Y chromosome appeared 180 million years ago

London: The Y chromosome, which distinguishes males from females at the genetic level, appeared some 180 million years ago, according to a new study. In humans and other mammals, the difference between sexes depends on one single element of the genome: the Y chromosome.

It is present only in males, where the two sexual chromosomes are X and Y, whereas women have two X chromosomes. Thus, the Y is ultimately responsible for all the morphological and physiological differences between males and females.