As I am nearing the closing chapter (I hope) of my book on rapid evolution in a chemical world – I am struggling to understand what this means for humans. If you search rapid evolution and humans, you will find all sorts of excitement in the popular press about how traits enabling humans to drink milk, or tolerate low oxygen or malaria, Evolved Rapidly! Or, Humans, we are still evolving! (All this, despite David Attenborough’s opinion.) It’s all very interesting but in the scheme of things, these traits are estimated to have evolved over the past several thousand years. Talk about relativity! That may well be fast for us long-lived, low fecundity species, but what does it mean for humans in today’s fast-paced world? And, since I’m focused on industrial age chemicals – what does it mean in the context of chemical exposures? Is there a chance we could be responding to industrial age chemicals?
An increasing number of studies are emerging supporting the idea that even in contemporary time frames – human populations can indeed evolve. That is, over a hundred or so years. This is just fascinating. For the past year even as I’ve written about Darwin’s finches evolving within a generation or two, the great majority of this book has focused on other species from bugs to minnows — highly fecund species that have adapted over the course of fifty or perhaps twenty-five generations. At twenty-five years a pop for humans, that would be somewhere around 1000 years give or take.
This is a time frames that trivializes chemical exposures, stretching well beyond the lifetime of most industrial-age chemicals (except for long-lived radioisotopes). As bad as we are with chemical management and regulation, we eventually figure out which are particularly egregious and rein them in. It may take decades to recognize our folly– as with organochlorines – but eventually we get it right. So even if any chemicals were to impose a powerful selective pressures, most likely they would be gone within a generation or two.Maybe industrial age chemicals just aren’t all that relevant to the question of human evolution? (Barring any discussion of epigenetics that’s a post for another day, for now check out this video, where as one participant says “everyone has their definition”.)
But what if the pressure was pervasive, reaching across large swaths of a population? And what if it hit us where it really hurt, reproduction?
Here’s a fun hypothetical really, JUST a hypothetical. Consider a chemical or group of chemicals that act upon reproductive output – not so hard to do considering the apparent wealth of industrial age chemicals that do so. Perhaps they interact with estrogen receptors. They might be subtle, not shutting down reproduction, but just making it that much more difficult to conceive. What if, rather than a scenario like PD James’ Children of Men (where sperm went downhill fast,) women’s estrogen receptors instead evolved just enough to “ignore” estrogen binding plastics and plasticizers or other chemicals that might influence fertility and fecundity? It’s a totally fascinating mental exercise. Because if that were to happen, given all the other stuff estrogen does – the trade offs would be anyone’s guess. Hopefully that is all this will ever be, an interesting mental exercise. But given the fast pace of human evolution – and this brave new chemical world we are creating, you just never know.