First sex determining genes appeared in mammals some 180 million years ago. Well, this is what a team from the Swiss Institute of Bioinformatics has figured out after conducting a study. Y chromosome is the single element of the genome that distinguishes sexes in humans and other mammals. Males have both X and Y chromosomes, but females have two X chromosomes only and no Y chromosome.

The researchers found X and Y were both identical a very long time ago, but Y began to differentiate from the X in males later on. It gradually progressed to such an extreme extent that it is presently left only with about 20 genes. First sex genes appeared concomitantly in mammals around 180 million years ago, found the study. The study took more than 29,500 computing hours and analyzed samples from 15 different mammals.

It was found by the researchers that X and Y chromosomes evolved from an ordinary pair of autosomes. However, the Y chromosome was ravaged by genetic decay and only 3% of its ancestral genes survived.

They affirmed that the Y chromosome is pretty weak in comparison to the X and has been left with 19 of the 600 genes it once shared with the X. The human Y had shed just one ancestral gene over the past 25 million years, found the scientists. The same genes were found in five more other mammals: the marmoset, mouse, rat, bull, and possum.

"This is not just a random sampling of the Y's ancestral repertoire. This is an elite bunch of genes. Evolution is telling us these genes are really important for survival. They've been selected and purified over time", said Whitehead Institute Director David Page.

The scientists are looking forward to conduct a study on unisex medicine as similar male and female cells work in different ways.