"Long before it's in the papers"
June 04, 2013


It’s true: cooking may have given us our big brains, study says

Oct. 22, 2012
Courtesy of PNAS
and World Science staff

New da­ta sup­port a the­o­ry that the in­ven­tion of cook­ing trig­gered a great in­crease in hu­man brain size, help­ing mak­ing us the in­tel­li­gent spe­cies we are to­day, re­search­ers have an­nounced.

The “cook­ing made us hu­man” hy­poth­e­sis has been cham­pi­oned by Har­vard bi­o­log­i­cal an­thro­po­l­o­gist Rich­ard Wrang­ham, who ar­gues that cook­ing pro­vid­ed such a nu­tri­tional boost that it in­creased brain size and freed up time for many oth­er ac­ti­vi­ties, such as hunt­ing and so­cial life. These in turn fur­ther en­cour­aged in­creased in­tel­li­gence in our an­ces­tors, by put­ting more de­mands on the grow­ing brain.

In the new stu­dy, Ka­rina Fon­seca-Azevedo and Su­­za­na Her­cu­lano-Hou­zel of the Uni­ver­si­dade Fed­er­al do Ri­o de Janeiro, Bra­zil, supply find­ings that they say back up the idea. The re­search ap­pears in this week’s early on­line edi­tion of the jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

Sci­en­tists have rec­og­nized that there are trade­offs be­tween body size and the num­ber of brain cells, simply be­cause there are lim­its to how much food an an­i­mal can eat, Fon­seca-Azevedo and Her­cu­lano-Hou­zel said. What makes the trade­off even tougher is that brains con­sume a lot of en­er­gy. “The hu­man brain, in par­tic­u­lar, has come to cost [a­bout] 20 per­cent of the to­tal body rest­ing met­a­bol­ic rate, even though it rep­re­sents only 2 per­cent of to­tal bod­y” weight, they wrote.

The two re­search­ers said their own con­tri­bu­tion has been to supply ev­i­dence that as the early hu­man line­age evolved, this trade­off was ac­tu­ally rel­e­vant and thus could have in­flu­enced how things turned out.

Cook­ing food eases the trade­off sig­nif­i­cantly, al­low­ing an an­i­mal to have a much big­ger brains with­out a smaller body, they added. Pre­vi­ous re­search hs shown that “the ad­vent of cook­ing food… greatly in­creases the ca­lor­ic yield of the di­et, as a re­sult of the great­er ease of chew­ing, di­ges­tion, and ab­sorp­tion of foods.”

“Although the ear­li­er ad­di­tion of raw meat to the di­et of ear­li­er ho­min­ins [hu­man an­ces­tors] may al­so have con­tri­but­ed to in­crease its ca­lor­ic con­tent, raw meat is dif­fi­cult to chew and in­gest, where­as cooked meat is eas­i­er to chew and has a high­er ca­lor­ic yield,” they went on.

“Cook­ing would al­so have in­creased the time avail­a­ble for so­cial and more cog­ni­tively de­mand­ing ac­ti­vi­ties,” the in­ves­ti­ga­tors con­tin­ued. This would im­pose new ev­o­lu­tion­ary pres­sures for still big­ger brains, with more brain cells, “now af­ford­a­ble by the new di­et.”

The re­search­ers based their ar­gu­ment about the rel­e­vance of the brain-body trade­off in ev­o­lu­tion by build­ing a math­e­mat­i­cal mod­el show­ing how much en­er­gy in­take is re­quired for dif­fer­ent body weights and num­bers of brain cells. “This trade­off is par­tic­u­larly clear in an­i­mals the size of great apes,” they wrote.

The new mod­el of the brain-body trade­off was changed from pre­vi­ous as­sump­tions in part, they added, be­cause ear­li­er work sug­gested that while big­ger brain sizes re­quire in­creased me­tab­o­lism, that in­crease is smaller than the in­crease in brain size. This sup­po­si­tion turned out to be wrong, they said; in real­ity, the met­a­bol­ic cost of the brain in­creases in a more or less one-to-one rela­t­ion­ship with the num­ber of nerve cells in the brain.

