Convergence is a theme that I have mentioned in the past. The important thought concerning convergence is that life is a lot more constrained than anticipated. Some argue that this restricted nature argues against an evolution that is completely random in nature.
"Striedter and Charvet show that parrots and songbirds also resemble primates in a key aspect of forebrain development: all three groups of animals have enlarged telencephalic subventricular zones, which contain specialized forebrain progenitor cells.
Striedter and Charvet further suggest that this enlargement of the forebrain's subventricular zone is due to a mechanical constraint arising from the cellular mechanisms governing forebrain development."
