Toxins are everywhere.
Key Vocabulary: Toxins, poison, toxicology, natural selection, convergent evolution,
Next Generation Science Standards
- HS-LS4-4. Construct an explanation based on evidence for how natural selection leads to adaptation of populations.
Snakes, frogs, octopi, platypi, spiders, milkweeds, hemlock and the humblest of bacterial colonies all produce enough toxins to kill a full-grown human. But what caused toxins to evolve in the first place? And why did we adapt to some toxins but not others? And how are other species evolving to cope with the chemicals that humans have put into the environment?
Like all kids, Emily Monosson was interested in poisons. But instead of just reaching for the chemicals underneath the sink, she used them to make potions to kill earwhips.That curiosity of compelled Dr. Monosson to become not only a researcher in toxicology but also one of the most enthusiastic communicators in her field.
After working some time researching the effects polychlorinated biphenyl (PCB) on fish reproduction, which included collecting flounder on the high seas and field work in mucky, contaminated creeks around old industrial sites, Dr. Monosson began writing. Her books have discussed topics ranging from how living things have evolved tolerance to toxic chemicals to how we can avoid over-dependence on traditional antibiotics and pesticides. I sat down to talk with Dr. Monosson about how natural toxins have evolved and how man-made toxicants have affected the environment.
Chris Anderson: Toxins include a wide variety of harmful chemicals found in nature. How common are natural toxins and what the different types?
Emily Monosson: They are really pretty common, from chemicals in plants to discourage grazing animals from eating them to tetrodotoxin in puffer fish, which is actually is made by a bacteria. Some familiar plant toxins include chemicals made by plants like milkweeds which are fed upon by Monarch butterflies or Queen Anne’s lace which can be eaten by Black Swallowtails. Both the Monarch butterflies and Black Swallowtails have evolved ways to detoxify or store the poisons. So plant, bacterial, and even animals make their own toxins, like some snakes.
You could also include things like radiation released from rocks containing radioactive elements and toxic metals like lead and mercury as “chemicals found in nature.” But, these tend to be locked away in the earth and humans are the ones who usually release them into the environment.
CA: What do we know about the evolutionary origins? What were the selection pressures that made natural toxins advantageous?
EM: That’s a good question that has some different explanations. Consider plants: rooted in the ground, with tasty and nutritious leaves. They can’t run, but they can chase away a predator by making them sick! Some research says that toxins evolved as a weapon to protect its producer. Other studies have shown that they may be chemicals used in communication, say, between microbes, much like our hormones.
Conversely, they can be helpful to predators, like snakes, by disabling their prey. There are some animals that produce toxins that act on similar targets, such as on nerve cells, which is an example of convergent evolution. The platypus, one of the few mammals that actually makes its own venom – is very similar to some snake and other venoms! Wild!
CA: So why have humans developed a tolerance to some toxins found in nature, while others remain lethal?
EM: We do metabolize and detoxify a lot of plant toxins, but humans didn’t evolve these tolerances and defenses, our long long ago ancestors did and we are the beneficiaries. That said, some human populations have evolved other kinds of tolerances – like the ability to digest milk throughout their lifetime.
CA: One of your focuses as a writer has been the effects of chemical exposures on the environment. Lead paint, BPA in plastics, industrial waste products, and heavy metals have begun to seriously change the chemistry of the air, land, and water. How have species responded to this?
EM: There is a growing list of species that have evolved resistance to many of our modern day toxicants such as metals and highly toxic pesticides. These tend to be animals that reproduce at a young age and produce lots of offspring – so highly fecund – species. Some fish, frogs, insects, even rats, have evolved resistance to certain chemicals that remain toxic to many other animals including ourselves. Just like bacteria evolve resistance to antibiotics.
But, those species that have longer lifetimes, and fewer offspring tend to remain sensitive. And once certain toxicants get into the environment, they may continue to cause damage years later. One group of “legacy” chemicals, the polychlorinated biphenyls or PCBs, were used in the electrical industry, but banned back in the 1970s, once we realized they were not only toxic, but persistent, or very slow to break down. While some fish seem to have evolved resistance, PCBs continue to poison other animals, like killer whales.
CA: Since humans have such a long generation time and have a relatively small number of offspring per mating pair, have we been able to adapt to any of these new chemicals?
EM: We don’t really have a lot of defenses against these which is why our body systems become quickly overloaded when exposed to toxicants. We do have proteins that bind to heavy metals because we need some such as zinc and iron for our own biological functioning. But mercury, cadmium, and lead are just toxic, and scientists are increasingly realizing that they are toxic in very small amounts.
CA: To that end, humans haven’t always been good stewards of the environment. How what are some things that everyday people can do to make sure that they are not adding more toxic chemicals to nature?
EM: Consider how to reduce releases into the environment. Even the microfibers from our clothes shed into the water during washing. Who would have thought? So thinking creatively and more broadly when developing new chemicals or products – about where they’ll end up and how. But that’s on the manufacturing end. I suppose we all can try to use less stuff altogether; choose products that are less toxic or have been produced more sustainable than other products and make sure you dispose of products correctly. For example, old pharmaceuticals should get returned or turned in, rather than flushed away down the toilet or sink.
Check out the episode of Nature Guys below to learn more about milkweed, a common toxic plant: