Indeed, it appears to derive from a deliberate introduction by humans. Though it is not clear whether this occurred in prehistoric times or more recently, this might be resolved by dating subfossil remains. Sequence evolution at least in mtDNA is known to proceed much more slowly in some turtles and tortoises than in others; the rate of the mitochondrial 12S rRNA gene in "Testudo" is probably a rather low 1.0–1.6% per million years (as this fits best the paleobiogeographical situation), limiting the resolution provided by molecular systematics.