Complex sentence: Whatever the case of their origins, it is likely that their grotesque appearance and unique constitution owes much to the race living generation after generation in an intensely hostile and radioactive environment, as a result of both natural selection and genetic drift.
Split into simpler sentences: Whatever the case of their origins, their grotesque appearance and unique constitution owes much to the race living generation after generation in an intensely hostile and radioactive environment. As eons passed they became more distinct from their ancestors as a result of both natural selection and genetic drift.