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Title: Chimp puberty helps recalculate when chimps and humans split
Source:
ars technica
URL Source: http://arstechnica.com/science/2016 ... -when-chimps-and-humans-split/
Published: Jan 27, 2016
Author: Cathleen O'Grady
Post Date: 2016-01-28 18:50:22 by cranky
Keywords: None
Views: 4371
Comments: 16
Recalibrated genetic evidence better matches the fossil record. A chimp in Leipzig Zoo contemplates its mutation rate. Genetic evidence is an incredibly useful tool for understanding evolutionary history. It has helped us build up our current picture of how humans migrated out of Africa and also estimate when chimps and humans parted ways from our last common ancestor. Estimates that used different methods have placed the chimp-human split anywhere from three million to 10 million years ago (mya), sometimes falling far from the estimated 6 to 7 mya suggested by the fossil record—an indication that something is wrong in the calculations. But as researchers improve their techniques, the estimates are revised over and over again, each time hopefully getting closer to the real picture. Having the dates provided by the fossil record match the genetic evidence would help us to be more sure of our understanding, so it’s important to try to work out where the mismatch is and why it’s happening. Two researchers at Columbia University, Guy Amster and Guy Sella, have suggested an important factor that has been missing: the timing of life events like puberty and reproduction and their effects on genetics. By building these factors into their calculations, they’ve come up with an estimate for the split that more closely matches the fossil records—around 6 mya. Slow or speedy mutation The genetic evidence that researchers used to estimate species splits like this comes from what's called a “molecular clock.” The principle is pretty simple: over time, DNA sees small mutations. Some of these changes make an important difference, but others are neutral—everything carries on the same before and after the mutation. These mutations happen at a certain rate. If we know that rate, we can see how many neutral mutations have accumulated in closely related species and use that to estimate when they started evolving separately. There are a couple of ways that the molecular clock can be calibrated: we can estimate it based on species that we’ve dated by other means (for instance, by the age of the rock layer they were found in) or by measuring the rates in species that are alive today. What Amster and Sella point out is that estimates of the chimp-human split haven’t taken into account some important complications in mutation rates, which means that the molecular clock isn’t properly calibrated. Most important, there are sex differences in mutation rates. One of these differences is in the creation of sperm cells and egg cells. The more often the germ cells that make them divide, the greater the chance of additional mutations. In female chimps and humans, as well as many other species, female sex cells do all their dividing before birth. This means that whether a female animal is twelve or twenty-four when it reproduces, it's using sex cell that has undergone the same number of divisions (with any ensuing mutations). As a result, the age of a female when it reproduces shouldn’t change the number of mutations passed onto its offspring. Males, on the other hand, see their sex cells undergoing divisions from puberty onwards. This means that a male reproducing later in life is likely to have a higher mutation rate in his sperm than if he reproduced earlier. This difference causes males to contribute more mutations than females, meaning that maternal and paternal lineages have differently calibrated clocks. There are also differences in mutation rates brought about by generation time. Because mutations accumulate from generation to generation, longer generation times mean fewer chances for mutations to build up. This means that the life cycles of different species will have different impacts on the mutation rates. So, when we estimate the human-chimp split based on the molecular clock, we need to take into account all this information, knowing when male chimps hit puberty (around 7.5 years, compared to humans’ 13 years), and what their generation times are like compared to humans. A recent division The authors found that when they included these factors in estimates of the mutation rates in humans and chimpanzees, the two lineages ended up with quite different rates, with humans experiencing a slower rate of mutation than our cousins. That in turn allows us to calculate a new estimate for when the human and chimpanzee lineages split. Far from the previous consensus date of 10 mya, the authors write, these techniques lead us to an estimate as early as 6.6 mya. It's exciting work, says Melissa Wilson Sayres, a computational biologist who wasn’t involved in the research. “This is a really brilliant paper,” she says, praising the authors for their “thoughtful and very clever approach.” It’s a problem she’s been thinking about for many years, she says, and this research is likely to be well-received by others in the field. Kevin Langergraber, who studies the behavioral and molecular ecology of chimpanzees and also wasn’t involved in the research, argues that it was premature to say that there was a consensus that the chimp-human split took place 10 million years ago. Wilson Sayres agrees, saying that rather than a consensus estimate, there’s been ongoing debate about the question. “The estimates have ranged from 5 million years to more than 10 million years ago,” she says. Nonetheless, says Langergraber, “this is a very interesting paper which will add to a growing body of literature,” exploring “more sophisticated estimates of split times.” Amster and Sella themselves point out that these estimates are still up for revision based on future work. For instance, there are certain kinds of mutations that could affect maternal sex cells, which would change the mutation rate for females. We also need to understand more about how sperm cells are generated in order to get an accurate estimate of timing there, too. 
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#6. To: cranky (#0)
Is that a ballpark figure?
I think the 6-7 mya is a ballpark figure. The 3-10 mya sounds more like a wag.
Good. What's one or two millions years with regards to scientific theory.
#9. To: redleghunter (#8)
Seems to depends on how much the grant was worth. For a couple more bucks, what number would you like?
Top • Page Up • Full Thread • Page Down • Bottom/LatestEstimates that used different methods have placed the chimp-human split anywhere from three million to 10 million years ago
#7. To: redleghunter (#6)
Is that a ballpark figure?
#8. To: cranky (#7)
I think the 6-7 mya is a ballpark figure.
Replies to Comment # 8. What's one or two millions years with regards to scientific theory.
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