Hunter-gatherers arrived in Japan in Paleolithic times, though little evidence of their presence remains, as Japan's acidic soils are inhospitable to the process of fossilization. However, the discovery of unique edge-ground axes in Japan dated to over 30,000 years ago may be evidence of the first Homo sapiens in Japan. Early humans likely arrived on Japan by sea on watercraft. Evidence of human habitation has been dated to 32,000 years ago in Okinawa's Yamashita Cave and up to 20,000 years ago on Ishigaki Island's Shiraho Saonetabaru Cave.