April 19, 2012
Meat-rich Diet Lead To Earlier Weaning, Helping Speed The Spread Of Humans
Diets rich in meat helped early mother´s wean their babies at an earlier age and allowed them to have more children, behaviors that may have helped humans spread more quickly across the world and had a profound effect on human evolution, according to new research.
Researchers from Lund University in Sweden, publishing their work in the open-access journal PLoS ONE, found a clear connection between eating meat and weaning at an earlier age. They discovered that all mammalian species stop suckling when their brains have developed to a particular stage, with carnivores reaching that stage more rapidly than herbivores or omnivores.“Eating meat enabled the breast-feeding periods and thereby the time between births to be shortened,” said Elia Psouni, lead author of the study. “This must have had a crucial impact on human evolution.”
Because human mothers wean their young so quickly, they “can potentially contribute a larger number of individuals to the human population during their reproductive years,” Psouni 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.”
Among natural fertility societies, the average duration of breast-feeding is 2 years and 4 months. This is a very small timeframe compared to the longevity of our species, more than 100 years. Chimpanzees, humans´ closest living relatives, which have a lifespan of around 60 years, suckle their young for 4 to 5 years.
Chimpanzees only receive about 5 percent of their calories from meat, compared to about 20 percent in humans.
To find out if the dietary shift from fruits and vegetables to meat is important in determining weaning age, the Lund team compared the developmental characteristics of 67 different mammals, including humans, apes, mice and killer whales. Using computer models and other analyses, they found that a combined body size, brain size and diet accounted for about 90 percent of the reasons for time of weaning.
“We are much more used to thinking of humans as aligned with other great apes in many aspects,” Psouni told Jennifer Welsh of LiveScience. But according to the study findings, she explained that “humans ought to be put together with tigers and killer whales,” because all these “animals wean their offspring sooner” than most apes.
Researchers in the past have tried to explain the relatively shorter suckling period seen in humans based on social and behavioral theories of parenting and family size. But the new research has shown that suckling stops when the brain has reached a particular developmental stage. And that stage is reached far sooner when the energy content of the diet comes from meat.
“That humans seem to be so similar to other animals can of course be taken as provocative,” Psouni noted. “We like to think that culture makes us different as a species. But when it comes to breast-feeding and weaning, no social or cultural explanations are needed; for our species as a whole it is a question of simple biology.”
Psouni added that it doesn´t matter if the meat is cooked or not, since the same early weaning trend is seen in lions, tigers and even killer whales. The confusion surrounding early human weaning only comes about when you compare humans to great apes, species of which are not carnivorous.
The early weaning seen in humans could have helped with human evolution and the spread across the world, the researchers said.
“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 added. “You wean faster, you can become pregnant faster and give birth to more offspring.”
Psouni cautioned that the research should not be received as a sign of what humans today should or should not eat.