[gizmo686]

Also, chromosome count is not as stable as you might intuitively think. There are hominidaes alive today with 48 chromosomes.

There are human men alive today without any Y chromosomes that present with standard male phenotypes without needing any medical intervention. Currently, the biggest symptom of this condition is infertility (which is often the point where most people learn they have it). However, on evilutionary timescales, it doesn't seem that far off for the remaining needed genes to migrate over as well. On a human timescale, it doesn't seem that far off for us to medically solve the infertility issue; although I imagine the ethics rules would prohibit such treatment if it leads to xx-male offspring.

Having said that, while chromosomal sex determination is not omnipresent, it does seem to be much more preserved than chromosome count.