How Prehistoric Primates Survived Deadly Snake Venom
The story of human evolution is one peppered with the acquisition of a variety of adaptive traits that have been acquired over thousands of years. While we take many of these for granted today, each of them was integral to our survival. Now, a new study published in BMC Biology has posited that, contact with certain neurotoxic snakes helped Afro-Asian primates develop resistance to snake venom.
The study argues that primates developed their vision in response venomous snakes, as a response-survival mechanism to snake venom. (jwjarrett / Adobe Stock)
Snake Detection Theory: Developing Vision as a Response to Snake Venom
Previous work on the Snake Detection Theory (SDT) has suggested that primates’ visual systems developed as a response-survival mechanism to snakes, allowing primates to quickly detect snakes. In fact, some studies have gone as far as suggesting that primates are able to detect the presence of snakes even without any overt visual cues, a sort of pre-conscious detection of snake stimuli. This group of Australian biologists have shown how our prehistoric ancestor evolved to survive snake venom and avoid snake bites.
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In her now seminal work, The Fruit, The Tree, and The Serpent: Why We See So Well (2009), Lynne A. Isbell, researcher and author at the University of California, Davis, explains this phenomenon rather proficiently. The “Snake Detection Theory” argues that snakes were ultimately responsible for the origin of primates by acting as a selective pressure in the modification and expansion of primate visual systems such that vision is now their predominant sensory interface with the environment.
“Predatory pressure from snakes is proposed to have contributed to primate visual modification and expansion by weeding out those individuals with poorer ability, and favoring those with better ability, to visually detect motionless snakes,” she later explained in a co-authored piece in Nature magazine, from 2017.
Associate Professor Bryan Fry faces off with a cobra at The University of Queensland. (The University of Queensland)
The Evolutionary Arms Race and Snake Venom Resistance
“As primates from Africa gained the ability to walk upright and dispersed throughout Asia, they developed weapons to defend themselves against venomous snakes, this likely sparked an evolutionary arms race and evolving this venom resistance,” said Richard Harris, a PhD candidate at the University of Queensland, and lead author on the current study.
They arrived at this conclusion after a breakthrough discovery that pointed to the fact that humans, chimps, and gorillas, all shared the exact same receptor sequence. This sequence had evolved to build a partial tolerance to snake venom, a sequence not found in other tree-faring species, like lemurs and South American monkeys. "Humans, chimps, and gorillas had the greatest resistance we found, and the reason for that was the whole group had the exact same receptor sequence," Harris said.
"It is important to note that this resistance is not absolute -- we are not immune to cobra venom, just much less likely to die than other primates," noted other lead author, Associate Professor Bryan Fry, from University of Queensland’s School of Biological Sciences. Professor Harris added that the descent from the treetops is another milestone in the development of human eyesight, as primates came down from the treetops and needed to see greater distances, reports Science Daily.
“This was just one of many evolutionary defenses – many primate groups appear to also have developed excellent eyesight, which is thought to have aided them in detecting and defending themselves against venomous snakes,” explained Professor Harris.
The study concludes that snakes have strongly influenced primate evolution. (Milan / Adobe Stock)
What About Primates Who Were Not Exposed to Snake Venom?
It’s important to note that other kinds of monkeys, such as Madagascan Lemurs, or monkeys from Central and South America, live in areas without close contact to so-called neurotoxic venomous snakes. This they use to explain why these species did not develop the same kind of snake venom resistance, while also having poorer eyesight. “It’s been long-theorized that snakes have strongly influenced primate evolution, but we now have additional biological evidence to support this theory,” highlighted Professor Harris.
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Finally, in the story of snakes and human evolution, resistance to snake venom comes with what is known as a fitness disadvantage, “wherein the receptors do not do their normal function as efficiently.” So, through a simple opportunity-cost metric, the gains must outweigh the losses. Here, partial resistance itself helped gained the evolutionary advantage, and the fitness advantage was not such an aberration. The chain of events this evolutionary mechanism sets off include changes in brain structuring, tool use, and the development of sophisticated communication and language.
Top image: snake venom Source: Mark Kostich / Adobe Stock