Carnivory is behind the evolutionary success of humankind. When early humans started to eat meat and eventually hunt, their new, higher-quality diet meant that women could wean their children earlier. Women could then give birth to more children during their reproductive life, which is a possible contribution to the population gradually spreading over the world.
The connection between eating meat and a faster weaning process is shown by a research group from Lund University in Sweden, which compared close to 70 mammalian species and found clear patterns.
Learning to hunt was a decisive step in human evolution. Hunting necessitated communication, planning and the use of tools, all of which demanded a larger brain. At the same time, adding meat to the diet made it possible to develop this larger brain.
"This has been known for a long time. However, no one has previously shown the strong connection between meat eating and the duration of breast-feeding, which is a crucial piece of the puzzle in this context. 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", says Elia Psouni of Lund University.
She is a developmental psychologist and has, together with neurophysiologist Martin Garwicz (also in Lund) and evolutionary geneticist Axel Janke (currently in Frankfurt but previously in Lund) published her findings in the journal PLoS ONE.
Among natural fertility societies, the average duration of breast-feeding is 2 years and 4 months. This is not much in relation to the maximum lifespan of our species, around 120 years. It is even less if compared to our closest relatives: female chimpanzees suckle their young for 4-5 years, whereas the maximum lifespan for chimpanzees is only 60 years.
Many researchers have tried to explain the relatively shorter breast-feeding period of humans based on social and behavioral theories of parenting and family size. But the Lund group has now shown that humans are in fact no different than other mammals with respect to the timing of weaning. If you enter brain development and diet composition into the equation, the time when our young stop suckling fits precisely with the pattern in other mammals.
This is the type of mathematical model that Elia Psouni and her colleagues have built. They entered data on close to 70 mammalian species of various types into the model - data on brain size and diet. Species for which at least 20 per cent of the energy content of their diet comes from meat were categorised as carnivores.
The model shows that the young of all species cease to suckle when their brains have reached a particular stage of development on the path from conception to full brain-size. Carnivores, due to their high quality diet, can wean earlier than herbivores and omnivores.
The model also shows that humans do not differ from other carnivores with respect to timing of weaning. All carnivorous species, from small animals such as ferrets and raccoons to large ones like panthers, killer whales and humans, have a relatively short breast-feeding period. The difference between us and the great apes, which has puzzled previous researchers, seems to depend merely on the fact that as a species we are carnivores, whereas gorillas, orangutans and chimpanzees are herbivores or omnivores.
A few years ago, the Lund group published an acclaimed study on the point at which the young of various animals start to walk. Here too, similar patterns were discovered between mammalian species that diverged in evolution millions of years ago. A particular stage in brain development seems quite simply to be the time to start to walk, independently of whether you are a hedgehog, a ferret or a human being.
"That humans seem to be so similar to other animals can of course be taken as provocative. 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. Social and cultural factors surely influence the variation between humans", says Elia Psouni.
She is careful to emphasise that their results concern human evolution. The research is about how carnivory can have contributed to the human species' spreading on earth and says nothing about what we should or should not eat today in order to have a good diet.
The article is entitled "Impact of carnivory on human development and evolution revealed by a new unifying model of weaning in mammals". It is published in the open access-journal PLoS ONE which is issued by the Public Library of Science, and can be accessed at www.plosone.org.
Elia Psouni can be reached by telephone 0046-46 2228503, 0046-733 14 28 76 and by email Elia.Psouni@med.lu.se