There's nothing more macho than a Y chromosome. It not only confers maleness on its recipient but is handed down from one generation to the next pretty much stubbornly unchanged. That's because the Y chromosome is kind of a loner. It doesn't talk much -- genetically speaking, that is -- with its lifetime mate, the X chromosome.
"Unlike all of a man's other 22 pairs of chromosomes, his Y chromosome doesn't have a nearly identical partner to pair with in the cell nucleus," Stanford genetic genealogy wizard Carlos Bustamante, PhD, told me recently. "Instead, the Y chromosome that a male inherits only from his father pairs up with its female counterpart, the X chromosome, passed down from his mother."
Because of this weak pairing, there can be no exchange of DNA between the two sex-chromosome partners. The only changes occurring on a Y chromosome arise from mutations. As long as a change doesn't affect a male offspring's ability to reproduce, it's passed down from father to son. As Y-chromosome mutations accumulate over the generations, a man's descendants' Y chromosomes increasingly diverge from one another.
"That allows us to get a very fine-grained view of how the human male population has developed over the millennia," said Bustamante, whose group teamed up with researchers at the UK-based Sanger Institute and well over a dozen other institutions to produce a study, published in Nature Genetics, of global genetic variation in the human Y chromosome.
Starting with 1,244 Y chromosomes from modern-day men, the scientists used the differences between them to construct an extended family tree. They focused on a type of mutation that's been shown in other studies to occur at a rate of about 1 per every million DNA letters (bases) every 1,300 years or so. This molecular clock made it possible to assign time frames to key branching events, evident in the genealogical tree.
In addition to confirming existing evidence of a common male ancestor who lived nearly 200,000 years ago, the study pinpointed numerous sudden increases in the number of human males roughly 50,000-55,000 years ago, corresponding in place and time to archaeologically documented population bursts such as the first successful expansion across Eurasia. It also located more-recent expansions in sub-Saharan Africa, Western Europe, South Asia and East Asia between 8,000 and 4,000 years ago.
While the earlier male population expansions probably resulted from the peopling of vast new resource-rich regions of unoccupied land by small bands of ancient explorers, the reason for the relatively recent Y-chromosome divergences is more speculative. But one reasonable possibility is that these latter bursts were fueled by the conquest, peaceable or not, of existing bands of humans (and impregnation of the women among them) by men rendered dominant by better access to food (due for example to the discovery of a way to grow a new crop), better transportation technologies such as horses or wheels, or superior war-making capacity accruing from, say, improved iron-working technology.
Previously: New genetic study: More evidence for modern Ashkenazi Jews' ancient Hebrew patrimony, Genetic study supports single migratory origin for aboriginal Americans, Kennewick man's origins revealed by genetic study and Iceman's origins discovered at Stanford
Photo by Gareth O'Neill