Biological anthropology researchers in СƬƵ’s College of Arts and Sciences have again shed new light on the very old topic of human origins.
In two new journal articles appearing this week in the Proceedings of the National Academy of Sciences, the researchers report likely explanations for the evolution of human social behavior and unparalleled intelligence. The human lineage is characterized by remarkable demographic success relative to our nearest relatives and by advanced social traits such as language, empathy, and altruism.
Explaining how human social behavior could have evolved through improving individual reproductive success has, until now, been difficult. However, the СƬƵ scientists led by Mary Ann Raghanti, Ph.D., compared neurochemical profiles in the striatum, a brain region that modulates social behavior, among humans, chimpanzees, gorillas, and monkeys and found a unique profile in humans. The levels of striatal dopamine and acetylcholine were essentially reversed in humans compared to other primates, and humans have dramatically increased dopamine levels. The human profile is consistent with exquisite sensitivity to social cues, social conformity, and reduced within-group aggression. Essentially, this shows that there has been a long history of intense selection for an extremely socially sensitive personality style, one end of the personality continuum. This “dopamine-dominated striatum” personality could have encouraged male provisioning and monogamy in early hominids, which according to the authors would have improved female and offspring survival.
This observation is in perfect accord with evidence about earliest humans provided by Ardipithecus ramidus (“Ardi”), which is about 4.4 million years old and probably lies near the origin of the human lineage. “Ardi” shows two special characters that make it differ profoundly from any other primate. Males show dramatic reduction of the sectorial (“slashing”) canine tooth—sometimes referred to as the “social tooth” because it plays a prominent role in communicating aggression in other primates. Its reduction in “Ardi” signals pronounced reduction in aggression among males, consistent with a dopamine-dominated striatum personality. Ardi also shows early evidence for upright walking, which could have played a prominent role in male provisioning of females and contributed to the success of the species. This would also be fully consistent with the dopamine-dominated striatum personality style because it would reflect both pair bonding and social cue sensitivity.
In a related study led by Richard S. Meindl, Ph.D., professor of anthropology at СƬƵ, the researchers examined the mortality and fertility of macaques, the most demographically successful primates after humans. They determined that a key to macaques’ reproductive success was elevated female survivorship, which is fully consistent with the changes in social structure likely produced by the dopamine-dominated striatum personality style. They suggest that a socially monogamous lifestyle would have elevated female life expectancy in early and later hominids.
“Humans are characterized by remarkable demographic success relative to our nearest relatives and by advanced social traits such as language, empathy, and altruism,” said Owen Lovejoy, Distinguished Professor of Human Evolutionary Studies at СƬƵ and one of the authors of the two studies. “Explaining how human social behavior could have evolved through improving individual reproductive success has been difficult.”
Modern chimpanzees and humans shared their last common ancestor somewhere between 7 and 9 million years ago. Chimpanzees and humans differ extraordinarily in their social behavior and also, these authors now report, in their striatum. This brain region contributes substantially to “personality style” and social behaviors. Early human ancestors had chimpanzee-sized brains, and this change in neurochemistry was able to have a dramatic impact in the absence of brain expansion.
“This difference in neurochemistry, when viewed from the perspective of the human fossil record, is likely to lie at the root of hominid origins, and to have been responsible for their early unqualified success,” Raghanti said.