Fry dismisses these arguments. Of course, toxins are recruited from ordinary physiological genes, he says. “How could it be any other way? They aren't magically created by the toxin fairy. They evolve.” So, it's obvious that toxin genes should be active in other tissues, and that doesn't refute their role in venom. Consider hyaluronidase—a protein that attacks sugars. Sperm cells use it to break into eggs and fertilize them. But snakes and scorpions use it to destroy flesh; if you inject it into another animal at very large concentrations, it starts breaking down the local tissues. “Such proteins could perform multiple functions in the same animal,” says Fry.
Okay, says Mulley, but you'd still expect these genes to be more strongly active in venom glands than other body parts, since their owners need to manufacture toxins at large concentrations. “And we were finding them at the same level all over the body,” he says.
That doesn't matter, says Kartik Sunagar at the Hebrew University of Jerusalem, who sides with Fry. “Venom is extremely expensive to produce so animals that don't use it lose it,” he says. For example, the marbled sea snake, which eats fish eggs rather than fish, has no teeth, shriveled venom glands, and venom that's about 100 times less toxic than those of its close relatives. So the fact that Toxicoferans are still activating these supposed toxin genes at all in their mouth glands speaks volumes.
To break this stalemate, Weinstein argues, the Toxicoferan supporters need to provide evidence that the things they call venom are actually being used to subdue and kill prey. Sure, says Fry, “just give us a couple more centuries, an army of research students, unlimited funding, and we will be with you on that.”
The problem is that venoms are highly targeted weapons, adapted to take out specific prey. Tree snakes, for example, wield neurotoxins that are up to 300 times more potent against birds (which they hunt) than mammals (which they don't). So, to work out if a potential toxin is actually toxic, you often need to test it against its owner’s natural prey, rather than just lab rodents.
Besides, it's not necessary, says Fry. The Toxicofera idea wasn't just based on a few genes, but also on studies of gland anatomy, feeding behavior, and evolutionary relationships. If that’s not enough, he says, you’d also have to dispute the status of most canonically venomous animals, including spiders, scorpions, cone snails, stonefish, and stingrays. “Anyone who contends that all these lines of corroborating evidence are still not enough seems to lack an understanding of toxinology specifically and science in general,” he adds.
That's a lot of people, it seems. This September, both sides presented their evidence in a public debate, at the International Society of Toxinology World Congress. To Mulley's surprise, he won by a landslide. “We expected to be defeated, or maybe scrape a draw,” he says. “Maybe the tide is turning.”