It’s been quiet at Lawnchair for a while, I’ve got my prelim exams (to become a candidate) coming up really soon, and I’ve been working on honing the topic of my dissertation (which is generally in a more fiery, furious and constant state of upheaval than the Earth’s inner-workings).
One thing I’m looking at right now, though, that I thought I’d share is how much human population size has increased in just a few thousand years. Below is a figure from John Hawks and colleagues (2007) on recent natural selection in modern humans (see also my post on evolution and why we’re still evolving).
Note that this is only part of the human population – it doesn’t include East Asia or the Americas. In addition, population size is on a logarithmic scale, so that each tic is an increase in order of magnitude. 30 thousand years ago there were probably only some 1.5 million people on the continents of Europe-West Asia and Africa. Those numbers slowly increased until around 10 thousand years ago, about when humans began farming. From there, population sizes sky-rocketed, such that there currently probably over 2 billion people on these continents alone. The global human population is estimated to number over 7 billion people right now. Seven billion – a number so unfathomably large you and all your friends couldn’t count that high before you expire (not to mention get bored from counting for no reason).
Our superlative success as a species is at once astounding, as well as disconcerting. Most animals reproduce at a rate such that their population sizes are either steady or declining over time. I want to say I’ve seen a figure somewhere to the effect that chimpanzee survival and reproduction rates are such that their numbers are likely declining. We, on the other hand, are reproducing like rabbits, to use the old saying. Have we figured out a way to circumvent the Malthusian constraint of limited resources in an expanding population? Or do we have yet to fall victim to our excessiveness?
On a lighter note what’s cool about this study is it shows that as a species we’ve experienced a great deal of natural selection in very recent times. This amount of evolution was made possible because our exploding population sizes, which meant the introduction of greater amounts of new mutations each generation, increasing the likelihood than an adaptive one pops up. Plus our social and food-gittin’ lives introduced us to new selective pressures most of our ancestors probably didn’t face, like crowded living conditions, first with more people and later with animals. We just might be an egregious experiment in extreme evolution.
Hawks, J., Wang, E., Cochran, G., Harpending, H., & Moyzis, R. (2007). Recent acceleration of human adaptive evolution Proceedings of the National Academy of Sciences, 104 (52), 20753-20758 DOI: 10.1073/pnas.0707650104