Are men headed for extinction?

I don't know that much about genes and chromosomes (here's what I do and don't know), but I do know that humans have 23 pairs of chromosomes, 46 in total, of which 45 are X chromosomes and only 1 is a Y chromosomes, and that only in men (women have 46 X chromosomes).

What I learned recently is that the Y chromosome in men is tiny, containing about 55 genes compared to the 900-1,600 or so in X chromosomes. Only one gene on the Y chromosome, known as SRY, is responsible for triggering a genetic pathway, starting with the SOX9 gene (which is NOT on the Y chromosome), which then switches on the development of the testes in utero, which results in maleness. Pretty much everything else on the Y chromosome is non-coding "junk" DNA, simple repetitive DNA that doesn't seem to actually do anything. Well, no-one could accuse human physiology of being straighforward and logical!

Be that as it may, what is even more strange is that the Y sex chromosome in men appears to be degenerating, and may actually disappear in a few million years (if humanity lasts that long). Monotremes (platypuses and echidnas) are the only mammals that have a full-sized Y chromosome, but then, as we know, platypuses are weird. Given that humans diverged from monotremes some 166 million years ago, this suggests that our Y chromosomes will disappear completely in less than five million years.

Of course, this is not assured. The degeneration may actually speed up as the Y chromosome weakens, or it could stabilize as the Y chromosome is stripped to essential (non-junk).genes. But, unless we evolve a new sex gene, it is possible that we will end up all female, which is not a good model for long-term survival, even if it may be preferable in other ways.

The good news is that there is a precedent for this in the animal kingdom. Two rodent lineages - the spiny rats of Japan and the mole voles of Eastern Europe - have already lost their Y chromosomes and and still surviving (albeit barely in the case of the spiny rat). They lack a Y chromosome (and therefore a SRY gene) completely, but what has been discovered is a tiny difference near the key sex gene SOX9 on chromosome 3. It only amounts to 17,000 base pairs out of about 3 million, but apparently that is enough to allow the SOX9 gene to kick-start the whole sex change pathway, even in the absence of the SRY trigger.

So, as so often, nature finds a way. But exactly how how this might play out in humans, who are spread throughout the world and come in a whole variety of shapes, sizes and colours, is anybody's guess, and the stuff of science fiction novels. Might we develop into different species? Might we develop some kind of parthenogenesis, like some lizards and snakes? Who knows?