Did the “Neanderthal Muscle Gene” Really Turn Modern Humans into Muscle Heads?
A real molecular effect, a relatively large single-SNP association, and the limits of a catchy label
On 5 August, Science reported on a new Current Biology paper by Philipp Kanis and colleagues, “Increased signaling of the Neanderthal growth hormone receptor.” It packaged the paper with a headline engineered to travel: “Neanderthal growth gene turns modern humans into muscle heads.” Its deck made the narrower claim that people carrying DNA from our evolutionary cousins tend to have more lean muscle mass. The visual framing pushed the stronger interpretation further: Science illustrated the story with two of the paper’s co-authors flexing their arms, and a caption suggesting that their visible difference in physique “might partly derive” from one of them carrying the Neanderthal variant. The distance between the underlying result and this presentation is the central problem. One reports the direction and magnitude of a population-average association; the other invites the reader to see the variant manifested in the physique of an individual. [1,2]
No participant in the study was shown to have been turned into a “muscle head.” The result closest to that claim was 150 g more arm-and-leg lean mass per haplotype copy (95% CI 64–236 g) and 121 g more fat-free trunk mass (50–192 g)—about 271 g combined. Across five biobanks, adult carriers also averaged 0.30 cm taller and 285 g heavier per copy. Those are persuasive associations and relatively large effects for one common-variant signal. They are not a bodybuilding phenotype. The study measured no muscular appearance, grip strength, torque, power or athletic performance. [2]
The news story also changes a cell-count result into a cell-size claim. It says cells with the Neanderthal receptor “grew about 40% larger.” The paper reports approximately 39% more cells by day five, following a 6% increase in exponential growth rate—not cells that were 40% larger. Nor did the experiment show a statistically significant shift in apparent pituitary-growth-hormone sensitivity: the dose-response EC50 values were 0.07 and 0.06 nM (P=0.13). It showed increased signaling output and proliferation in a particular engineered cell system. That is biologically interesting, but it is a different result. [1,2]
Science’s visual framing turns a population-level association into an individual before-and-after-style contrast. Maričić is larger and carries one copy of the haplotype; Kanis is slighter and does not. But a staged comparison of two co-authors cannot separate genotype from height, age, training, diet, body fat, ancestry or the rest of their genomes. The evidence is the adjusted carrier–noncarrier comparison across cohorts, not which scientist looks more muscular in a photograph. The possibility suggested by the caption cannot be tested from the photograph itself. [1,2]
To its credit, the Science report quotes three researchers warning readers not to explain Neanderthal morphology with one variant. Those cautions are right; the headline, opening rhetoric, cell-size error and flexing photograph nevertheless push in the opposite direction. Correcting that framing does not require pretending the variant is trivial. The molecular effect is real, and the lean-mass and height associations are relatively large by single-SNP standards. What they do not provide is an explanation of strength, individual physique, population differences or the overall Neanderthal body plan. [1,2]
The mistake is not believing that the variant matters. It is confusing a relatively important SNP with an explanation of a highly polygenic phenotype.
But how important is “relatively important” for lean mass? The direct benchmark is Pei et al.’s 2020 genome-wide association study (GWAS) of 450,243 UK Biobank participants. [3] It measured appendicular lean mass—the estimated fat-free mass of the arms and legs—and selected 1,059 primary and conditional associations at a stringent genome-wide threshold. rs6184 was not among them. Yet its standardized point estimate was larger than 80.6% of that selected set, equivalent to rank 206 by magnitude. Its P value, 6.35×10⁻⁴, was far above the study’s threshold.
How can rs6184 look fairly large by single-SNP standards, fail genome-wide significance in the lean-mass GWAS and explain only a few thousandths of one percent of lean-mass variation? And why does the broad population score point in the opposite direction from the haplotype’s frequency? Resolving those apparent contradictions requires separating effect magnitude from statistical certainty, one allele from the polygenic background, and lean mass from actual strength. [3]