Costly Transitions:

Randolph M. Nesse:

"the transition to bipedality and its legacy of vulnerability to hernias, hemorrhoids, back pain, knee pain, plantar fasciitis, varicose veins, and omental torsion. It is painful to imagine how prevalent these problems must have been in the first million years of bipedality.

The wrenching transition to the cognitive social niche may have created even more severe problems, considering the path-dependent interactions of multiple alleles that influence brain development pathways. Imagine a new allele changing the chemical gradients that influence neuronal migration during brain development in ways that give a benefit, perhaps something like more expressive vocalization. If this gives a net selective advantage, the allele will be selected for, despite negative effects that slightly disrupt multiple other adaptations that evolved previously.”