Men are slowly losing their Y chromosome, but a new sex gene discovery in spiny rats brings hope for humanity.

The sex of human and other mammal babies is decided by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years, leading to our extinction unless we evolve a new sex gene.

The good news is two branches of rodents have already lost their Y chromosome and have lived to tell the tale. A new paper in Proceedings of the National Academy of Science shows how the  rat has evolved a new male-determining gene.

How the Y chromosome determines human sex

In humans, as in other mammals, females have two X chromosomes and males have a single X and a puny little chromosome called Y. The names have nothing to do with their shape; the X stood for 'unknown'.

The X contains about 900 genes that do all sorts of jobs unrelated to sex. But the Y contains few genes (about 55) and a lot of non-coding DNA - simple repetitive DNA that doesn't seem to do anything.

Some lizards and snakes are female species and can make eggs out of their own genes via what's known as parthenogenesis. But this can't happen in humans or other mammals because we have at least 30 crucial 'imprinted' genes that work only if come from the father via sperm.

To reproduce, we need sperm and we need men, meaning that the end of the Y chromosome could herald the extinction of the human race.

The new finding supports an alternative possibility - that humans can evolve a new sex determining gene. Phew! However, evolution of a new sex determining gene comes with risks. What if more than one new system evolves in different parts of the world? A 'war' of the sex genes could lead to the separation of new species, which is exactly what happened with mole voles and spiny rats.

So, if someone visited Earth in 11 million years, they might find no humans - or several different human species, kept apart by their different sex determination systems.