Most species are locked in an evolutionary arms race with those above or below them in the food chain. As one evolves new weapons or defenses, the other must adapt to survive. However, for 10 million years some Australian snakes have kept almost the same venom. Tiger snakes have found a formula their prey cannot evolve resistance to, so they've had no need to improve it, and this could actually have medic benefits for humans
Snake venom varies by preferred prey. “Those that feed on warm-blooded prey have venoms that hit the blood, while the ones that feed on cold-blooded prey hit the nerves,” Dr Bryan Fry of the University of Queensland told IFLScience. Exceptions exist, with some venoms used to scare off threats, but, Australian snakes feeding largely on mammals or birds mostly produce venom that prevents blood clotting.
Some prey species usually contain individuals that have some capacity to resist venoms. These individuals multiply, eventually reaching a point where the snakes need to come up with some new sort of venom or starve.
However, “If the animals had variation in their blood clotting proteins, they would die because they would not be able to stop bleeding,” Fry said in a statement. “Blood clotting is a very complex cascade. If you change one part in the middle you disrupt everything downstream,” he further explained to IFLScience.
Mutations are usually disastrous in the wild, producing effects like hemophilia. Squid and insects have fundamentally different circulatory systems to our own, but no vertebrates has managed to suddenly jump to an entirely different system to avoid snake predation.
Having developed the perfect system for targeting blood clotting, many Australian snakes have rested on their laurels. Where other venoms evolve twice as fast as their makers, these are very similar across three genuses and many species, including the Stephen's banded snake and the Sydney broad-headed snake.
The finding explains something that has been widely anecdotally reported, but had not been rigorously confirmed until Fry’s work: tiger snake antivenom is highly effective against many other snake bites. Australia has such an abundance of venomous snake species, many of them quite rare, there are not antivenoms for all of them. Consequently, doctors confronted with a snake-bite victim for a species that lacks an antivenom often use the tiger snake version.
In Comparative Biochemistry and Physiology Part C, Fry shows that this is generally an effective approach, including for some species so rare there are no recorded cases of antivenom being required.
Obtaining venom samples from 16 isolated tiger snake populations and 11 other species is no easy feat, but Fry was assisted by his discovery of a long-lost treasure-trove of venom samples collected up to the 1950s, including from subspecies that are now extinct. Remarkably, most of the venoms remained effective.