One reason to stress the prevalence of toxins in our nat- ural environment, and our evolutionary adaptation to them, is to provide a perspective on the medical signifi- cance of novel toxins. Novel toxins are a special prob-
lem not because artificial pesticides such as DDT are intrinsically more harmful than natural ones but because some of them are extremely different chemically from those with which we are adapted
to cope. We have no enzymatic machinery designed to deal with PCBs or organic mercury complexes. Our livers are ready and wait- ing for many plant toxins, but they don’t know what to do with some novel substances. Furthermore, we have no natural inclination to avoid some novel toxins. Evolution equipped us with the ability to smell or taste common natural toxins and the motivation to avoid such smells and tastes. In psychological jargon, the natural toxins tend to be aversive stimuli. But we have no such machinery to protect us from many artificial toxins, like DDT, that are odorless and taste- less. The same is true of potentially mutagenic or carcinogenic radioisotopes. Sugar synthesized from radioactive hydrogen or car- bon tastes as sweet as that made with ordinary stable isotopes, but we have no way of detecting its dangers.
It is not always easy to tell what the effects of a novel environ- mental factor may be. For instance, the debate about the possible dangers of mercury in dental fillings has gone back and forth, but Anne Summers and her colleagues at the University of Georgia have recently found that mercury fillings increase the number of gut bac- teria that are resistant to common antibiotics, apparently because the mercury acts as a selective factor for bacterial genes that protect against mercury and some of these same genes confer resistance to antibiotics. The clinical significance of this finding is uncertain, but it nicely illustrates the unexpected means by which novel toxins can affect our health.