DNA Sample from Ancient Bear Skull Offers Insight on How Their Ancestors Migrated to Japan

Extracted DNA samples from an ancient bear skull, dated 32,500 years old, offer insight into how Ice Age bears migrated to Honshu, lived near what is now Tokyo, and eventually died out.

Today, the only bears native to Japan are brown bears (Ursus arctos) that live in Hokkaido, the East Asian nation's northernmost island. A previous study has shown that the ancestors of these bears most likely came from Sakhalin, an island north of Hokkaido and is now a part of Russia. Land bridges that used to connect a lot of modern-day islands could have provided these ancient bears the pathway to cross the island of Hokkaido at different parts of the Pleistocene (2.6 million years ago to 11,700 years ago).

Now, a new study comes after the discovery of multiple remains belonging to brown bears across the island of Honshu, Japan's main island, where the cities of Kyoto and Tokyo are located. The fossils, ranging between 340,000 to 20,000 years old, raise the question of how brown bears that are now only in Japan's northernmost island got to Honshu a little more to the south. Possible explanations on the brown bears' lineages based on extracted DNA samples are presented in the report "Ancient DNA Reveals Multiple Origins and Migration Waves of Extinct Japanese Brown Bear Lineages," appearing in the Royal Society Open Science journal, August 4.

(Photo: Photo credit should read TORU YAMANAKA/AFP via Getty Images)
People look at a Hokkaido brown bear in an enclosure at Ueno Zoo in Tokyo on December 25, 2011. Many people visited the zoo on the last Sunday of this year.

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A Surprising Find on Ancient Brown Bears in Japan

Lead author of the study Takahiro Segawa, who is a senior assistant professor from the Center of Life Science Research Institute at the University of Yamanishi, tells LiveScience that remains belonging to brown bears from the Pleistocene in Japan are "scarce," with the country having less than ten incomplete specimens.

One of the recovered samples coming from a cave in the Gunma Prefecture located northwest of Tokyo includes a well-preserved brown bear skull, still with both of its petrosals, which are dense sections of the temporal bone at the base of the skull that surrounds the inner ear.

More importantly, these petrosals helped protect DNA samples within the skulls from degrading, as detailed in a 2015 report on the excavation, appearing on PLOS One. Moving forward with this knowledge, the researchers on the new study collected a small amount of powdered petrosal from the ancient brown bear skull for further analysis of the DNA samples.

Revealing Multiple Origins, Migration Waves for Brown Bears' Lineages

Dating the recovered DNA samples, the researchers suggest that it is 32,700 to 32,200 years old, comparing its genetic sequence from petrosals with 95 near-complete sequences from other brown bears. These include the extant lineages of brown bears in Hokkaido.

Researchers found that the ancient bear skull they tested belongs to a previously unknown lineage that diverged from its more common sister lineage, the Southern Hokkaido brown bear clade. This divergence between the two brown bear lineages is estimated to have happened some 160,000 years ago, with this line crossing what is now the Tsugaru strait that separates the islands of Hokkaido and Honshu.

The new theory surrounding black bears is also consistent with previous studies showing how other large mammals, particularly Naumann's elephants (Palaeoloxodon naumanni) and the giant deer (Sinomegaceros yabei), both crossed a little later, about 140,000 years ago, during a time when sea levels were significantly lower, as reported in a 2005 study in the Paleoceanography and Paleoclimatology journal.