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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.

Elia Psouni »

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.

Elia Pounsi 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 above news item is reprinted from mate­ri­als avail­able at Lund Uni­ver­sity via Alpha­Galileo. Orig­i­nal text may be edited for con­tent and length.

(Source: Lund Uni­ver­sity, 20.04.2012)