[Delvo]

We're not all females at first. We're all sexually undefined at first. (And even if that myth were true, Jurassic Park would still have gotten it wrong, because sex determination in birds is the opposite of the way it is in mammals anyway...)

The idea of "holdovers" is better illustrated not with whole organs existing or not existing, but with things that are arrangements in odd ways or have features that don't make sense. For example, a bunch of the muscles in our limbs twist and bend around the joints in funny ways that only make sense if you figure they got stretched that way from some other original position when our limbs were attached differently and moved differently... and the junction of our digestive and respiratory systems is just weird unless you consider the history of how it got that way, especially in humans, in whom that junction (the throat) seems to be specially modified to increase the odds of choking.