human brain needs more food than the brains of other animals as well, sort of like a linebacker scarfing down 10 mondo-burgers at MacJumbos. A recent article in Scientific American suggests the ever-hungry human brain consumes more sugar at the expense of our muscles. The developing human brain needs that extra sugar to grow as well as a mutation in a glucose transporter impairs brain growth causing microcephaly.
Of course other mutations, such as in my favorite gene, N-Myc, also cause microcephaly and our work in this case suggests that mutation of N-Myc causes microcephaly via impairing stem cell function (of course I have to tie everything back to stem cells , huh?)
In fact driving extended proliferation of neural stem and progenitor cells in mice via enhanced beta-catenin signaling, most likely feeding through N-Myc which is a beta-catenin effector molecule, the cortex of mouse brains start growing more and folding up like a human brain.
Beyond the cellular and molecular levels, there is also the evolutionary question of why the human brain has in essence become a super computer over the recent history. From an evolutionary perspective for humans at least, brain size does matter.
More simply put, one could argue that the human brain is so freaking big because thinking “better” gives a creature an advantage over others.
There are certainly costs to having a brain like we do.
The human brain literally eats up more sugar than any other organ in the body so perhaps relatively speaking human’s muscles are weaker or as former California Governator, Arnold might say “human muscles are like those of a ‘girly man'”. It is also possible that our big brains, which require a more super charged stem cell-driven growth period, could make us more susceptible to brain cancers than other species…a notion that is not proven, but intriguing.
It’s not just that the human brain is so jumbo, but more specifically our cerebral cortexes (cortices) are like monster trucks. The cortex is associated with so-called executive functions such as language and abstract reasoning so those human abilities presumably give us an advantage over other animals.
One way of boiling this all down is that our human big-brained smartness is a result of the sugar-addicted stem cells of the brain being like hyperactive genius kids in a candy store munching on humungo lollipops, swelling in size, tricking other “dumber” animals.
Interestingly and somewhat paradoxically, exercise seems to enhance human cognitive function by supplying the brain with even more sugar.
So to build and keep the human brain working well, all roads lead to sugar!