Two new surveys have now reconstructed the full history of the Y chromosome back to its evolutionary origin. One research group was led by Daniel W. Bellott and David C. Page of the Whitehead Institute in Cambridge, Mass., and the other by Diego Cortez and Henrik Kaessmann of the University of Lausanne in Switzerland. Their findings were reported on Wednesday in the journal Nature.

In the past 12 years, with the help of the genome sequencing centers at Washington University in St. Louis and the Baylor College of Medicine in Houston, Dr. Page’s group has decoded the DNA sequence of the Y chromosome of eight mammals, including the rhesus monkey and humans. The Y chromosome is so hard to decode that many early versions of the human genome sequence just omitted it. Dr. Kaessmann’s group, on the other hand, devised a quick method of fishing out Y chromosome genes by simply comparing the X and Y DNA of various species and assuming that any genetic sequences that did not match to the X must come from the Y.

Dr. Kaessmann calculates that the Y chromosome originated 181 million years ago, after the duck-billed platypus split off from other mammals but before the marsupials did so.

In some reptiles, sex is determined by the temperature at which the egg incubates. Genetic control over sex probably began when a gene on one of the X chromosomes called SOX3 became converted to SRY, the gene that determines maleness, and thus the Y chromosome came into being.

Until this time, the predecessors of the X and the Y had been an equal pair of chromosomes just like any of the others. Humans have 23 pairs of chromosomes, with one member of every pair being inherited from each parent. People with an XX pair among their 23 are female; those with an XY pair are male.