Developmental biology has a whole lot of plasticity and works primarily by self-organization of cell populations. It’s not a computer program where you have to

Developmental biology has a whole lot of plasticity and works primarily by self-organization of cell populations. It’s not a computer program where you have to allocate memory for skulls, eyes, ears and more — it’s actually more like a population of organisms that align themselves towards specific goals (“hey team, we need a hand — now time for fingers! You guys in the middle, die off!”) You can think of a gene for i.e. brain size just being an instruction that’s “you can press harder on the other populations to maximize your growth” and the other populations are likely to accommodate the stress during development, and change size and shape accordingly. There are of course harmful mutations that are unable to be compensated for and lead to failure to thrive, but when you see a successful morphological mutation it’s because the systems are very adept at integrating new stimulus during growth. (Similarly, social structures are emergent properties as well.)