The SMN2 gene can alleviate spinal muscular atrophy, a neurological disease caused by mutations of SMN1 (the ancestral paralog of SMN2) (Parsons et al., 1996; Lorson et al., 1999; Watihayati et al., 2009), and thus can be regarded as an, albeit inefficient, SMN1 back-up specific to modern humans. This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.