When early humans became carnivores, their higher-quality diet allowed mothers to wean babies earlier and have more children, altering the course of human evolution.
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Humans' meat-eating habits help separate them from other great apes, new research suggests. A meat-heavy diet lets people wean younger babies and have more offspring, which may have contributed to the population explosion, the researchers say.
Because human females wean their young so quickly, they "can potentially contribute a larger number of individuals to the human population during their reproductive years," study researcher Elia Psouni, an associate professor at Lund University in Sweden, told LiveScience. "We are suggesting that this has had a very big impact on the survival and spreading of the species and the way it happened."
Studies of "reproductively natural" populations (that is, societies that don’t use birth control) showed that mothers stop giving breast milk to their baby when the baby reaches about 2 years and 4 months of age. That surprised the researchers, since other great apes take about four times as long to wean their offspring (proportionate to their maximum lifespans). [Birth Control Quiz: Test Your Contraception Knowledge]
These other apes have diets dominated by fruits, vegetables and other plant materials. Chimpanzees, humans' closest living ancestors, get only about 5 percent of their calories from meat, compared with about 20 percent for humans.
To find out if this dietary shift is important in determining weaning age, researchers compared the developmental characteristics of 67 different mammals. With computer models and other analyses, they found that, together, body size, brain size and diet accounted for about 90 percent of the reasons for time of weaning.
The young of all mammal species stop suckling when their brains reach a specific developmental stage, the researchers said, and this stage seems to come earlier in carnivores (species for which at least 20 percent of their calories come from meat).
"We are much more used to thinking of humans as aligned with other great apes in many aspects," Psouni said. In this instance, though, "the pattern is one where humans ought to be put together with tigers and killer whales — all these animals wean their offspring sooner." [In Photos: A Lion's Life]
It doesn't matter if meat is cooked or not, Psouni said, since the same early-weaning trend is seen in lions, tigers and killer whales. The confusion about humans weaning "early" really only comes about when you compare humans with other great apes, species that aren't carnivorous.
The researchers think this younger-age weaning could have helped humans spread throughout the world. A quicker age to weaning means a woman can have more children throughout her lifetime.
"The access to a diet that is rich with animal protein is what makes it possible for that species to [over many generations] shorten the time between births," Psouni said."You wean faster, you can become pregnant faster and give birth to more offspring."
The study was published today (April 18) in the journal PLoS ONE.