Snake Vs Ape: Humans’ Partial Resistance To Venom Shaped By Biological Arms Race

If you wish your reaction times were faster, blame snakes. Slower reactions are a consequence of evolving partial resistance to cobra venom.

Coming down from the trees meant our ancestors encountered ground-dwelling snakes more often. In a two-stage process, we evolved partial resistance, allowing most snakebite victims to survive, although often losing a limb in the process, but some sacrifices were required. Snakes account for 100,000 deaths a year, and cobras are one of the four main killers. This, however, is a small proportion of the million or so people bitten by venomous snakes each year.

Dr Bryan Fry of the University of Queensland is exploring how snakes and primates have influenced each others' evolution. In BMC Biology, he reveals things would be much worse if we were descended from animals who hadn't faced the cobra threat earlier in their evolution.

Primates living in continental Africa and Asia are exposed to snakes, particularly cobras, whose venom is based on ?-neurotoxins.

Fry has helped develop synthetic nerve receptors that mimic those of different species, allowing scientists to investigate the way venoms (and potential medications) affect different animals without the ethical problems of live animal experimentation. He used these to test how different members of the primate family are affected by cobra venom, without having to expose any animals to the toxins.

“It’s been long-theorised that snakes have strongly influenced primate evolution, but we now have additional biological evidence to support this theory,” PhD student Richard Harris said in a statement.

Harris and Fry showed these toxins are deadly in much smaller doses to American monkeys and Madagascan lemurs. Madagascar has no ?-neurotoxin-producing snakes, while in the Americas they are small and nocturnal.

Great apes and African or Asian monkeys have acetylcholine receptors that bind less easily to these neurotoxins, allowing many to survive a bite. “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,” Harris said.

However, once our ancestors left the trees cobras became a bigger threat. Somewhere between the point where our line diverged from orangutans, and where we split from gorillas, our cobra defenses took a step up to combat this, Fry told IFLscience.

“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,” Fry noted in the statement. 

Efforts to evolve complete resistance were hindered by cobras increasing their own capacities. Indeed, “It's thought the spread of humans was the trigger for the evolution of defensive spitting,” Fry told IFLScience.

Moreover, Fry noted, “We have shown in other studies that resistance to snake venoms comes with what’s known as a fitness disadvantage, whereby the receptors don’t do their normal function as efficiently.” This results in slower reaction times; the difference between life and death in some circumstances, he explained.

“There is a fine balance to be struck where the gain has to outweigh the loss,” Fry concluded. 

Humans who migrated to regions – such as the Americas or Australia – inhabited by snakes with other types of venoms would no longer have benefited from their partial cobra resistance. However, Fry said there are no signs inhabitants of these locations have traded in cobra resistance to regain faster reactions.

“We are increasingly recognizing the importance snakes have played in the evolution of primates, including the way our brain is structured, aspects of language and even tool use,” Fry said. 

“This work reveals yet another piece in the puzzle of this complex arms race between snakes and primates.”