Some people seem to pack on muscle remarkably fast, even with the same exercise intensity. While training methods, diet, and a complex mix of genetic factors all play a role, new research suggests that a specific gene variant inherited from Neanderthals—who went extinct around 47,000 years ago—may contribute to these differences.
An international research team from Germany’s Max Planck Institute for Evolutionary Anthropology (MPIEA), Sweden’s Karolinska Institute, and other institutions published findings in the journal Current Biology on the 5th (local time) showing that a “growth hormone receptor” gene variant passed from Neanderthals to modern humans has a statistically significant impact on muscle mass and jaw and tooth morphology in people today.
“This study reveals the intriguing fact that a single genetic change inherited from Neanderthals can still influence the human body today,” said co-corresponding author Hugo Zeberg, a professor at the Karolinska Institute. He cautioned, however, that “this is just one of many genetic factors affecting growth and body shape,” adding, “this gene alone cannot explain the Neanderthal physique, much less determine an individual’s overall appearance.”
The research team focused on a key pathway governing human growth. If the growth hormone secreted by the pituitary gland acts as a “signal” commanding muscles and bones to grow, the growth hormone receptor serves as the “antenna” on the cell surface that receives this signal. The Neanderthal version of this receptor, however, contains two amino acid variants that differ from the form commonly found in modern humans. Specifically, the amino acids at positions 422 and 561 of the receptor are altered, and in modern humans, these two variants are inherited together as a single package.
To determine how these genetic differences affect actual cell function, the researchers divided mouse-derived cells lacking growth hormone receptors into two groups, introducing the modern human-type receptor into one group and the Neanderthal-type receptor into the other. They then administered the same concentration of pituitary growth hormone and observed the responses. While there was no difference in the receptors’ ability to bind to growth hormone, after five days, cells implanted with the Neanderthal-type receptor proliferated approximately 39% faster than those with the modern human-type receptor. This indicates that the Neanderthal-type receptor transmits growth hormone signals more strongly into the cell interior.
To verify whether this difference observed in cell experiments manifests in actual human bodies, the team conducted a large-scale analysis of genomic and anthropometric data from 1,134,174 adults registered in biobanks in the UK, Finland, Japan, South Korea, and Taiwan. The results showed that individuals who inherited one copy of the Neanderthal-type growth hormone receptor gene variant from one parent tended to be, on average, 3 mm taller and 285 grams heavier than those without the variant. This weight difference primarily stemmed from a 150-gram increase in limb muscle mass and a 121-gram increase in trunk lean mass (the weight of muscle, bone, organs, etc., excluding fat). The team estimated that each copy of the gene variant increases muscle mass by roughly 271 grams.
Interestingly, the variant’s effects appeared more pronounced after puberty rather than during fetal development or early childhood. When cells were stimulated with growth hormone secreted by the placenta during pregnancy, there was no difference in proliferation between the two receptor types. Furthermore, a longitudinal study tracking 6,602 children from birth to age 11 found no clear association between this gene variant and height or weight.
Traces of Neanderthal inheritance were also subtly evident in facial and dental morphology, not just muscle mass. Combining data from four previous studies, the researchers found that carriers of the variant had a mandibular ramus—the part of the lower jaw extending from below the ear to the jaw angle—that was, on average, 3.6 mm shorter. A small-scale study of Japanese individuals also reported that some front teeth and upper canine teeth had shorter roots. This aligns with fossil evidence indicating that Neanderthals had shorter tooth roots than modern humans. However, the researchers stressed that the tooth root study had a small sample size requiring further validation, and that this single variant does not make a modern human face resemble that of a Neanderthal.
The distribution of this gene variant varies by region. It is almost entirely absent in sub-Saharan African populations but appears at relatively high frequencies in South Asian and East Asian populations. The researchers suggest that the DNA segment containing this gene was likely introduced through interbreeding between Neanderthals and Homo sapiens.
This study is drawing attention in the scientific community for corroborating statistical associations derived from the vast physical data of over one million individuals with cell-based experiments. However, the variant’s effects on height and weight—just 3 mm and 285 grams, respectively—make it difficult to view as a decisive factor in determining an individual’s physique. The researchers emphasized that a person’s physique results from the complex interplay of exercise, nutrition, and numerous other genetic and environmental factors, and that the Neanderthal-type variant should not be simplistically labeled a “muscle gene.”
