But maybe you needed a pre-human womb to bootstrap the first human, and we no longer have the blueprint for that...

It is known that that is not true, due to the distinct genetic code of mitochondria and known epigenetic influences of mothers on their children in utero.

You could say “well that's the last 10% of the details, maybe 90% is in the DNA,” but I think I would be suspicious that it's that high, because one of the things we know about humans is that we are born with all of the ova that we will ever have, rather than deferring the process until puberty. I should think that if it could be deferred it would have been, “you will spend the energy to make these 15 years before you need to for no real reason” seems very unlike evolution whereas “my body is going to teach you how to make these eggs, just the same as my mother's body taught me,” sounds quite evolutionarily reasonable.