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ScienceDaily, April 20, 2012
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 emphasize 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.
Story Source:
The above story is reprinted from materials provided by Lund University, via AlphaGalileo.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- Elia Psouni, Axel Janke, Martin Garwicz. Impact of Carnivory on Human Development and Evolution Revealed by a New Unifying Model of Weaning in Mammals. PLoS ONE, 2012; 7 (4): e32452 DOI:10.1371/journal.pone.0032452
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