Generation Gaps Suggest Ancient Human-Ape Split
We aren’t the only primates with a big generation gap. Human parents are, on average, a whopping 29 years older than their kids. That had been considered unusually long for a primate, but a new study reveals that chimpanzees and gorillas have their own large generation gaps, about 25 years and 19 years, respectively. The findings also indicate that our ancestors split from those of chimpanzees at least 7 million to 8 million years ago, more than 1 million years earlier than previously thought.
For the past 45 years, geneticists have suggested that the ancestors of today's humans and chimps went their separate ways about 4 million to 6 million years ago, and the ancestors of gorillas diverged about 7 million to 9 million years ago. There are almost no fossils of chimps and gorillas, however, so these dates were calculated by counting the number of DNA sequence differences between the three species and dividing that number by an estimated "mutation rate" for primates—or how fast mutations arise over time. The problem is that scientists often calculate the mutation rate using dates from fossils of other primate species, then applying this rate to the African apes and humans. The approach is subject to error because it relies on the accuracy of the ages of fossils and assumes that mutation rates are similar across ape species.
There is a better way, says molecular anthropologist Linda Vigilant of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Instead of looking at fossils and other primates, she says, researchers can use data from recent genome sequencing in humans, which estimates more precisely the average number of mutations that arise per generation in human families. Then, scientists can use the new generation time estimates to derive the yearly mutation rates in humans and apes to calculate how long ago the lineages split. Until recently, however, researchers didn’t have DNA samples from enough chimps and other primates in the wild to prove paternity so that they could calculate average generation times accurately.
Now, after a decade of analyzing the patterns of reproduction in chimpanzees and gorillas in Africa, Vigilant and former postdoc Kevin Langergraber—now a primatologist at Boston University—say they have the data they need. Working with almost 20 collaborators, the duo gathered data recording the age of mothers and fathers at the birth of 226 offspring in eight different chimpanzee communities in the wild, and 105 offspring of mountain gorillas from two different research sites in Africa—and they verified those relationships with DNA paternity tests on coprolites gathered in the field. As they report today in the Proceedings of the National Academy of Sciences, chimpanzee mothers ranged in age from 11.7 to 45.4 years at the birth of their offspring. The average age of reproduction was 25 years for females and 24 years for males, giving them an average generation time of about 25 years. Gorilla females ranged in age from 7.3 to 38 years when they gave birth, and the average generation time for both sexes was about 19.3 years.
The rate in chimpanzees was particularly surprising to primatologists: Although females gave birth as early as age 11 and males as early as age 9, the average age over their reproductive lifespan was much older. These lengthy generation gaps negate a long-standing rule of thumb that the larger the body of an animal, the longer its generation time—and slower its mutation rate. While this may be true for some mammals, it doesn’t seem to apply when comparing African great apes to humans. "Larger body size does not necessarily mean a lower mutation rate," says population geneticist David Reich of Harvard University, who was not involved in the study. "Even people who study gorillas and chimps are surprised," Vigilant says.
To get the mutation rates, the team divided the number of mutations between parents and their offspring (collected by analyzing DNA from coprolites sent to the lab in Leipzig) by the newly calculated generation times. The researchers got a high and low mutation rate for each species per year—rates that were slower than previously estimated using fossils to calibrate the molecular clock. When they applied the new rates to the history of all three species, they calculated that humans and chimps split earlier than expected—at least 7 million to 8 million years ago and possibly as early as 13 million years ago. They estimate the split between gorillas and the lineage leading to humans and chimpanzees to 8 million to 19 million years ago. Those dates have such wide ranges, Vigilant explains, because they assume the mutation rates seen today have been constant over time in all three lineages. So a key remaining question is whether mutation rates were faster in the past.
If the ancestors of humans, chimps, and gorillas parted company earlier than expected, it would give more validity to claims that the fossils purporting to be the earliest members of the human family, such as the 6 million to 7 million-year-old Sahelanthropus tchadensis from Chad, really are hominins, the group that includes humans and their ancestors but not the African apes. The longer generation times will also influence evolutionary models on skeletal and dental development that assume that human ancestors had more rapid growth patterns, similar to apes, than to living humans.
The study has yielded "the most accurate estimates of generation length yet possible for wild chimpanzees and gorillas," says paleoanthropologist John Hawks of the University of Wisconsin, Madison, who was not involved in the work. While these precise dates for both generation times and the split between lineages may be modified as more data is collected from more apes, adds evolutionary biologist Wen-Hsiung Li of the University of Chicago, the new work is significant because it "provides a novel approach to the long-standing issue of the divergence time between human and chimp."