The poorly designed Y chromosome that makes men is degrading rapidly and will disappear, even if humans are still around.
Evolutionary geneticist Jenny Graves says while the process is likely to happen within the next five million years it could have begun in some isolated groups.
"As long as something came along it its stead, we would not even suspect without checking the chromosomes," she told AAP on Tuesday.
Professor Groves, who first made the prediction some years ago, was in Canberra to give a public lecture on the subject for the Australian Academy of Science.
There have been dissenting research papers, but her prediction hasn't changed.
"Its very bad news for all the men here," she told her audience.
Prof Graves has been studying sex-determining genes in Australian animals to shed light on human genetics.
"You would think that sex is so important it wouldn't change a lot. But it changes all over the place and the Y chromosome sort of self-destructs," she told AAP.
Y is always in the male and active mostly in the testes - making sperm.
That is a "very dangerous" place because there's a lot of cell division going, she says.
With every split there's a chance for a mutation or gene loss.
"The X chromosome is all alone in the male, but in the female it has a friend so it can swap bits and repair itself. If the Y gets a hit it's a downward spiral."
The X has about 1000 genes left, too many relating to sex and intelligence, she says.
The smaller Y started with about 1700 genes but only has 45 left, and that's mostly "junk".
"It is a lovely example of what I call dumb design," she told AAP.
"It is an evolutionary accident."
If humans don't become extinct, new sex-determining genes and chromosomes will evolve, maybe leading to the evolution of new hominid species.
This had happened in the Japanese spiny rat, which had survived the loss of its Y, she said.
Professor Graves is thinker-in-residence at Canberra University's Institute for Applied Ecology, a distinguished professor at the La Trobe Institute of Molecular Science, an ANU emeritus professor and University of Melbourne Professorial fellow.