Evolution is not error-free. For instance, amounts of testosterone differ. Most males get amounts adequate for reproduction; a few get toxic amounts.

My observation here would be that Mother earth is 4.5 billion years old. At the beginning it was a bloody hot mess. It took a while for the right conditions to get set for life to start somehow. Based on that, I would guess 3.8 billion years is not that far off. Of course it's easy to confuse this figure with 13.8 billion years which is the age of the universe, a different thing.