Which genes make us human?
Posted: Thursday, September 03, 2009 8:15 PM by Alan Boyle
European Parliament |
Researchers have identified three genes that appear to have been activated in humans alone, adapted from DNA that serves no function in other species.
But are these the genes that make us human?
That's not likely. At best, they're just part of our genetic story.
The best guess is that the human genetic code takes in somewhere around 24,000 genes - bits of chemical code that provide the instructions for building the proteins used in our bodies. Many of these genes are shared with other species. In fact, geneticists have found that humans and their closest living relatives in the animal kingdom, chimpanzees, hold about 96 percent of their DNA in common.
Most of the differences arise from old genes that have been copied and tweaked over time to create a larger store of genes - feeding a merry-go-round of mutations that keep the evolutionary process spinning. Biologists have long surmised that such mutations add up over time to produce different species.
In addition to our genes, there are other long stretches of DNA in the genome that don't figure in the production of proteins. Those stretches are known as "non-coding DNA" or "junk DNA," and every species has some. It's only been in the last few years that scientists realized that junk DNA may not be junk at all but instead can play an essential role in our genetic workings.
Three years ago, scientists discovered that bits of non-coding DNA in fruit flies actually turned into protein-coding genes. In this week's issue of the journal Genome Research, David Knowles and Aoife McLysaght of Trinity College Dublin say they found at least three human genes that appear to have gone through a similar conversion process.
Knowles and McLysaght found the genes by running a computerized comparison of the human and chimp genomes and checking the sections that didn't have anywhere near a close match. They identified 644 protein-producing genes in humans that didn't produce a corresponding hit in the chimp genome. Then they took a closer look at those sections.
In 425 cases, there were gaps in the chimp genome sequence big enough to account for the missing human gene. In 150 other cases, the researchers found a match that was missed the first time around. They looked at other species as well - eventually winnowing down their list of "uniquely human" genes to just three, known as CLLU1, C22orf45 and DNAH10OS.
That wasn't the end of the exercise. "We needed to demonstrate that the DNA in human is really active in the gene," McLysaght said in a news release. She and Knowles verified that the genes really did play a role in producing proteins for humans, and that the protein-producing capacity was disabled for other primates.
Then they were left with a mystery: What specific function do these three genes have in humans that would be missing in every other species? Right now, no one knows - although one of the genes, CLLU1, appears to be linked to leukemia.
It's virtually certain that these three aren't the only uniquely human genes, because of all the limitations in the analysis method. Knowles and McLysaght figure that they could survey only about one-sixth of the total human genome - meaning that, statistically speaking, there might be 18 human genes that arose from junk DNA since our family tree diverged from that of other primates.
The researchers suspect these genes are important in determining traits that are specific to humans. "They are unlike any other human genes and have the potential to have a profound impact," McLysaght said in the news release. In ScienceNOW's report on the findings, she's quoted as saying that "the distinction between humans and other apes must lie somewhere in the small genetic differences between the species."
But is McLysaght saying that these three, or 18, genes out of 24,000 account for all the differences between humans and other species? That's highly doubtful.
You can't exclude the roles played by the many more genes that have been tweaked and twisted over millions of years - genes such as PDYN, which has been associated with brain evolution; or FOXP2, which has been linked to speech and language. Moreover, the idea that each gene does its own thing in isolation has given way to the view that ensembles of genes work together, producing effects that are bigger than the sum of their parts.
My bet is that the quest for the genes that make us human won't end with the discovery of a holy grail, but with the discovery that myriads of genes work together holistically.
It could be that the rise of uniquely human genes, cooked up from the seeming leftovers of other species' DNA, gave a slight push to our humanness. But the fact is that every generation gets another little genetic push - as evidenced by other findings published last week in Current Biology.
Yali Xue of Britain's Wellcome Trust Sanger Institute and her colleagues analyzed Y chromosomes from two men separated by 13 generations, finding four mutations in the 10 million or so nucleotides within that one chromosome. Based on statistics, they estimated that each one of us carries 100 to 200 new mutations in our DNA.
In other words, according to the institute's news release as well as the BBC's report on the research, we are all mutants. X-Men (and X-Women), unite!
More on genetics and human evolution:
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