The Y chromosome may run out, but men could adapt
Men may be on the road to extinction as their genes shrink and slowly fade away, medical students have been told.
A researcher in human sex chromosomes said the male Y chromosome may run out within five million years.
But Professor Jennifer Graves said men may follow the path of a type of rodent which reproduces despite not having the genes that make up the Y chromosome.
She told students at the Royal College of Surgeons in Ireland a second species of human could be born in the future.
"You need a Y chromosome to be male," said Prof Graves.
"Three hundred million years ago the Y chromosome had about 1,400 genes on it, and now it's only got 45 left, so at this rate we're going to run out of genes on the Y chromosome in about five million years.
"The Y chromosome is dying and the big question is what happens then."
The male Y chromosome has a gene (SRY) which switches on the development of testis and pumps out male hormones that determine maleness.
In her lecture, The Decline and Fall of the Y Chromosome and the Future of Men, Prof Graves discussed the disappearance of the Y chromosome and implications for humans.
She said it was not known what would happen once the Y chromosome disappeared.
"Humans can't become parthenogenetic, like some lizards, because several vital genes must come from the male," she continued.
"But the good news is that certain rodent species - the mole voles of Eastern Europe and the country rats of Japan - have no Y chromosome and no SRY gene.
"Yet there are still plenty of healthy male mole voles and country rats running around. Some other gene must have taken over the job and we'd like to know what that gene is."
The scientist said there were several candidate genes which could take over from SRY, adding whichever one did take over was sheer chance.
"It is even possible that two or more different sex-determination systems based on different genes could arise in different populations," she added.
"These could no longer reproduce with each other, leading to two different species of humans."
The work of Prof Graves, of the Australian National University, Canberra, on the past evolution of sex determination has paved the way for developments in diagnosis of gender disorders and gender-related disease in humans.