CrossMark COMMENTARY COMMENTARY Thank your intelligent mother for your big brain Holly M.Dunsworth31 Inventors,artists,and scientists are the usual suspects artifacts indicate that behavioral complexity did too for symbolizing and celebrating the brainy human pri- These hallmarks of humanity could have been ratcheted mate.However,what if babies,mothers,and other up accordingly. caregivers were the real stars in the story of human This spin on well-known patterns of variation and intelligence?That possibility is one implication of a development among primates and fossil hominins recent study in PNAS from Piantadosi and Kidd(1). raises questions specific to the study and beyond, Among primates,greater adult brain size and new and old.For instance,what renders human babies behavioral complexity are correlated with heightened helpless?Is it all just relative brain size?Why must an offspring dependency,which are all exaggerated in increase in adult brain size require a decrease in humans.Scientists have long emphasized the signifi- neonatal brain size?Finally,is human parenthood cance of the coevolution of those traits in humans (2-4), more intelligent than parenthood in other primates? and Piantadosi and Kidd have now provided new insight regarding how that coevolution occurred. Baby Brains and Building Them First,they modeled their assumptions of hominin Today,neonatal humans have the largest absolute evolution:Those individuals with larger heads grow brain and overall body mass for primates and are born faster after birth than those individuals with smaller after a longer than expected gestation for a mother heads,those individuals with larger heads have a primate of our body size (5).So,the notion that hu- greater drop-off in survival as gestation lengthens, mans are born early is not supported by matemal in- and those individuals with larger heads have a vestment.Most often,the claim that human babies are higher probability of survival throughout develop- underdeveloped is based on their relative brain size ment because of greater parental intelligence. Because adult humans are so encephalized,human Then,Piantadosi and Kidd (1)built those assump- babies have the smallest relative brain size of all of the tions into an evolutionary fitness landscape in which a primates.With only about 30%of brain growth child's probability of survival to reproductive age was achieved at the time of birth,humans experience highest in two regions:one where pregnancy is long more brain maturation while under the care of others and neonatal heads are small and another where preg- than our closest relatives do.With a gestation length nancy is short and neonatal heads are large.The latter nearly as long as ours,chimpanzees have the next phenomenon fits with the hypothesis that larger smallest relative newborn brain among primates at brained hominin species bore their infants earlier in only about 40%,and they too are burdens on their development.So,based on the models,Piantadosi intelligent caregivers.Capuchin monkeys,known to and Kidd(1)provide a scenario for the evolution of be quite brainy,are born with only 50%of their adult human intelligence and infant dependency.They are brain mass and are notably needy as infants as well, inevitable adaptations to one another.This type of lagging in thermoregulation,for example.So,regard- "runaway"selection would have occurred if natural less of whether it is fair to say that humans are born selection for big,intelligent adult brains meant that "early,"the link that Piantadosi and Kidd (1)make hominin babies were born with relatively small brains between relative brain size at birth and intensity of and,because this diminished brain size rendered parenthood is a fair one.However,is the neonatal them more dependent,they benefited from the care brain the entire cause of human offspring neediness? of intelligent,big-brained hominin parents who had The loss of the grasping foot must have played a even smaller brained babies,and so on.Starting as role in the evolution of hominin parental care because early as the genus Australopithecus and then gradually it would have limited an infant's ability to cling,es- over the roughly 2.5 million y of the fossil record for the pecially to an upright standing,walking,and running genus Homo,adult hominin brain size increased and mother.Nonhuman primate mothers count on their "Department of Sociology and Anthropology,University of Rhode Island,Kingston,RI 02881 Author contributions:H.M.D.wrote the paper. The author declares no conflict of interest. See companion article on page 6874. Email:holly_dunsworth@uri.edu. 6816-68181PNAS1June21,20161vol.1131no.25 www.pnas.org/cgi/doi/10.1073/pnas.1606596113
