Possession of a genetic variant inherited from Neanderthals may contribute to increased muscle mass. A genetic variant, originally from Neanderthals, changes how cells respond to growth hormone. Almost all Neanderthals had this variant, while only about 10 percent of modern humans do, with notable differences depending on ancestry. The interbreeding between Neanderthals and modern humans has left a lasting impact on our DNA. A study published on August 5 in the journal Current Biology identified a variant in the gene that codes for the growth hormone receptor, making cells more responsive to the hormone. This variant entered the human genome about 47,000 years ago. Two new studies have refined the timeline of when interbreeding between Homo sapiens and Neanderthals began, showing that Eurasian populations today are the result of 7,000 years of hybridization. The distribution of this variant is uneven: nearly all Neanderthals carried it, compared to about 10 percent of modern humans. Among people of European descent, it is found in about 1 percent, while up to 25 percent of people from South Asia or Southeast Asia carry it. It appears to be absent in people of African descent, likely because interbreeding occurred in Eurasia. Hugo Zeberg, a geneticist at the Karolinska Institute in Sweden and co-author of the study, estimates the most visible effect of the variant: about 270 grams of additional muscle mass per copy of the variant. Someone who inherits two copies, one from each parent, could have more than half a kilogram of extra muscle. However, this doesn’t necessarily translate to a more muscular physique. Instead, the effect is more subtle, including higher levels of a key signaling protein in response to growth hormone. Other traits include slightly shorter dental roots and small variations in jaw shape, characteristics observed in Neanderthals. The study shows that some modern humans have inherited a Neanderthal gene that promotes greater muscle mass and craniofacial features similar to those of Neanderthals. To investigate the effects of this variant, the researchers analyzed DNA from ten Neanderthal samples and two Denisovan samples, the latter from a group related to Neanderthals, found in archaeological sites across Europe, the Balkans, Siberia, and Asia. These data were compared to biobanks containing over 1.1 million human genomes, each accompanied by physical characteristics that allow researchers to evaluate the effects of the variant. While the genomes of Neanderthals and Denisovans have been sequenced, many genes remain unexplored. A laboratory experiment complemented the study: cells modified to express either the Neanderthal variant or the modern human version were exposed to growth hormone. Those with the Neanderthal variant multiplied about 40 percent more frequently. A surprising finding is the timing of the variant’s effect. The placenta produces a distinct form of growth hormone, different from that secreted by the pituitary gland. The Neanderthal variant does not respond to the placental hormone, meaning Neanderthal babies were born the same size as modern human babies but grew faster afterward. Data on modern humans support this: among children under 11 carrying the variant, no effect is visible. The effect only appears in older individuals, suggesting a limited response to pituitary hormone. This finding challenges previous assumptions about Neanderthal evolution. By examining 250,000-year-old teeth from the Payre cave, researchers found a mix of anatomical traits that suggest Neanderthals may not have evolved along a strictly linear path. As Hugo Zeberg notes, the growth hormone plays a significant role during puberty, when its levels rise sharply. Therefore, the effect of the variant likely occurs during and after this period, not before birth. This discovery is a piece of a larger puzzle. Neanderthals were generally more stocky than modern humans, but this variant alone does not explain their overall body shape, according to Zeberg. Their robustness likely resulted from multiple genes and factors like development, lifestyle, and environment. Kornelius Kupczik, a paleoanthropologist at the University of Chile, who was not involved in the study, points out that the high prevalence of the variant among people of South Asian and Southeast Asian descent, compared to its rarity among Europeans, raises questions about the complexity of early human migrations. He emphasizes that this finding highlights the need for further research into the movement and interactions of early human populations.