(a)
(b) How does the brain works to save us from a snake ?
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Answer: By selecting for traits that helped animals avoid them, snakes ultimately endowed us with forward-facing eyes
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The sinuous shape triggers a primal jolt of recognition: snake! A new study of the monkey brain suggests that primates are uniquely adapted to recognize the features of this slithering threat and react in a flash. The results lend support to a controversial hypothesis: that primates as we know them would never have evolved without snakes.
A tussle with a snake meant almost certain death for our preprimate ancestors. The reptiles slithered through the forests of the supercontinent Gondwana roughly 100 million years ago, squeezing the life out of the tiny rodent-sized mammalian ancestors of modern primates. About 40 million years later, likely after primates had emerged, some snakes began injecting poison, which made them an even deadlier and more immediate threat.
Snakes were “the first and most persistent predators” of early mammals, says Lynne Isbell, a behavioral ecologist the University of California, Davis. They were such a critical threat, she has long argued, that they shaped the emergence and evolution of primates. By selecting for traits that helped animals avoid them, snakes ultimately endowed us with forward-facing eyes, for example, and enlarged visual centers deep in our brains that are specialized for picking out specific features in the world around us, such as the general shape of a snake’s body camouflaged among leaves.
Isbell published her “Snake Detection Theory” in 2006. To support it, she showed that the rare primates that have not encountered venomous snakes in the course of their evolution, such as lemurs in Madagascar, have poorer vision than those that evolved alongside snakes.
“It is a very bold theory,” says Arne Öhman, a psychologist at the Karolinska Institute in Sweden who uses brain imaging and behavior studies to test how humans respond to visual threats. But thus far, he says, there has been little neurobiological evidence for it.
Two years ago, neuroscientists at the University of Toyama in Japan and the University of Brasilia in Brazil contacted Isbell, hoping to join her in a search for brain-based evidence. In a paper published online today in the Proceedings of the National Academy of Sciences, the team describes how images of snakes affect the pulvinar—a cluster of neurons in an evolutionarily ancient part of the brain called the thalamus. Pulvinar neurons are believed to help direct our attention using our eyes and recognize a potential threat. Primates have much larger pulvinars than other animals and certain parts of the pulvinar are even unique to primates.
According to Isbell’s hypothesis, other mammals that had to contend with snakes were mostly burrowing creatures, and they didn’t rely as heavily on vision as early primates, which rested in trees during the day. While some mammals developed resistance to snake venoms, primates opted for a better detection strategy.
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