COMMENTARY Thank your intelligent mother for your big brain Holly M. Dunswortha,1 Inventors, artists, and scientists are the usual suspects for symbolizing and celebrating the brainy human primate. However, what if babies, mothers, and other caregivers were the real stars in the story of human intelligence? That possibility is one implication of a recent study in PNAS from Piantadosi and Kidd (1). Among primates, greater adult brain size and behavioral complexity are correlated with heightened offspring dependency, which are all exaggerated in humans. Scientists have long emphasized the significance of the coevolution of those traits in humans (2–4), and Piantadosi and Kidd have now provided new insight regarding how that coevolution occurred. First, they modeled their assumptions of hominin evolution: Those individuals with larger heads grow faster after birth than those individuals with smaller heads, those individuals with larger heads have a greater drop-off in survival as gestation lengthens, and those individuals with larger heads have a higher probability of survival throughout development because of greater parental intelligence. Then, Piantadosi and Kidd (1) built those assumptions into an evolutionary fitness landscape in which a child’s probability of survival to reproductive age was highest in two regions: one where pregnancy is long and neonatal heads are small and another where pregnancy is short and neonatal heads are large. The latter phenomenon fits with the hypothesis that larger brained hominin species bore their infants earlier in development. So, based on the models, Piantadosi and Kidd (1) provide a scenario for the evolution of human intelligence and infant dependency. They are inevitable adaptations to one another. This type of “runaway” selection would have occurred if natural selection for big, intelligent adult brains meant that hominin babies were born with relatively small brains and, because this diminished brain size rendered them more dependent, they benefited from the care of intelligent, big-brained hominin parents who had even smaller brained babies, and so on. Starting as early as the genus Australopithecus and then gradually over the roughly 2.5 million y of the fossil record for the genus Homo, adult hominin brain size increased and artifacts indicate that behavioral complexity did too. These hallmarks of humanity could have been ratcheted up accordingly. This spin on well-known patterns of variation and development among primates and fossil hominins raises questions specific to the study and beyond, new and old. For instance, what renders human babies helpless? Is it all just relative brain size? Why must an increase in adult brain size require a decrease in neonatal brain size? Finally, is human parenthood more intelligent than parenthood in other primates? Baby Brains and Building Them Today, neonatal humans have the largest absolute brain and overall body mass for primates and are born after a longer than expected gestation for a mother primate of our body size (5). So, the notion that humans are born early is not supported by maternal investment. Most often, the claim that human babies are underdeveloped is based on their relative brain size. Because adult humans are so encephalized, human babies have the smallest relative brain size of all of the primates. With only about 30% of brain growth achieved at the time of birth, humans experience more brain maturation while under the care of others than our closest relatives do. With a gestation length nearly as long as ours, chimpanzees have the next smallest relative newborn brain among primates at only about 40%, and they too are burdens on their intelligent caregivers. Capuchin monkeys, known to be quite brainy, are born with only 50% of their adult brain mass and are notably needy as infants as well, lagging in thermoregulation, for example. So, regardless of whether it is fair to say that humans are born “early,” the link that Piantadosi and Kidd (1) make between relative brain size at birth and intensity of parenthood is a fair one. However, is the neonatal brain the entire cause of human offspring neediness? The loss of the grasping foot must have played a role in the evolution of hominin parental care because it would have limited an infant’s ability to cling, especially to an upright standing, walking, and running mother. Nonhuman primate mothers count on their a Department of Sociology and Anthropology, University of Rhode Island, Kingston, RI 02881 Author contributions: H.M.D. wrote the paper. The author declares no conflict of interest. See companion article on page 6874. 1 Email: holly_dunsworth@uri.edu. 6816–6818 | PNAS | June 21, 2016 | vol. 113 | no. 25 www.pnas.org/cgi/doi/10.1073/pnas.1606596113 COMMENTARY
ability to remain hands-free while carrying their dependent infant(s) from traditional societies.In addition,that shorter time to for months to years,and grasping infant feet are part of this weaning is marked by fast-paced infant growth and a high cost equation.According to fossil footprints in Tanzania,grasping feet of lactation.However,to really compare human dependence were gone from part of the hominin clade by 3.6 million y ago with the dependence of nonhuman primates,we need to build in This is the genus Australopithecus,which may have also birthed the time and cost of growing children after they are weaned, large infants(6).A big,heavy baby ups the parental ante,both as something that is comparatively absent in nonhuman primates. an organism to nourish with milk and as a load to carry,and that is Both the human brand of offspring dependency and intelligence even when