Boy cooks a steak (Photo: woodleywonderworks via Flickr/Creative Commons)

Boy checks on his steak (Photo: woodleywonderworks via Flickr/Creative Commons)

Vegetarians and animal right groups may take issue with this, but a new study has revealed that when humans started to include meat in their diets, mothers were able to wean their children earlier, which allowed them to have more children, which in turn helped the human species spread throughout the world.

Researchers from Lund University in Sweden, conducting a comparison study of 67 mammalian species that included humans, apes, mice, and killer whales and others, say they found a solid connection between meat eating and shorter periods of weaning their offspring.

“Eating meat enabled the breast-feeding periods and thereby the time between births to be shortened,” said Elia Psouni, a clinical psychologist and the study’s lead author. “This must have had a crucial impact on human evolution.”

Compared to the mostly plant-eating chimpanzee, man’s closest evolutionary cousin, the study showed that humans tend to breastfeed their young for a shorter average duration, despite having nearly twice the maximum lifespan. Female chimpanzees breastfeed their young for four-to-five years, but humans only do so for an average of two years and four months.

Over the years, a number of researchers tied the human’s relatively shorter breastfeeding period to other aspects such as social and behavioral theories of parenting along with family size.

But this new research shows that breastfeeding for all species generally ends once the brains of the young reach a certain stage of development.

For their study, the Swedish researchers categorized the species into three categories: carnivores, or species whose diets take at least 20 percent of their energy content from meat, herbivores, or species whose diets only consist of plant life, and omnivores, or species who take their nourishment from both plant life and meat. For this study, the authors considered humans carnivores, despite that human diets often consist of both plant life and meat.

In comparing the weaning times for humans to the great apes such as gorillas, orangutans and chimpanzees, the researchers say that the differences seem to come down to the differences in diet composition.

“That humans seem to be so similar to other animals can of course be taken as provocative,” Psouni says. “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 did emphasize that the results of her study only provided insight into how the addition of meat to their diets may have contributed to early humans spreading on Earth, and that it doesn’t advocate whether humans should or should not eat meat.

The study is published in the journal PLoS ONE.