Recent human evolution: height changes and the ice age

The Last Glacial Maximum (LGM) occured 26,000-20,000 years ago and was – as the name might suggest – the last time glaciers reached their full extent.

The LGM was harsh, with rainfall decreasing by up to 69% in some places breaking up the Amazon rainforest. Meanwhile sea levels dropped dramatically as water was trapped in glaciers. These frozen monstrosities extended far into Europe, reaching Southern Britain.

Grey marks ice sheets, pink tundra.

Obviously this period had a dramatic effect on animals and plants living in regions affected by these changes. Many species of animals can trace their ancestry back to a few isolated populations in Southern Europe that managed to survive the LGM.

Interestingly, humans seem to have been amongst the animals influenced by these dramatic climate conditions. Prior to the LGM Europeans were all tall, gracile individuals reaching heights not seen until industrialised societies.

After the ice had retreated humans looked a lot different. They weren’t as tall and their limbs were short too. These are all features in common with cold adaptation.

Being tall has its advantages. Longer legs allow for more efficient movement whilst the larger surface area means you can loose heat faster which is hand if you’re in a hot environment. In a cold environment, however, this isn’t the best adaptation to have. By reducing limb length and overall size your surface area shrinks and you retain heat more efficiently.

A similar pattern is seen in modern populations across the globe (x axis is mean temperature)

In other words, we’re watching humans evolve before our (archaeological) eyes.

Of course, the picture is a little more complicated than this. Diet, gene flow and a handful of other factors also seem to have played a role in changing our ancestors stature.

Indeed, this multitude of factors have again acted on us: improving diet, amongst other things, has allowed many European populations to start being as tall as they were before the last ice age.

However, that does not change the fact that relatively recently the environment did indeed shape modern human evolution.