it has clingy feet.So,following Piantadosi and Kidd's make direct comparisons with nonhuman primates difficult.Thus, arguments(1),it may not be coincidence that adult hominin brain much of what links the runaway scenario to the supporting evidence size took its first steps toward remarkability (suggesting that in- is the untested assumption that human parental intelligence is es- telligence did too)around this time in the Pliocene epoch when pecially important.To explore this issue further,we can continue hominin infants may have become more costly. our consideration of the costliness of human babies The evolution of hominin parental behavior was surely more Whether the trend actually began with Australopithecus or complicated than the evolution of neonatal brain size alone.However with early Homo,human offspring are exceptionally large.As we are still left wondering why offspring independence and brain size should decrease just because adult brain size increases. In support of the runaway hypothesis,Piantadosi The most prominent hypothesis for a limit on human neonatal and Kidd plot nonhuman primate intelligence brain size is the "obstetrical dilemma,"whereby the birth canal constrains fetal growth because it is limited by anatomical adap- and time to weaning,showing that the former tations for bipedal locomotion.Thus,it is often assumed that the predicts the latter,and that as one increases,so male pelvis,which is narrower in the dimensions that make up the does the other. birth canal,is better adapted to bipedalism.Observations of the difficulty of human childbirth lead many to hypothesize that discussed already,for a primate of our body size,humans are born the bipedal pelvis was a unique selective pressure on fetal brain after a long gestation,are remarkably fat,and are larger than size while brains were expanding in hominin history.However, expected in body and brain size.It is likely that pregnant mothers there is another,nonpelvic explanation for why neonatal brain size are able to endure this costly experience,at least partly,because decreases when adult brain size does. humans have a higher basal metabolic rate and expend more Unlike humans,our closest relatives,chimpanzees,do not give energy per day than other apes [about 400 kcal/d more than birth when the fetal cranium is approaching the limits of the birth chimpanzees and bonobos(7).This heightened metabolism may canal.However,they have the next smallest relative brain size at factor into both parties in the human lactation relationship too, birth among primates.So,if the pelvis is not limiting chimpanzee given that the fastest brain growth rate occurs during the first 3 y gestation and fetal growth,it is possible that something else is, of life(8)while babies are gaining calories,fat,and many other and it is possible that humans share it with our evolutionary kin.It important factors from mother's milk before weaning.Such a high is difficult to measure,so it remains to be learned whether it works cost of a rapidly growing infant brain is likely contributing to our this way in chimpanzees or other primates,but humans give birth early weaning relative to other apes(9),despite additional re- just before fetal energetic demands outstrip a mother's metabolic sources available to and from human mothers.What is more,the ceiling-the sustainable limit to her physiology(5).So,although a high-energy human condition,buffered by enhanced fat de- mother increases her basal metabolic rate during her pregnancy, position,may support the physical costs of carrying absolutely she reaches a point where she cannot continue to increase it big,heavy babies.It may also support the excessive costs of further to accommodate any more fetal growth,especially meta- provisioning weaned children who are too immature in their bolically expensive brain tissue.If metabolism is the fundamental musculoskeletal and cognitive development to forage for and constraint on pregnancy,then birth canal dimensions need only process food entirely for themselves(10). remain adequate for childbirth. Childhood's slow period of postweaning growth is argued to So,if brains are just too metabolically or energetically costly to be a hallmark biocultural adaptation in humans because it eases a increase in utero,then at a certain point,encephalization would mother's burden and allows her to invest relatively sooner in her occur postnatally.This pattern is what we see:The larger the next costly infant,and because factors like kinship and marriage mother's brain,the smaller is the fraction of hers that emerges are significant factors in childcare (4).These and other benefits from her womb.So,whether one applies the pelvic or the meta- and complexities to a long period of juvenile dependency are bolic explanation,or both,for neonatal brain size,one is left missing from the runaway scenario,but they should not be ignored. wondering whether a runaway scenario focused on parenting The accelerated human metabolism likely fuels the big pa- is necessary,given the many existing hypotheses for hominin rental brain (7),which is associated with significant diet-related encephalization. behavioral shifts during hominin