Complex sentence: The first recorded mutant on Earth is Selene Gallio (active as early as the Hyborian Age); as her particular mutant powers grant her immortality, however, she may not necessarily be the "first" mutant.
Split into simpler sentences: The first recorded mutant on Earth is Selene Gallio (active as early as the Hyborian Age), as her particular mutant powers grant her immortality. However, she may not necessarily be the "first" mutant.