If early humans had been vegans we might all still be living in caves, Swedish researchers suggested in an article Thursday.

When a mother eats meat, her breast-fed child’s brain grows faster and she is able to wean the child at an earlier age, allowing her to have more children faster, the article explains. That provided a distinct competitive advantage for early humans when limited resources and a small population made it difficult for them to thrive. “Eating meat enabled the breast-feeding periods and thereby the time between births to be shortened,” said psychologist Elia Psouni of Lund University in Sweden. “This must have had a crucial impact on human evolution.”

Women generally do not have a second child until they have finished breast-feeding their first. Humans typically suckle their children for a little over two years, a relatively short time compared with their maximum 120-year lifespan. Female chimpanzees, in contrast, suckle their young for four to five years out of their maximum lifespan of only 60 years. Chimp populations are thus much smaller than human populations.

Previous researchers have speculated that the short breast-feeding period is based on cultural norms. But Psouni and her colleagues looked at a variety of species and concluded that mothers of any species stop suckling when the child’s brain reaches a certain developmental stage and that this stage is reached earlier for carnivores, who have a higher quality diet.

The team reported in the journal PLoS One that they studied 67 mammalian species, including humans, apes, mice and killer whales. They concluded that there is a direct correlation between the amount of meat consumed in the diet and how early weaning occurs. Thus chimps, with their four-to-five-year suckling period, are among the longest for mammalian species primarily because their diet consists largely of nuts and berries.

“We like to think that culture makes us different as a species,” Psouni said. “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.”

She notes, however, that the results say nothing about what humans today should or should not eat.

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