evolutionary history.Processing (or predigesting)foods with stone tools(11),increasing acquisition Parents'Brains and Using Them of fat-and protein-rich animals in the diet(12),and cooking(13) In support of the runaway hypothesis,Piantadosi and Kidd(1)plot may have been the most beneficial to the lives of the youngest, nonhuman primate intelligence and time to weaning,showing most dependent,and most vulnerable members of hominin so- that the former predicts the latter,and that as one increases,so cieties,while being the most beneficial to their parents'and re- does the other.So,among nonhuman primates,caring for off- lated caregivers'fitness.It is commonly assumed that these spring appears to be a brainy activity.However,although time to technological and ecological behavioral shifts arose in conjunc- weaning is a sound measure of infant dependency,Homo sapiens tion with enhanced cognitive ability.What is more,the highly deviates from the pattern by weaning infants early,not late,for social and emotional nature of human reproduction between an ape of our body size,between 2.5 and 3 y as estimated child and caregiver and also between caregivers,a situation often Dunsworth PNAS I June 21,2016 I vol.113 I no.25 I 6817
ability to remain hands-free while carrying their dependent infant(s) for months to years, and grasping infant feet are part of this equation. According to fossil footprints in Tanzania, grasping feet were gone from part of the hominin clade by 3.6 million y ago. This is the genus Australopithecus, which may have also birthed large infants (6). A big, heavy baby ups the parental ante, both as an organism to nourish with milk and as a load to carry, and that is even when it has clingy feet. So, following Piantadosi and Kidd’s arguments (1), it may not be coincidence that adult hominin brain size took its first steps toward remarkability (suggesting that intelligence did too) around this time in the Pliocene epoch when hominin infants may have become more costly. The evolution of hominin parental behavior was surely more complicated than the evolution of neonatal brain size alone. However, we are still left wondering why offspring independence and brain size should decrease just because adult brain size increases. The most prominent hypothesis for a limit on human neonatal brain size is the “obstetrical dilemma,” whereby the birth canal constrains fetal growth because it is limited by anatomical adaptations for bipedal locomotion. Thus, it is often assumed that the male pelvis, which is narrower in the dimensions that make up the birth canal, is better adapted to bipedalism. Observations of the difficulty of human childbirth lead many to hypothesize that the bipedal pelvis was a unique selective pressure on fetal brain size while brains were expanding in hominin history. However, there is another, nonpelvic explanation for why neonatal brain size decreases when adult brain size does. Unlike humans, our closest relatives, chimpanzees, do not give birth when the fetal cranium is approaching the limits of the birth canal. However, they have the next smallest relative brain size at birth among primates. So, if the pelvis is not limiting chimpanzee gestation and fetal growth, it is possible that something else is, and it is possible that humans share it with our evolutionary kin. It is difficult to measure, so it remains to be learned whether it works this way in chimpanzees or other primates, but humans give birth just before fetal energetic demands outstrip a mother’s metabolic ceiling—the sustainable limit to her physiology (5). So, although a mother increases her basal metabolic rate during her pregnancy, she reaches a point where she cannot continue to increase it further to accommodate any more fetal growth, especially metabolically expensive brain tissue. If metabolism is the fundamental constraint on pregnancy, then birth canal dimensions need only remain adequate for childbirth. So, if brains are just too metabolically or energetically costly to increase in utero, then at a certain point, encephalization would occur postnatally. This pattern is what we see: The larger the mother’s brain, the smaller is the fraction of hers that emerges from her womb. So, whether one applies the pelvic or the metabolic explanation, or both, for neonatal brain size, one is left wondering whether a runaway scenario focused on parenting is necessary, given the many existing hypotheses for hominin encephalization. Parents’ Brains and Using Them In support of the runaway hypothesis, Piantadosi and Kidd (1) plot nonhuman primate intelligence and time to weaning, showing that the former predicts the latter, and that as one increases, so does the other. So, among nonhuman primates, caring for offspring appears to be a brainy activity. However, although time to weaning is a sound measure of infant dependency, Homo sapiens deviates from the pattern by weaning infants early, not late, for an ape of our body size, between 2.5 and 3 y as estimated from traditional societies. In addition, that shorter time to weaning is marked by fast-paced infant growth and a high cost of lactation. However, to really compare human dependence with