The findings, presented Friday (Nov. 9) here at the American Society of Human Genetics' annual meeting, dispute the notion that historically most men in a generation have not passed on their genes while a few lucky guys fathered hordes of children.
"Averaged over many hundreds of thousands of years, there's likely been a small skew in the number of males to females," said Melissa Wilson Sayres, a researcher at the University of California Berkeley. "There is some skew, but it's not very large."
Fewer alpha males?
Every cell in the human body contains 23 pairs of chromosomes (threadlike structures into which DNA is packed), with one of those pairs comprising the sex chromosomes (an X and Y for males and XX for females). The DNA in the Y chromosome represents about 2 percent of the DNA in human's cells, compared with the X chromosome's 5 percent.
The Y chromosome also has much less variation in its DNA than other types of DNA, meaning that the sex chromosomes in two men will look more alike than other chromosomes do.
Some scientists have argued the Y chromosome is so uniform because throughout evolution, relatively few men passed on their genes compared with women - in other words, alpha males hogged all the women, while less successful men lost out in the mating game. That would mean that modern-day people have far fewer male ancestors than female ones. But nobody had looked to see if that theory jibed with the genetic makeup of the Y chromosome.
To test this theory, Wilson Sayres and her team looked at genetic variations in eight African and eight European men. Next, they did computer simulations to see if they could match the variation seen in that sample by skewing the ratio of reproducing males to females over hundreds of thousands of years of evolution.
They found a polygamous mating system, in which a few select men mate with lots of women, couldn't explain just how little the Y chromosome differs between individuals.
"Other people have suggested it's as few as four females for one male," Wilson Sayres told LiveScience. "We find that it's probably skewed, but it's more like four females to three males."
The researchers' models showed that evolution probably weeded out a lot of the variation that happened randomly throughout history. But evolution didn't just cull harmful gene changes: it also reduced the variation in the one-third of the Y chromosome composed of highly repetitive strings of letters.
Those strange regions don't code for proteins, which carry out a gene's instructions in the body, but Wilson Sayre's results suggest they are probably doing something useful or evolution would have not have reduced how much variation they contain. Some scientists have suggested those regions prevent the Y chromosome from degenerating, but their exact purpose remains a mystery.
The results also suggest that some estimates of when humans first left Africa may be too recent, Wilson Sayres said. That's because many researchers only look at variation in the Y chromosome to estimate when people spread geographically, with less diversity on the sex chromosome implying a more recent common ancestor, Wilson Sayres said.
"But if selection is reducing that diversity, then they're probably estimating more recent timings than actually occurred," she said.