From microbes to mushrooms, spiders to snakes, urchins to us, all life engages in chemical warfare. Toxins produced by organisms are enormously varied in their origins, functions, and evolutionary histories. In this episode, we discuss the nasty chemicals that make up Venom and Poison.
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Poison, Toxin, Venom
Vocabulary time! Generally speaking, when it comes to biology and medicine:
Poisons are substances that cause chemical harm if too much is introduced onto or into a living thing. Poisoning can be caused by plants, animals, gases, metals, medications, and even too much of common things like water.
Toxins are poisons produced by the metabolic processes of living organisms.
Venoms are typically combinations of multiple toxins that are injected by one organism into another via a bite or sting.
Practically every living thing produces some form of toxin, and their functions are incredibly diverse: toxins can cause discomfort, pain, or death; various toxins can damage cells and tissues, disrupt nerve activity or digestion, impact blood flow, or cause any number of other effects; most toxins have specific effects on specific lifeforms, from bacteria to animals.
Toxins are often used defensively, as with the toxins used by our immune systems to ward off bacteria or the poisonous secretions that protect organisms like frogs and mushrooms. Other toxins are used offensively, as with rattlesnakes whose venom kills their prey, or parasitoid wasps whose venom paralyzes the soon-to-be hosts of their eggs.
Toxins are ubiquitous across life, but venom more specifically is also very widespread, having evolved numerous times in many groups of animals.
Poisons of the Past
As you might image, direct evidence of venomous, poisonous, or otherwise toxic organisms in the fossil record is pretty rare, but it’s not unheard of!
Fossil examples of vipers, stingrays, wasps, and many more animals feature the same venom-delivery adaptations seen in their living relatives. There are also some well-known cases like the “proto-mammal” Euchambersia, whose skull features a possible venom gland-and-delivery mechanism, and the reptile Uatchitodon, whose teeth were hollow tubes like some venomous snake fangs. Among dinosaurs, Sinornithosaurus was famously suspected to be venomous, although this is disputed.
Sometimes we can infer toxicity from other evidence. A fossil plant closely related to modern-day Strychnos can be inferred to have been similarly toxic; a cockroach from the Cretaceous shows the same warning color patterns as living species with chemical defenses; and any organism with an immune system would have produced some toxins!
And sometimes we’re lucky enough to catch a chemical in the act, as in the case of a Cretaceous soldier beetle caught in amber while secreting defensive compounds to ward off offenders.
Many, many studies have addressed questions of toxin evolution. Here are some examples we discussed in the episode.
Probably the best-studied group of venomous animals are snakes, which is due in large part to how medically important they are to humans. Popular hypotheses suggest that snake venom evolved one time, far back in their evolution, and that their toxins are derived from substances all over the body that have been repurposed for venom, although this 2014 paper disagrees with both of those hypotheses, and provides a nice overview of snake venom research.
One 2014 study experimentally found how a relatively simple gene mutation can change an antimicrobial toxin into a potentially dangerous neurotoxin. The toxin in question belongs to a group called defensins, which are extremely widespread and extremely ancient, found in the immune systems (and more) across plants, animals, and fungi.
Toxins and Venoms (pdf), from a 2009 edition of Current Biology is a nice, not-too-technical overview of toxin diversity.
Complex cocktails: the evolutionary novelty of venom, 2013, is a good overview of venom evolution research, though a bit more technical and paywalled.
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