the dependence of nonhuman primates, we need to build in the time and cost of growing children after they are weaned, something that is comparatively absent in nonhuman primates. Both the human brand of offspring dependency and intelligence make direct comparisons with nonhuman primates difficult. Thus, much of what links the runaway scenario to the supporting evidence is the untested assumption that human parental intelligence is especially important. To explore this issue further, we can continue our consideration of the costliness of human babies. Whether the trend actually began with Australopithecus or with early Homo, human offspring are exceptionally large. As In support of the runaway hypothesis, Piantadosi and Kidd plot nonhuman primate intelligence and time to weaning, showing that the former predicts the latter, and that as one increases, so does the other. discussed already, for a primate of our body size, humans are born after a long gestation, are remarkably fat, and are larger than expected in body and brain size. It is likely that pregnant mothers are able to endure this costly experience, at least partly, because humans have a higher basal metabolic rate and expend more energy per day than other apes [about 400 kcal/d more than chimpanzees and bonobos (7)]. This heightened metabolism may factor into both parties in the human lactation relationship too, given that the fastest brain growth rate occurs during the first 3 y of life (8) while babies are gaining calories, fat, and many other important factors from mother’s milk before weaning. Such a high cost of a rapidly growing infant brain is likely contributing to our early weaning relative to other apes (9), despite additional resources available to and from human mothers. What is more, the high-energy human condition, buffered by enhanced fat deposition, may support the physical costs of carrying absolutely big, heavy babies. It may also support the excessive costs of provisioning weaned children who are too immature in their musculoskeletal and cognitive development to forage for and process food entirely for themselves (10). Childhood’s slow period of postweaning growth is argued to be a hallmark biocultural adaptation in humans because it eases a mother’s burden and allows her to invest relatively sooner in her next costly infant, and because factors like kinship and marriage are significant factors in childcare (4). These and other benefits and complexities to a long period of juvenile dependency are missing from the runaway scenario, but they should not be ignored. The accelerated human metabolism likely fuels the big parental brain (7), which is associated with significant diet-related behavioral shifts during hominin evolutionary history. Processing (or predigesting) foods with stone tools (11), increasing acquisition of fat- and protein-rich animals in the diet (12), and cooking (13) may have been the most beneficial to the lives of the youngest, most dependent, and most vulnerable members of hominin societies, while being the most beneficial to their parents’ and related caregivers’ fitness. It is commonly assumed that these technological and ecological behavioral shifts arose in conjunction with enhanced cognitive ability. What is more, the highly social and emotional nature of human reproduction between child and caregiver and also between caregivers, a situation often Dunsworth PNAS | June 21, 2016 | vol. 113 | no. 25 | 6817
referred to as communal or cooperative breeding,would benefit benefit survival if they were not born into such a handy and from an energy-fueled brain,including its role in the development intelligent species. of language(14).It is too easy to emphasize a baby's deficient Given the large literature dedicated to the territory covered motor skills,and thus to forget how intelligent and unlike other here,Piantadosi and Kidd's powerful scenario(1)is probably too primates they are from the very earliest moments of life.Al- simple to depict the complex evolutionary processes that brought though they are relatively small,neonatal human brains are the us big brains,intelligence,and costly babies.Regardless,their absolute largest for primates,as previously stated.So,if brain research underscores the importance of child-rearing in the size is linked to intelligence in adults,why not in babies? evolution of humankind,an importance that is often overlooked. Through gaze,facial expressions,gestures,and more,human Likewise,the work that goes into raising children is woefully infants and young children manipulate parents and other care- undervalued both socially and economically in the United States givers into investing so carefully and intensely (2).Perhaps It is unlikely that an evolutionary appreciation for childcare will hominin babies have cleverly manipulated their intelligent care- lead to massive culture and societal change,but such change givers into relaxing selection on many of the traits that would may arise by a slow and gradual process. 1 Piantadosi ST,Kidd C(2016)Extraordinary intelligence and the care of infants.Proc Natl Acad Sci USA 113:6874-6879. 