Blokes could be completely wiped out in the future as women will not longer need them to reproduce, scientists are claiming.
The bombshell comes after an endangered rat species that no longer has a Y chromosome was found on a Japanese island.
The genetic marker, which determines animals are male, has been shrinking in mammals – including humans – for millions of years.
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And experts now fear it could eventually disappear, bringing about the "end of man".
The spiny rats, which live on the island of Amami Oshima, can still reproduce despite their lack of Y chromosomes.
Researchers believe a new gene has evolved that gives female embryos male genitals.
Study leader Asato Kuroiwa, of Hokkaido University, added: "There is no reason to think our Y chromosome is any more robust than the spiny rat’s."
But it's not all bad news – a paper published in the journal PNAS by scientists at the university has explained this may not be the beginning of the end for the human male.
This is because in humans, the Y chromosome isn't key in the development of male traits – instead it's a gene found on the chromosome, the SRY gene, sex chromosome geneticist Jenny Graves explained.
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"The Y [chromosome] makes no sense in terms of function, but is easy to understand in terms of evolution," Graves told Newsweek.
"The X and Y were once upon a time just an ordinary pair of chromosomes. Then one partner acquired a variant gene (SRY) that determines maleness.
"The human Y is in the very last stages of degeneration, and the big question is how long till it, too, gets lost, and what will happen when it does.
"If it goes on degenerating at the same rate it has over the last 150 million years, it has only a few million years to go," Graves explained.
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In humans, the SRY gene is on the Y chromosome – but Graves added that even if the Y chromosome were to disappear, it wouldn't necessarily spell the end for men.
"Kuroiwa's team have minutely examined the genome of the Y-less spiny rats and discovered that a gene that is the target of SRY (called SOX9) has a changed sequence in its control region which is only in males," she explained.
"This makes sense, because an upregulated SOX9 would not need to be turned on by SRY."
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