One cartoon-cliché of “prehistoric” time is that everything was bigger. We grow up absorbing a picture of the vague deep past as including dinosaurs, cavemen, and big versions of everything. While this is an oversimplification, and tends to mash vastly different periods into one (the past), the notion that mammals, at least, tended to be bigger in the past is accurate. When mammals replaced dinosaurs as the dominant megafauna on land, they became really big (although not as big as the biggest dinosaurs they replaced). But then over the last 2 million years average mammals sizes have been steadily decreasing. The big question is – why?

I recently wrote about the North American megafauna, and the debate between whether their extinction about 12 thousands years ago was caused primarily by humans (the overkill hypothesis) or climate change. But this debate exists for the entire planet – every continent except Antarctica. This continues to be a debate because large-scale cause and effect is difficult to prove in evolutionary history. We can test various hypotheses by predicting correlations and patterns in the fossil record and then looking for them. Sometimes we can extrapolate from current observable trends, or even changes in the laboratory. But the steady reduction in the average size of mammals requires looking for large correlations – what factor does this trend most have in common around the world? Many scientists think the clear answer is – humans.

Starting with Homo erectus, wherever humans go around the world they are followed by a wave of megafauna extinction. Could this just be a general trend unrelated to humans? Perhaps the world’s climate has been changing in such a way over the last two million years that explains a selective advantage to small size. Climate cannot be ruled out as a factor, but the evidence increasingly supports the notion that human hunting played a major factor.

I am always suspicious of nice clean stories in evolution, so I will add a huge caveat that any hypothesis about A causing B is likely to be a massive oversimplification. But some factors can have dominant effects. Around 2 million years ago we start to see the first evidence of Homo erectus using fire. At this same time erectus clearly dramatically ramped up their hunting, and also this was the first time a human species spread throughout the world. Cooking meat would have been a huge energy and nutritional advantage, and hunting requires a lot of land, and hence the need to spread out. This shift in our human ancestors correlates nicely with dramatic reductions in the average size of mammals, wherever humans went.

A recent study adds another layer to this story. Dr. Miki Ben-Dor and Prof. Ran Barkai from the Jacob M. Alkow Department of Archaeology at Tel Aviv University suggest in their paper that this trend is all about energy efficiency. Early humans would get the most energy back for their effort by hunting larger game. It’s just a matter of energy economics, which is a recognized factor in evolutionary pressure. Once humans develops the tools to kill big game, and the cooperative hunting, the trend was set in motion. They killed the biggest game because it was most energy efficient. Combine that with cooking, and they essentially conquered the world.

This would create a clear steady evolutionary pressure wherever humans went against large body size, and so body size steadily decreases. The factor is also huge – species that go extinct are between 100 and 1,000 times larger than those that survive. In North Amercia, for example, megafauna extinction resulted in a decrease in average body size from 216 pounds to 17.  Researchers favoring this hypothesis also point out you would not have to kill every single last member of a species to make them go extinct. Just adding a significant stressor, especially to species that tend to have few young, would be enough. Sure, there were likely other environmental stressors as well, but persistent human hunting eventually drove them to extinction.

The Tel Aviv researchers then add another factor – as humans started working their way to smaller and smaller animals, they would necessarily have to become more intelligent, and this does correlate with increased brain size. Hunting smaller game is more challenging, you have to be fast and clever. You also get less energy back for your effort, so you have to be really efficient, which requires more cleverness. Eventually, they argue, average animal size decreased so much as a result of human hunting that this strategy was no longer sustainable. That is when the shift to agriculture and domesticating animals happened. This, of course, created a separate set of pressures on animals, such as land use, water diversion, and disruption of habitats.

One important bit of evidence for the general association of human hunting and reduced animal size is the fact that, absent humans, there is no such trend observable in the fossil record. In other words, there is no trend separate humans where animal size decreases steadily. Climate change itself does not result in either an increase or reduction in average animal size. Only human hunting seems to be a consistent factor.

While the evidence seems to be piling up for the overkill hypothesis to explain the general trend of decreased mammal size over the last two million years, this debate is certainly not over. As I said, the evidence is mostly correlational, with leaves room for alternate interpretations. Humans may have been following this trend (hunting smaller and smaller game) rather than causing it. But right now there are solid arguments to be made for the overkill hypothesis.

There is nothing we can do about this now, of course, but – this trend of increasing extinction rate due to human activity has not ended. It is, in fact, accelerating. The background extinction rate is about 1 species per million per year, which means we should be experiencing about 10 extinctions per year. Instead, we are observing about 30,000 extinctions per year. Some scientists argue this means were are in the middle of the sixth mass extinction, which seems a reasonable conclusion. This is no longer due to direct hunting, but due to loss of habitat and human-caused climate change. Those are things we can do something about, if we wanted to.