2 Hrdy S(2009)Mothers and Others (Harvard Univ Press,Cambridge,MA) 3 Trevathan W,Rosenberg K,eds(2016)Costly and Cute:Helpless Infants and Human Evolution(School for Advanced Research Press,Santa Fe,NM). 4 Bogin B,Bragg J,Kuzawa C(2016)Childhood,biocultural reproduction,and human lifetime reproductive effort.Childhood:Origins,Evolution and Implications, eds Meehan CL,Crittenden AN (School for Advanced Research Press,Santa Fe,NM). 5Dunsworth HM,Warener AG,Deacon T,Ellison PT,Pontzer H(2012)Metabolic hypothesis for human altriciality.Proc Natl Acad Sci USA 109(38):15212-15216. 6 DeSilva JM(2011)A shift toward birthing relatively large infants early in human evolution.Proc Natl Acad Sci USA 108(3):1022-1027. 7 Pontzer H,et al.(2016)Metabolic acceleration and the evolution of human brain size and life history.Nature 533:390-392 8 Leigh SR(2004)Brain growth,life history,and cognition in primate and human evolution.Am J Primatol 62(3):139-164. 9Kennedy GE(2005)From the ape's dilemma to the weanling's dilemma:Early weaning and its evolutionary context.JHum Evol 48(2):123-145. 10 Humphrey LT (2010)Weaning behaviour in human evolution.Semin Cell Dev Biol 21(4):453-461. 11 Zink KD,Lieberman DE (2016)Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans.Nature 531(7595):500-503. 12 Ferraro JV,et al.(2013)Earliest archaeological evidence of persistent hominin carnivory.PLoS One 8(4):e62174. 13 Groopman EE,Carmody RN.Wrangham RW(2015)Cooking increases net energy gain from a lipid-rich food.Am JPhys Anthropol 156(1):11-18. 14 Falk D(2016)Evolution of brain and culture:The neurological and cognitive joumey from Australopithecus to Albert Einstein.J Anthropol Sci 94:1-14. 6818 I www.pnas.org/cgi/doi/10.1073/pnas.1606596113 Dunsworth
referred to as communal or cooperative breeding, would benefit from an energy-fueled brain, including its role in the development of language (14). It is too easy to emphasize a baby’s deficient motor skills, and thus to forget how intelligent and unlike other primates they are from the very earliest moments of life. Although they are relatively small, neonatal human brains are the absolute largest for primates, as previously stated. So, if brain size is linked to intelligence in adults, why not in babies? Through gaze, facial expressions, gestures, and more, human infants and young children manipulate parents and other caregivers into investing so carefully and intensely (2). Perhaps hominin babies have cleverly manipulated their intelligent caregivers into relaxing selection on many of the traits that would benefit survival if they were not born into such a handy and intelligent species. Given the large literature dedicated to the territory covered here, Piantadosi and Kidd’s powerful scenario (1) is probably too simple to depict the complex evolutionary processes that brought us big brains, intelligence, and costly babies. Regardless, their research underscores the importance of child-rearing in the evolution of humankind, an importance that is often overlooked. Likewise, the work that goes into raising children is woefully undervalued both socially and economically in the United States. It is unlikely that an evolutionary appreciation for childcare will lead to massive culture and societal change, but such change may arise by a slow and gradual process. 1 Piantadosi ST, Kidd C (2016) Extraordinary intelligence and the care of infants. Proc Natl Acad Sci USA 113:6874–6879. 2 Hrdy S (2009) Mothers and Others (Harvard Univ Press, Cambridge, MA). 3 Trevathan W, Rosenberg K, eds (2016) Costly and Cute: Helpless Infants and Human Evolution (School for Advanced Research Press, Santa Fe, NM). 4 Bogin B, Bragg J, Kuzawa C (2016) Childhood, biocultural reproduction, and human lifetime reproductive effort. Childhood: Origins, Evolution and Implications, eds Meehan CL, Crittenden AN (School for Advanced Research Press, Santa Fe, NM). 5 Dunsworth HM, Warrener AG, Deacon T, Ellison PT, Pontzer H (2012) Metabolic hypothesis for human altriciality. Proc Natl Acad Sci USA 109(38):15212–15216. 6 DeSilva JM (2011) A shift toward birthing relatively large infants early in human evolution. Proc Natl Acad Sci USA 108(3):1022–1027. 7 Pontzer H, et al. (2016) Metabolic acceleration and the evolution of human brain size and life history. Nature 533:390–392. 8 Leigh SR (2004) Brain growth, life history, and cognition in primate and human evolution. Am J Primatol 62(3):139–164. 9 Kennedy GE (2005) From the ape’s dilemma to the weanling’s dilemma: Early weaning and its evolutionary context. J Hum Evol 48(2):123–145. 10 Humphrey LT (2010) Weaning behaviour in human evolution. Semin Cell Dev Biol 21(4):453–461. 11 Zink KD, Lieberman DE (2016) Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans. Nature 531(7595):500–503. 12 Ferraro JV, et al. (2013) Earliest archaeological evidence of persistent hominin carnivory. PLoS One 8(4):e62174. 13 Groopman EE, Carmody RN, Wrangham RW (2015) Cooking increases net energy gain from a lipid-rich food. Am J Phys Anthropol 156(1):11–18. 14 Falk D (2016) Evolution of brain and culture: The neurological and cognitive journey from Australopithecus to Albert Einstein. J Anthropol Sci 94:1–14. 6818 | www.pnas.org/cgi/doi/10.1073/pnas.1606596113 Dunsworth