From ScienceDaily:
The study shows that the same sex-determining gene, named SRY, in placentals and marsupials had formed in the common ancestor of both lineages around 180 million years ago. Another gene, AMHY, is responsible for the emergence of Y chromosomes in monotremes and appeared some 175 million years ago. Both genes, which according to Henrik Kaessmann are “involved in testicular development ,” have thus emerged ” nearly at the same time but in a totally independent way .”
Just an accident.
The nature of the sex-determination system present in the common ancestor of all mammals remains unclear, given that mammalian Y chromosomes did not yet exist at that time — at least not those discovered in this study. So what triggered back then that an individual was born male or female? Was this determination linked to other sex chromosomes, or even environmental factors such as the temperature? The latter is not an unreasonable scenario, given that temperature determines sex in present-day crocodiles. As far as mammals are concerned, “the question remains open ,” concludes Diego Cortez.
Hmmm/ That question may well remain open fora long time. How, exactly, would we find out without recreating the habitat and its inhabitants?
See also: Junk DNA files: Genes on the Y chromosome a must for male survival (The Y chromosome had been reprieved earlier from its status as a vestige about to disappear, but this is newish)
See: Diego Cortez, Ray Marin, Deborah Toledo-Flores, Laure Froidevaux, Angélica Liechti, Paul D. Waters, Frank Grützner, Henrik Kaessmann. Origins and functional evolution of Y chromosomes across mammals. Nature, 2014; 508 (7497): 488 DOI: 10.1038/nature13151
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