If you sometimes wonder why you pack on muscle more easily than your friends, your DNA might have something to say about it — specifically, genes you inherited from Neanderthals tens of thousands of years ago. Scientists have discovered that people who carry a Neanderthal version of a growth hormone receptor gene have, on average, 271 additional grams of muscle compared with those who do not carry it. That's roughly the weight of a small can of soup. And people who inherited the variant from both parents tend to have even more.

The discovery comes from a study that combined two types of evidence: laboratory experiments on cells and data from more than one million living people. That double approach is what makes the finding especially strong. "It is rare to see laboratory and population evidence fit together so clearly," said Philipp Kanis, the study's first author and a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The growth hormone receptor is a protein on cells that helps regulate growth throughout the body. When researchers compared modern human and Neanderthal versions of this gene, they found clear differences. In cell cultures, the Neanderthal receptor produced more growth than the modern human version — which makes sense given that Neanderthals were known for their sturdy, muscular builds.

The variant is surprisingly common. Most people alive today carry between one and two percent Neanderthal DNA, thanks to interbreeding between modern humans and Neanderthals roughly 50,000 to 60,000 years ago. This particular growth hormone receptor variant shows up most often in South and East Asian populations, reaching up to 24 percent in some parts of South Asia.

Researchers are still mapping out how deeply Neanderthal ancestry shapes modern human bodies. Effects on the immune system are already well documented. Muscle mass may now join that list. But one of the study's own authors was quick to keep things in perspective. "Even a Neanderthal growth hormone receptor is no substitute for training," said Miriam Berreiter, a co-author and PhD student at the Karolinska Institute in Sweden.

So if you happen to carry this ancient genetic gift, congratulations — but your gym membership still comes in handy.