Eating meat was the key to expanding the human race because the "higher quality" diet of early carnivorous females allowed them to stop breastfeeding earlier and produce more children, according to Swedish scientists.
The offspring of all species stop breastfeeding when their brain reaches a particular developmental stage, and in carnivores this point is reached much faster than herbivores or omnivores.
Studies of natural fertility societies -- where birth control is not used -- found the average duration of breastfeeding is two years and four months, compared to four to five years for chimpanzees, our closest relatives.
Previous studies have suggested that carnivores wean earlier than herbivores and omnivores because of differences in social or behavioral patterns, but researchers from Lund University believe that meat-eating is the key.
Lead researcher Dr. Elia Psouni said, "Eating meat enabled the breast-feeding periods and thereby the time between births to be shortened. This must have had a crucial impact on human evolution."
She added, "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."
The research, published in the scientific journal PLoS ONE, examined 67 mammal species, including humans, apes, mice, and killer whales.
The researchers emphasized that their findings related to early humans populating the earth and did not impact on what should or should not be eaten by modern humans.