Differences between the sexes started taking shape about 180 million years ago. That, say researchers from Switzerland and Australia, is the approximate time when there was a biological split between females, which have two X sex chromosomes, and males, which sport one X and also one Y chromosome.

Actually, the X and Y chromosomes used to be identical, explains Henrik Kaessmann, associate professor at the Center for Integrative Genomics and group leader at the Swiss Institute of Bioinformatics, both in Switzerland. The first "sex genes," he said, apparently showed in different species at about the same period, an estimated 180 million years ago.

Then, for reasons that still need to be determined, the Y chromosome, which is ultimately responsible for all the morphological and physiological differences between males and females, started to differentiate from the Xs in males, progressively shrinking to such an extent that, today, it only contains about 20 genes, compared to the thousand or so in the X.

By studying samples taken from the testicles of several male species, the researchers recovered the Y chromosome genes from the three main mammal lineages: placentals, which include humans, apes, rodents and elephants; marsupials, such as opossums and kangaroos; and monotremes, which are egg-laying mammals, such as the platypus and the Australian porcupine echidna.

The team pieced together the largest gene atlas of the "male" chromosome ever created.

The study found that one type of sex-determining gene, known as SRY, formed in the common ancestor of placentals and marsupials around 180 million years ago. Another gene, AMHY, caused the emergence of Y chromosomes in monotremes approximately 175 million years ago.

Both genes, said Kaessmann, are "involved in testicular development," and emerged "nearly at the same time but in a totally independent way."

The research said the exact nature of the sex-determination system found in the mammalian common ancestor -- in other words, why the sexual break happened -- remains unclear, since it's assumed, based on the data so far collected, that Y chromosomes did not yet exist prior to that specific time.