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Brain cells for snake-spotting found?

Oct. 28, 2013
Courtesy of University of California - Davis
and World Science staff

Did snakes drive the ev­o­lu­tion of high-qual­ity vi­sion in our an­ces­tors? Work by neu­ro­sci­en­tists in Ja­pan and Bra­zil is sup­port­ing that the­o­ry, orig­i­nally pro­posed by Uni­vers­ity of Cal­i­for­nia, Da­vis an­thro­po­l­o­gist Lynne Is­bell.

In a pa­per pub­lished Oct. 28 in the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences, those sci­en­tists, work­ing with Is­bell, re­ported that spe­cif­ic nerve cells in the brains of rhe­sus ma­caque mon­keys re­spond to im­ages of snakes.

The snake-sen­si­tive neu­rons, or nerve cells in the brain, were more plen­ti­ful, and re­sponded more strongly and rap­id­ly, than oth­er nerve cells that were ac­tive in re­sponse to im­ages of ma­caque faces or hands, or to ge­o­met­ric shapes, the sci­en­tists said.

Is­bell said she was sur­prised that more neu­rons re­sponded to snakes than to faces, giv­en that pri­mates—or hu­mans and their mon­key and ape kin—are highly so­cial. “We’re find­ing re­sults con­sist­ent with the idea that snakes have ex­erted strong se­lec­tive pres­sure on pri­mates,” that is, ev­o­lu­tion­ary pres­sure that drove the de­vel­op­ment of good snake-tracking abil­ity, she said.

Is­bell orig­i­nally pub­lished her hy­poth­e­sis in 2006, fol­low­ing up with a book, “The Fruit, the Tree and the Ser­pent” (Har­vard Uni­vers­ity Press, 2009) in which she ar­gued that our pri­mate an­ces­tors evolved good, close-range vi­sion pri­marily to spot and avoid dan­ger­ous snakes.

Mod­ern mam­mals and snakes big enough to eat them are be­lieved to have evolved at about the same time, 100 mil­lion years ago. Ven­om­ous snakes are thought to have ap­peared about 60 mil­lion years ago—“ambush preda­tors” that have shared the trees and grass­lands with pri­mates.

“The re­sults show that the brain has spe­cial neu­ral cir­cuits to de­tect snakes, and this sug­gests that the neu­ral [brain] cir­cuits to de­tect snakes have been ge­net­ic­ally en­cod­ed,” said Hisao Nishijo of To­ya­ma Uni­vers­ity in Japan, a col­la­bo­ra­tor in the new stu­dy. Nishijo be­came in­volved af­ter read­ing of Is­bell’s the­o­ry.

The mon­keys tested in the ex­pe­ri­ment were reared in a walled col­o­ny and nei­ther had pre­vi­ously en­coun­tered a real snake, the re­search­ers said. “I don’t see anoth­er way to ex­plain the sen­si­ti­vity of these neu­rons to snakes ex­cept through an ev­o­lu­tion­ary path,” Is­bell said.


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Did snakes drive the evolution of high-quality vision in our ancestors? Work by neuroscientists in Japan and Brazil is supporting that theory, originally proposed by University of California, Davis anthropologist Lynne Isbell. In a paper published Oct. 28 in the journal Proceedings of the National Academy of Sciences, those scientists, working with Isbell, reported that specific nerve cells in the brains of rhesus macaque monkeys respond to images of snakes. The snake-sensitive neurons, or nerve cells in the brain, were more plentiful, and responded more strongly and rapidly, than other nerve cells that were active in response to images of macaque faces or hands, or to geometric shapes, the scientists said. Isbell said she was surprised that more neurons responded to snakes than to faces, given that primates—or humans and their monkey and ape kin—are highly social. “We’re finding results consistent with the idea that snakes have exerted strong selective pressure on primates,” that is, evolutionary pressure that drove the development of good snake-tracking ability, she said. Isbell originally published her hypothesis in 2006, following up with a book, “The Fruit, the Tree and the Serpent” (Harvard University Press, 2009) in which she argued that our primate ancestors evolved good, close-range vision primarily to spot and avoid dangerous snakes. Modern mammals and snakes big enough to eat them are believed to have evolved at about the same time, 100 million years ago. Venomous snakes are thought to have appeared about 60 million years ago — “ambush predators” that have shared the trees and grasslands with primates. “The results show that the brain has special neural circuits to detect snakes, and this suggests that the neural [brain] circuits to detect snakes have been genetically encoded,” said Hisao Nishijo of Toyama University, a collaborator in the new study. Nishijo became involved after reading of Isbell’s theory. The monkeys tested in the experiment were reared in a walled colony and neither had previously encountered a real snake, the researchers said. “I don’t see another way to explain the sensitivity of these neurons to snakes except through an evolutionary path,” Isbell said.