Not everyone starts at the same point when trying to build muscle. The genes we inherited from our ancestors can influence how well we build muscle, no matter how much we exercise. Some of those genes may even come from ancient interbreeding with Neanderthals.

Neanderthals generally had more robust bodies than modern humans, with larger muscles, prominent brow ridges, and barrel-shaped chests. Although they evolved largely in isolation, some of their genetic traits still survive in people today.

Now, a study led by the Karolinska Institutet, Sweden, and published in Current Biology, has found that a Neanderthal version of the growth hormone receptor boosts muscle cell growth in the lab, which is also linked to slightly greater muscle mass in people who carry it today. The variant is especially common in some South Asian populations, where up to 24 percent of people carry it.

“What struck me most was that two very different types of evidence told the same story,” said the study's first author Philipp Kanis, researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, in a press statement. “Cells grown in the laboratory responded more strongly, while data from more than a million people showed signs of the same effect in the body. It is rare to see laboratory and population evidence fit together so clearly.”

Muscle Cells With Neanderthal Growth Hormone Receptor Grow Better

The human growth hormone helps regulate the growth of muscles and bones. Produced by the pituitary gland, it travels through the bloodstream and binds to receptors on the surface of cells, triggering signals that promote growth.

To investigate whether differences in this system contributed to Neanderthals' stockier build, researchers compared growth hormone receptor genes from modern humans and Neanderthals. They found that the Neanderthal receptor differs by two amino acid changes and one small deletion.

When the team tested the receptor in laboratory-grown cells, those carrying the Neanderthal version showed an increased response to growth hormone, growing about 40 percent more than cells with the modern human receptor. Most of that effect appeared to come from one of the two Neanderthal-specific changes.

Read More: Genomes Across Japan Reveal Traces of Neanderthal and Denisovan DNA — and a Hidden Third Ancestry

Neanderthal Receptor Variant Most Common in South Asian Populations

Cell experiments are great to observe potential biochemical effects; however, they do not always translate to real-life anatomical observations. But because modern humans still carry this receptor variant through interbreeding with Neanderthals around 47,000 years ago, physical differences are still measurable today.

The research team described in their study how the growth hormone receptor variant can still be found primarily in people with non-African ancestry. More precisely, the variant is especially common in South and East Asia, where nearly one in four people carries the variant. In contrast, the receptor variant is only found in about 0.5 percent of Europeans.

Additionally, the researchers found no differences in growth among more than 6,000 children through age 11, suggesting the receptor's effects emerge later, when growth hormone levels rise during puberty. In adults, carriers had an average of about 0.6 pounds (270 grams) more muscle mass.

“As someone who does CrossFit, I was delighted to find Neandertal genetics and muscle mass coming together in the same study,” said study co-author Miriam Berreiter from the Karolinska Institutet, Sweden, in the release. “But even a Neandertal growth hormone receptor is no substitute for training.”

People With Neanderthal Receptor Also Have Shorter Jaws and Tooth Roots

Beyond changes in muscle mass, the scientists also saw differences in the skull and teeth of those carrying the growth hormone receptor variant, including a slightly shorter lower jaw and shorter tooth roots. These are traits that were commonly seen in Neanderthals.

“It is fascinating that one genetic change inherited from Neandertals can still have an effect on the human body today,” added senior study author Hugo Zeberg in the statement. “But this is only one of many genetic influences on growth and body shape. It cannot explain the Neandertal body type on its own, and it certainly does not determine a person’s overall appearance.”

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• This article references information from a study published in Current Biology:Increased signaling of the Neanderthal growth hormone receptor