Origins of Y Chromosome Revealed: Male 'Genetics' Emerged 180 Million Years Ago

We all know that while there are many differences between men and women, the main element boils down to the genome: the Y chromosome. This particular chromosome is only present in males. Now, though, researchers have found exactly when the Y chromosome first emerged in our evolutionary history, revealing a bit more about men.

In the past, the X and Y chromosomes were identical. Slowly, the Y chromosome began to differentiate from the X chromosome in males; it shrank to such an extent that these days, it only contains about 20 genes whereas the X contains about one thousand.

In order to find out when the Y chromosome first emerged, the researchers studied samples from several male tissues-in particular, testicles. They focused on several different species, including placentals (which include humans, apes, rodents and elephants), marsupials (such as opossums and kangaroos), and monotremes (such as platypus and echidna).  In all, the researchers looked at 15 different mammals.

The scientists compared the genetic sequences from male and female tissues, and then eliminated all of the sequences found in both sexes. This allowed them to focus on the sequences that only corresponded to the Y chromosome.

So what did they find? It turns out that the same sex-determining gene, named SRY, in placentals and marsupials formed in the common ancestor of both lineages about 180 million years ago. Another sex-determining gene, called AMHY, was responsible for the emergence of the Y chromosome in monotremes, and appeared about 175 million years ago. Both of the genes involved testicular development, but evolved in a totally independent way.

Currently, the researchers are unsure what exactly triggered this evolution. They hope to continue their studies to answer what triggered an individual to be born male or female, and whether this determination was linked to other sex chromosomes or environmental factors. While these questions remain open for now, the scientists hope to answer them in the future.

The findings are published in the journal Nature.