Tens of thousands of years ago, Neanderthals mated with modern humans and passed along a gene variant that enables us to pack on more muscle, a new study finds. Researchers found that this specific Neanderthal gene, a variant of the growth hormone receptor gene, causes cells to respond strongly to growth hormone. The researchers think it entered the modern human gene pool through interbreeding, probably about 47,000 years ago . Neanderthals went extinct around 40,000 years ago, but passed along key genetic features to modern humans. The placenta produces a particular form of growth hormone for a fetus, which differs from the form of growth hormone produced by the pituitary gland.