Thanksgiving not included of course.

all it's quacked up to be.

Think about it this way:

Mammals and birds diverged before sexual differentiation was established. Then they developed sex through convergent evolution.

Sex is such an evolutionary advantageous strategy that it has evolved multiple times throughout life.

Or, more accurately put, entirely necessary or observationally adequate.

It can be claimed that they they diverged before the two current systems were established.

Or maybe not.

https://en.wikipedia.org/wiki/Sex-determination_system#Evolution

Since argumentatio ex wiki seems in vogue.

Thanks for posting this it gave me some interesting things to read about.

Learned there are 3 main systems, XY where the male has two different chromosomes and determine the sex of the offspring; X0 where the male gives an X or no chromosome; XW where the male is XX and the female determines sex by whether the egg has an X or W chromosome

Also read a little about X inactivation which prevents too much transcription of those genes in XY systems.

What surprised me most is it sounds like these systems exist in species that are unrelated. There are mammals with X0 systems and reptiles with XY. ZW systems exist in insects, reptiles and birds.

That sounds like these systems must have evolved multiple times? The wiki articles didn’t talk about about that aspect, at least I didn’t see it.

Sorry if I got any of this wrong. Interesting stuff.