The sci­en­tists al­so cit­ed stud­ies in­di­cat­ing the daily feed­ing time spent by sev­er­al early hu­man an­ces­tors was si­m­i­lar to that of mod­ern great apes in the wild. That sug­gests this feed­ing time—se­ven to eight hours—is about the most that an ape can af­ford to spend eat­ing. There­fore, these an­ces­tral crea­tures would pre­sumably not have been able to in­crease their brain sizes with­out ei­ther dras­tic­ally re­duc­ing body size, or in­vent­ing some oth­er ma­jor change such as cook­ing.

Wrang­ham es­ti­mates the ad­vent of cook­ing as oc­cur­ring about 1.8 mil­lion years ago. “While much re­mains to be dis­cov­ered… the adop­tion of cook­ing would have led to an im­por­tant rise in en­er­gy avail­abil­ity,” he wrote in an pa­per in the Oct. 2009 is­sue of the Jour­nal of Hu­man Ev­o­lu­tion, with Ra­chel Car­mody. “We pre­dict that cook­ing had sub­stanti­al ev­o­lu­tion­ary sig­nif­i­cance.”

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New data support a theory that the invention of cooking triggered a great increase in human brain size, helping making us the intelligent species we are today, researchers have announced. The “cooking made us human” hypothesis has been championed by Harvard biological anthropologist Richard Wrangham, who argues that cooking provided such a nutritional boost that it increased brain size and freed up time for many other activities, such as hunting and social life. These in turn further encouraged increased intelligence in our ancestors, by putting more demands on the growing brain. In the new study, Karina Fonseca-Azevedo and Suzana Herculano-Houzel of the Universidade Federal do Rio de Janeiro, Brazil, supply findings that they say back up the idea. The research appears in this week’s early online edition of the journal pnas. Scientists have recognized that there are tradeoffs between body size and the number of brain cells, simply because there are limits to how much food an animal can eat, Fonseca-Azevedo and Herculano-Houzel said. What makes the tradeoff even tougher is that brains consume a lot of energy. “The human brain, in particular, has come to cost [about] 20% of the total body resting metabolic rate, even though it represents only 2% of total body” weight, they wrote. The two researchers said their own contribution has been to supply evidence that as the early human lineage evolved, this tradeoff was actually relevant and thus could have influenced how things turned out. Cooking food eases the tradeoff significantly, allowing an animal to have a much bigger brains without a smaller body, they added. Previous research hs shown that “the advent of cooking food… greatly increases the caloric yield of the diet, as a result of the greater ease of chewing, digestion, and absorption of foods.” “Although the earlier addition of raw meat to the diet of earlier hominins may also have contributed to increase its caloric content, raw meat is difficult to chew and ingest, whereas cooked meat is easier to chew and has a higher caloric yield,” they went on. “Cooking would also have increased the time available for social and more cognitively demanding activities,” the investigators continued. This would impose new evolutionary pressures for still bigger brains, with more brain cells, “now affordable by the new diet.” The researchers based their argument about the relevance of the brain-body tradeoff in evolution by building a mathematical model showing how much energy intake is required for different body weights and numbers of brain cells. “This tradeoff is particularly clear in animals the size of great apes,” they wrote. The new model of the brain-body tradeoff was changed from previous assumptions in part, they added, because earlier work suggested that while bigger brain sizes require increased metabolism, that increase is smaller than the increase in brain size. This supposition turned out to be wrong, they said; in reality, the metabolic cost of the brain increases in a more or less one-to-one relationship with the number of nerve cells in the brain. The scientists also cited studies indicating the daily feeding time spent by several early human ancestors was similar to that of modern great apes in the wild. That suggests this feeding time—seven to eight hours—is about the most that an ape can afford to spend eating. Therefore, these ancestral creatures would presumably not have been able to increase their brain sizes without either drastically reducing body size, or inventing some other major change such as cooking. Wrangham estimates the advent of cooking as occuring about 1.8 million years ago. “While much remains to be discovered… the adoption of cooking would have led to an important rise in energy availability,” he wrote in an paper in the Oct. 2009 issue of the Journal of Human Evolution, with Rachel Carmody. “We predict that cooking had substantial evolutionary significance.”