Results show modern humans, Neandertals diverged 660,000 years ago
An international consortium of researchers reports in the Aug. 8 Cell that for the first time the complete sequence of mitochondrial DNA from a Neandertal has been deciphered. Comparison of the Neandertal sequence with mitochondrial sequences from modern humans confirms that the two groups belong to different branches of humankindâ€™s family tree, diverging 660,000 years ago.
That date is not statistically different from previous estimates of the split between humans and Neandertals, says Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis. The sequence also doesnâ€™t reveal what happened to drive Neandertals to extinction, but it does clear up some discrepancies in earlier studies. …
At 16,565 bases long, the new sequence is the largest stretch of Neandertal DNA ever examined. The DNA was isolated from a 38,000-year-old bone found in a cave in Croatia.
â€œItâ€™s a nice accomplishment and the next important step toward completing the Neandertal genome,â€ says Stephan Schuster of Pennsylvania State University in University Park. Schuster is part of a group that is sequencing the genomes of the mammoth and other extinct animals, but was not involved in the current study. â€œItâ€™s a nice landmark on the way to saying what makes modern humans so special.â€
In order to know exactly how modern humans and Neandertals differ, scientists will need to examine DNA from the Neandertalâ€™s entire genome. The sequence reported in the new study was generated as part of a project to decode Neandertal DNA, but it contains information only about DNA from mitochondria.
Mitochondria are organelles that generate energy for a cell. Inside each mitochondrion is a circular piece of DNA that contains genes encoding some of the key proteins responsible for power generation. Mitochondria are passed down from mothers to their children. Scientists use variations in mitochondrial DNA as a molecular clock to tell how fast species are evolving.
Scientists have previously examined a short piece of Neandertal mitochondrial DNA known as the hypervariable region, but this new complete sequence helps clear up some ambiguities from studies comparing Neandertals and humans, says John Hawks, a biological anthropologist from the University of Wisconsinâ€“Madison.
Some modern humans have several changes in the hypervariable region that made it seem as if Neandertals are more closely related to modern humans than humans are to each other.
â€œComparing the complete mitochondrial DNA genomes of a Neandertal and many recent humans presents a very different picture,â€ Hawks says. â€œHumans are all more similar to each other, than any human is to a Neandertal. And in fact the Neandertal sequence is three or more times as different, on average, from us as we are from each other. This change from the earlier picture is a purely statistical one, but it makes a clearer picture.â€
Human and Neandertal mitochondrial DNAs differ at 206 positions out of the 16,565 examined, while modern humans differ at only about 100 positions when compared with each other.
08 Aug 2008