多巴胺幫助社交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

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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.

多巴胺讓人類占據社交優勢

作者：宗華 來源：科學網 www.sciencenet.cn 發布時間：2018/1/23 19:18:48

 

當雄性黑猩猩露出大的獠牙時，這象征著侵略行為。相比之下，人類微笑時會露出小小的牙齒。

圖片來源：Sergey Uryadnikov/shutterstock.com

人類是終極的社會性動物，擁有和伴侶結合、通過語言溝通以及在擁擠的公共汽車上同陌生人閑聊的能力。一項最新研究顯示，人類獨特的社會智力的進化可能開始於一種簡單的大腦化學物質。

美國肯特州立大學生物人類學家Mary Ann Raghanti和同事收集了來自腦器官庫和動物園的組織樣本。其包括死於正常原因的6個物種的38個個體，其中6個物種分別是：人類、卷尾猴、豚尾猴、橄欖猴、大猩猩和黑猩猩。他們切下了部分基底核——位於腦基底部、被稱為紋狀體的神經細胞和纖維群。這裏是一個交換所，負責傳遞大腦不同部分發出的運動、學習和社交行為信號。研究人員用對不同神經傳導物質（包括多巴胺、血清素和神經肽Y）作出反應的化學物質將這些切片著色。上述化學物質均同對社交提示和合作行為的敏感度相關。隨後，他們分析了這些切片，以衡量靈長類動物活著時釋放的神經傳導物質的水平。

研究人員在日前出版的美國《國家科學院院刊》上報告稱，和猿類相比，人類在大腦紋狀體中擁有更多的多巴胺。同時，人類擁有較少的乙酰膽堿——一種同統治欲和領地行為存在關聯的神經化學物質。Raghanti表示，這種結合是“將人類同所有其他物種區分開來的關鍵差異”。

肯特州立大學古人類學家、最新研究共同作者Owen Lovejoy介紹說，這些神經化學上的差異可能調動起其他漸進式變化，比如人類的一夫一妻製和語言的發展。關於原始人類起源，Lovejoy提出了一種新的“神經化學假設”，即女性會更多地同外向而又沒那麽強勢的男性結合。同時，同其他男性進行良好合作的男性可能成為更成功的狩獵者。隨著人類祖先變得越來越擅長合作，他們會共享製造工具的專門技能並且最終發展出語言。而所有這些都位於一個由水平激增的多巴胺推動的反饋環路中。

Lovejoy認為，這些神經化學上的變化在440多萬年前便已到位。當時，人類家族的早期成員——拉密達猿人已在埃塞俄比亞居住。和在具有侵略性的展示中將獠牙露出的黑猩猩相比，男性拉密達猿人的尖牙已經減少。這意味著當他們笑起來時，很有可能在發出合作的信號。（宗華）

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