A brown bear skull (Not exactly the same as the one used in the new study). Image credit: Shutterstock
A 32,500-year-old skull of a bear suggests that ice age brown bears moved to Honshu, Japan’s largest island. They lived close to Tokyo until eventually dying.
Today, Japan's only brown bear (Ursus arcos) lives on Hokkaido. It is the island with the most northern islands in the Japanese archipelago. There is evidence that these bears' ancestors migrated from Sakhalin (an island north of Hokkaido, now part of Russia), to reach the island. They likely walked over the land bridge connecting Sakhalin and Hokkaido during the Pleistocene, which lasted approximately 2.6 million years to 11,700 years ago.
Researchers noted that brown bears are no longer seen in Tokyo but their fossils, which date between 340,000 to 20,000 years ago, were found in several places on Honshu Island. The new report was published in Royal Society Open Science. This raises questions about when and how Honshu bears arrived on the island. Unfortunately, fossil evidence is not available to support this hypothesis.
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"The number of brown bear fossils excavated in Japan's Pleistocene is very small, with less that ten incomplete specimens," Takahiro Segawa (a senior assistant professor at University of Yamanashi’s Center for Life Science Research) told Live Science via email.
One unique specimen was found in a cave in Gunma Prefecture. It is located northwest of Tokyo. It contains the skull of a bear.
According to a 2015 journal PLOS One report, the dense structure of petrosals protects ancient DNA from being damaged. Therefore, these bone fragments are more likely to retain DNA than other fossilized bones. The research team gathered a small amount of powdered petra from the skull of a brown bear and brought it to the lab for DNA analysis.
The team found that the samples ranged in age from 32,700 to 32,000. The team then compared the genetic sequences of the petrosals to 95 nearly-complete genomes derived from brown bears in the Hokkaido area.
This analysis led them to conclude that the Honshu bear was part of a previously unknown lineage that split from its sister lineage, Southern Hokkaido brown bear clan, around 160,000 years ago. According to the authors, the bears may have crossed the Tsugaru Strait, which divides Hokkaido from Honshu at some point around that time.
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According to a 2005 study in Paleoceanography and Paleoclimatology, fossil evidence indicates that large mammals such as Naumann's elephants, Palaeoloxodon numanni, and the giant deer Sinomegaceros, crossed from Hokkaido and Honshu around 140,000 years ago. This was during a period of low sea levels. The authors speculate that brown bears might have used the same shallow waters to reach Honshu.
According to the authors, 340,000 years is the oldest known brown bear fossil found in Honshu. The fossil was found in a limestone quarry near the northernmost point Honshu. It predates the Honshu bear skull as well as the Southern Hokkaido Clade. This suggests that different lines of brown bears traveled to Honshu once more than 340,000 years ago and again in the Pleistocene.
The authors explained that the bears disappeared from the island as soon as they arrived. It is still unknown what caused these animals to disappear.
Original publication on Live Science