1. Chemicals that were once lethal can become essential for life The human body can repair DNA, defuse toxic substances, and metabolize plant-based pharmaceuticals because our earliest ancestors—in some cases, single-celled organisms—developed defense systems to protect themselves against dangerous chemicals. Some chemicals that were once lethal to living things are no longer harmful, and some (like oxygen) have actually become essential. But this relationship developed over hundreds of millions of years. Some of these defense systems kick into action even when faced with new, synthetic chemicals. Yet we are altering the world’s chemical landscape at an unprecedented rate and we do not always know how our bodies will respond. Tracing how our defense systems evolved can teach us a great deal about which systems are surprisingly resilient to pollutants and other chemicals, and which are exceedingly sensitive. 2. Evolution can happen rapidly and in response to toxic chemicals We once believed that evolution happens slowly, too slowly to observe in our lifetime. But in the past hundred years, scientists have observed rapid evolution, not just in bacteria, but also in fish and birds. For example, killifish living in high concentrations of PCBs and dioxins have evolved resistance to these pollutants. We don’t know yet how quickly evolution can happen in humans, but it may well be much faster than we think. 3. We have benefitted from millions of years of plant-insect chemical warfare While fending off predators, some plants became increasingly toxic, and predators in turn, became increasingly resistant, often through the evolution of detoxification enzymes. Many of these enzymes are critical for metabolism of foreign chemicals, including plant-based pharmaceuticals. When pharmaceuticals like Coumadin (warfarin) concentrations rise dangerously after consuming grapefruit juice, or dogs are poisoned by common plant chemicals like the theobromine in chocolate, evolutionary history may help to explain and predict these drug-drug, drug-nutrient interactions or interspecies reactions. 4. Understanding our natural defenses can lead to better cancer treatment Cancerous cells are often referred to as renegade cells – because they break free from the cellular level controls which evolved along with multicellular life. Understanding the evolution of these controls, and the breakdown of these controls, may help cancer researchers both detect and design more effective chemotherapy drugs. 5. Testing single chemicals isn’t enough Toxicology is traditionally a “single chemical science,” which means we know a lot about exposures to individual chemicals like mercury or PCBs, but very little about how these chemicals act in combination with lead, or atrazine, or BPA. In reality we are exposed to many different potentially toxic chemicals through consumer products, food, water, and the air we breathe. The more we know about how our internal defense systems interact, the better we can predict our responses to complex chemical exposures. Studying the evolution of our chemical defense and stress response networks will improve our understanding of toxicology. That scientific understanding is critical for better regulation and management of myriad chemicals.