New research published in Nature Communications this month shows that among the genes modern humans have inherited from Neaderthals are those that are specifically involved in lipid catabolism. It's proposed that the excess fat was evolutionarily advantageous for early humans in the relatively cool climes of Europe.
Modern humans and Neanderthals diverged on their evolutionary pathways about 800 - 400,000 years ago. Neanderthals went on to evolve in Europe and Asia, while early humans evolved in Africa. The migration of modern humans out of Africa, however, occurred when Neanderthals were still occupying Europe and Asia and the two coexisted for thousands of years before the eventual Neaderthal extinction. During those thousands of years, things happened as things do and now modern (as in currently existing) humans share about 1 - 4% of their genome with Neanderthals.
The authors suggest that the genes controlling lipid catabolism were acquired by modern humans through "genetic flow" (heh heh heh) from Neanderthals and then spread throughout the population due to positive selection. Humans able to store more fat could survive better after the migration from Africa to Europe than those that couldn't. Their theory of geographic-based selection is supported by the fact that modern humans of East Asian descent do not have these genes, despite living in a region once populated by Neanderthals.
Proportion of Neanderthal genome still present in various modern human populations. The blue represents total genome and the red represents lipid catabolism genes specifically. Reproduced with permission from Nature Communications (2014), 5, 3584.
So the next time someone tells you to drop a little weight, you can helpfully inform them that it's not your fault — your Neanderthal genes made you this way.
Because this is another biology-type paper, I probably missed a whole bunch of potentially interesting details. Check out the original (which is free!), Neanderthal ancestry drives evolution of lipid catabolism in contemporary Europeans, here.