I already explained that one. You weren't paying attention. Brain size is determined by Sox genes during development.
Brain size is a competition between the number of neurons and the boundary defined by the skull, including the meninges let's say (vasculature, protective layer, etc).
That's why the brain "folds", it tries to fit lots of neurons into a small space. The SOX-11 gene is responsible for neural growth and differentiation. If you remove it, you actually get a smaller brain. With fewer folds.
There's a reservoir of pluripotent neural stem cells just below the ventricles in the brain. There are other Sox gene family members responsible for maintaining it, like Sox-2, Sox-4 and Sox-9.
In the cerebral cortex near the hippocampus the neurons get replaced every few weeks. That too is under the control and guidance of the Sox genes.
On the flip side of that we have another population of genes that determines skull size. Which is body plan. We already talked about that. Pay attention this time, and go back and read the earlier discussion.
In another one of these dumbass anti-evolution threads I already posted lots of images of what mutated Hox and Sox genes look like. Do you need to see them again?
We know how these genes work now. They make proteins, which in turn go back and sit in the DNA. You can read all the scientific details right here:
It shows you what the protein looks like and how it sits in the DNA. More importantly it shows you the mechanism, which is "DNA bending".
Figure 2 is particularly enlightening, it shows you some of the ways the various Sox proteins operate.
None of this is mysterious, and we already know a lot about the dynamics. Brain size can be changed by a single mutation in a Sox gene. If you change brain size and skull size at the same time, you can get a bigger head and a bigger brain with more folds.
Of COURSE the brain grows in humans. You didn't know about that before you discovered Lucy? In humans multiple Sox genes have longer time constants, so you get more bone growth in the skull and more neurons in the brain.
In this context fossils are uninteresting, they're just snapshots of history. We want to control where along the spectrum an organism ends up. We can already do this in frogs, we can make their heads grow bigger relative to their bodies by manipulating Hox and Sox genes.
In 5 years we'll have this all figured out.
