Human Sex Chromosomes and the Future of Men
In humans and other mammals, females have two X chromosomes, and males a single X and a Y. The Y chromosome is male determining because it bears a gene (SRY) that switches on the development of testis, which pumps out male hormones. The X is a decent, ordinary chromosome, though it has more than its fair share of genes involved in male sex and reproduction - and maybe sexual behaviour and intelligence. But the Y is a genetic wasteland - small and full of genetic junk, bearing only 45 genes, mostly active only in testis.
The X and Y evolved from an ordinary chromosome pair as the Y degraded progressively. Comparing Y chromosomes between distantly related mammals, as well as their antecedents in birds and reptiles, reveals links to ancient bird/reptile sex chromosomes. The Y chromosome looks like a degraded relic of the X because most of its genes - even those with important male-specific functions - have partners on the X from which they evolved.
The human Y chromosome is running out of time. At the rate it is degrading, it will lose its last 45 genes in just 10 million years, even takiing complicatiing factors into account. What happens when SRY disappears? Since imprinting means we can't go in for a female-only lifestyle, would this be the end of the line for our species? The good news is that SRY has been lost in at least two groups of rodents. Somewhere else in the genome, a new sex determining gene must have taken over the function of SRY. So as the human Y runs out of options, new sex determining genes may evolve, maybe leading to different hominid species.