The science has a clear cut answer to this, not only with twin studies but also with different genes having been tied to specific body features that directly affect athletic ability.
If genetics didn't play a meaningful role in athleticism, we wouldn't have evolved it. But even if that was true at some point in history, that would only be temporary, because without selective pressure, genetic drift would move the population to some point where there was selective pressure.
Obviously environment plays a role, with climate giving kids in California more practice time, or the annual cut-off date in team sports affecting who's in what age group.