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.1606596113COMMENTARY 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 pri￾mate. 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 signifi￾cance 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 develop￾ment because of greater parental intelligence. Then, Piantadosi and Kidd (1) built those assump￾tions 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 preg￾nancy 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 hu￾mans are born early is not supported by maternal in￾vestment. 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, regard￾less 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, es￾pecially 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