多巴胺帮助社交Dopamine gives humans our social edge

Dopamine may have given humans our social edge over other apes

 

  Ann Gibbons   a contributing correspondent for Science

http://www.sciencemag.org/news/2018/01/dopamine-may-have-given-humans-our-social-edge-over-other-apes

• Twitter
•  

 

Male chimpanzees signal their aggression when they display their big canines, in contrast with humans, who show small canines when they smile.

Sergey Uryadnikov/shutterstock.com

Humans are the ultimate social animals, with the ability to bond with mates, communicate through language, and make small talk with strangers on a packed bus. (Put chimpanzees in the same situation and most wouldn’t make it off the bus alive.) A new study suggests that the evolution of our unique social intelligence may have initially begun as a simple matter of brain chemistry.

Neuroanatomists have been trying for decades to find major differences between the brains of humans and other primates, aside from the obvious brain size. The human brain must have reorganized its chemistry and wiring as early human ancestors began to walk upright, use tools, and develop more complex social networks 6 million to 2 million years ago—well before the brain began to enlarge 1.8 million years ago, according to a hypothesis proposed in the 1960s by physical anthropologist Ralph Holloway of Columbia University. But neurotransmitters aren’t preserved in ancient skulls, so how to spot those changes?

One way is to search for key differences in neurochemistry between humans and other primates living today. Mary Ann Raghanti, a biological anthropologist at Kent State University in Ohio, and colleagues got tissue samples from brain banks and zoos of 38 individuals from six species who had died of natural causes: humans, tufted capuchins, pig-tailed macaques, olive baboons, gorillas, and chimpanzees. They sliced sections of basal ganglia—clusters of nerve cells and fibers in a region at the base of the brain known as the striatum, which is a sort of clearinghouse that relays signals from different parts of the brain for movement, learning, and social behavior. They stained these slices with chemicals that react to different types of neurotransmitters, including dopamine, serotonin, and neuropeptide Y—which are associated with sensitivity to social cues and cooperative behavior. Then, they analyzed the slices to measure different levels of neurotransmitters that had been released when the primates were alive.

Compared with other primates, both humans and great apes had elevated levels of serotonin and neuropeptide Y, in the basal ganglia. However, in line with another recent study on gene expression, humans had dramatically more dopamine in their striatum than apes, they report today in the Proceedings of the National Academy of Sciences. Humans also had less acetylcholine, a neurochemical linked to dominant and territorial behavior, than gorillas or chimpanzees. The combination “is a key difference that sets apart humans from all other species,” Raghanti says.

Those differences in neurochemistry may have set in motion other evolutionary changes, such as the development of monogamy and language in humans, theorizes Kent State paleoanthropologist Owen Lovejoy, a co-author. He proposes a new “neurochemical hypothesis for the origin of hominids,” in which females mated more with males who were outgoing, but not too aggressive. And males who cooperated well with other males may have been more successful hunters and scavengers. As human ancestors got better at cooperating, they shared the know-how for making tools and eventually developed language—all in a feedback loop fueled by surging levels of dopamine. “Cooperation is addictive,” Raghanti says.

Lovejoy thinks these neurochemical changes were already in place more than 4.4 million years ago, when Ardipithecus ramidus, an early member of the human family, lived in Ethiopia. Compared with chimpanzees, which display large canines when they bare their teeth in aggressive displays, A. ramidus males had reduced canines. That meant that when they smiled—like male humans today—they were likely signaling cooperation, Lovejoy says.

However, it’s a big leap to prove that higher levels of dopamine changed the evolution of human social behavior. The neurochemistry of the brain is so complex, and dopamine is involved in so many functions that it’s hard to know precisely why natural selection favored higher dopamine levels—or even whether it was a side effect of some other adaptation, says evolutionary geneticist Wolfgang Enard at Ludwig Maximilian University of Munich in Germany. But he says this painstaking research to quantify differences in neurochemistry among primates is important, especially as researchers study differences in gene expression in the brain. Raghanti agrees and is now writing a grant to study the brain tissue of bonobos.

多巴胺让人类占据社交优势

作者：宗华 来源：科学网 www.sciencenet.cn 发布时间：2018/1/23 19:18:48

 

当雄性黑猩猩露出大的獠牙时，这象征着侵略行为。相比之下，人类微笑时会露出小小的牙齿。

图片来源：Sergey Uryadnikov/shutterstock.com

人类是终极的社会性动物，拥有和伴侣结合、通过语言沟通以及在拥挤的公共汽车上同陌生人闲聊的能力。一项最新研究显示，人类独特的社会智力的进化可能开始于一种简单的大脑化学物质。

美国肯特州立大学生物人类学家Mary Ann Raghanti和同事收集了来自脑器官库和动物园的组织样本。其包括死于正常原因的6个物种的38个个体，其中6个物种分别是：人类、卷尾猴、豚尾猴、橄榄猴、大猩猩和黑猩猩。他们切下了部分基底核——位于脑基底部、被称为纹状体的神经细胞和纤维群。这里是一个交换所，负责传递大脑不同部分发出的运动、学习和社交行为信号。研究人员用对不同神经传导物质（包括多巴胺、血清素和神经肽Y）作出反应的化学物质将这些切片着色。上述化学物质均同对社交提示和合作行为的敏感度相关。随后，他们分析了这些切片，以衡量灵长类动物活着时释放的神经传导物质的水平。

研究人员在日前出版的美国《国家科学院院刊》上报告称，和猿类相比，人类在大脑纹状体中拥有更多的多巴胺。同时，人类拥有较少的乙酰胆碱——一种同统治欲和领地行为存在关联的神经化学物质。Raghanti表示，这种结合是“将人类同所有其他物种区分开来的关键差异”。

肯特州立大学古人类学家、最新研究共同作者Owen Lovejoy介绍说，这些神经化学上的差异可能调动起其他渐进式变化，比如人类的一夫一妻制和语言的发展。关于原始人类起源，Lovejoy提出了一种新的“神经化学假设”，即女性会更多地同外向而又没那么强势的男性结合。同时，同其他男性进行良好合作的男性可能成为更成功的狩猎者。随着人类祖先变得越来越擅长合作，他们会共享制造工具的专门技能并且最终发展出语言。而所有这些都位于一个由水平激增的多巴胺推动的反馈环路中。

Lovejoy认为，这些神经化学上的变化在440多万年前便已到位。当时，人类家族的早期成员——拉密达猿人已在埃塞俄比亚居住。和在具有侵略性的展示中将獠牙露出的黑猩猩相比，男性拉密达猿人的尖牙已经减少。这意味着当他们笑起来时，很有可能在发出合作的信号。（宗华）

更多阅读

《科学》相关报道（英文）