> many valid sequences are relatively nothing compared to the infinitely many invalid ones, right?
There’s finite invalid sequences, DNA isn’t infinitely long. It’s also not a question of valid or invalid we live with sub optimal DNA, so yea most people aren’t born with some new beneficial mutation. However, not winning the lottery isn’t the same things a dying, and even smaller wins still benefit us.
As to our long reproductive cycle, there’s a reason we share so much in common with other primates. Most of our DNA has been worked out for a long time. We share 98% of our DNA with pigs, and 85% of it is identical in mice which has practical application in drug development. Of note common ancestors were more closely related to us because both branches diverged.
Hell 60% is shared with chickens, and half of it’s shared with trees.
> only reproduce sexually once every decade or so at best?
Many sperm and fetuses die from harmful mutations, live babies are late in the process here. Also, because order doesn’t matter you get multiple chances to roll the same sequence for every birth.
PS: There’s also quite a bit of viral insertion into our DNA, it’s mostly sexual reproduction but we have some single cell ‘ancestors’ in our recent history.