With this being the big old Darwin anniversary extravaganza year, one of the issues scientists are talking about at this meeting is how evolution shaped human beings into what they are. What’s fascinating to me is the emerging debate over whether some uniquely human traits may have resulted from what is essentially genetic detritus.
For instance, scientists have discovered in the past few years that the human genome is littered with genetic parasites that have no known mission in life apart from colonizing our DNA. More than half of our genome consists of large tracts that feature these parasites, which are called transposons. A big question of biology today is how much of an impact transposons have had on human evolution.
Jef Boeke of Johns Hopkins University School of Medicine in Baltimore, who spoke today, thinks transposons may be crucial to both normal human diversity and, possibly, to disease. He talked about one particular type of transposon called an L1 element, which has copied itself more than 100,000 times in the human genome. Boeke’s lab has devised a way to count the number of L1 elements in individual people’s genomes, and to figure out where the transposons fall in a person’s DNA.
Boeke’s lab used this method to count L1 elements on the X chromosomes of 117 people. The team found 34 differences in how the element had incorporated itself on just this one chromosome among the different individuals. Extrapolating that finding to the rest of the genome, and performing a rough back-of-the envelope calculation, Boeke estimated that L1 alone contributes to a “staggering” amount of human variation. Therefore, “We can say that everyone in this room – unless there are identical twins in the room – is different,” Boeke said. “There’s no question these elements are a major part of human diversity.”
But so what – what do transposons actually do? Boeke has asked that question by examining L1 elements in 70 men with mental retardation that has no known cause, but that appears to be related to mutations on their X chromosomes. Boeke’s group found seven L1 insertions that don’t appear in 500 normal men. Some of these insertions interrupt genes that are expressed in the brain, indicating they might somehow be involved in contributing to mental impairment. Of course, Boeke’s group still needs to figure out whether or not the insertions really are causing disease. But he is encouraged by these results: “We’re very excited about this as a potential approach to find new disease alleles” he said.