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Fossil, Genomic Evidence Pinpoints Bison Arrival in North America

NEW YORK (GenomeWeb) – Bison first arrived in North America from Asia between 195,000 years and 135,000 years ago, according to a combined paleontological and genomic analysis.

As they reported today in the Proceedings of the National Academy of Sciences, researchers led by the University of California, Santa Cruz's Beth Shapiro analyzed two fossil-rich locations in Canada's Yukon as well as sequenced the mitochondrial genomes of a bison bone found in the Yukon and one previously unearthed in Colorado. By comparing these bison mitochondrial genomes to those from some 44 other bison, they began to piece together the history of bison in North America.

"After their arrival, bison quickly colonized much of the rest of the continent, where they rapidly diversified phenotypically, producing, for example, the giant long-horned morphotype Bison latifrons during the last interglaciation," Shapiro and her colleagues wrote in their paper.

For this study, the researchers characterized two fossils assemblages in the Old Crow area of the Yukon, CRH11 and Ch'ijee's Bluff. Both sites are marked by the same tephra, or layer of volcanic ash, that dates back 124,000 years, give or take 10,000 years. This timeframe spans the transition from a glacial to interglacial period, but paleoecological evidence gives greater support to the deposition occurring during the late glacial period, the researchers noted.

Nearly 300 vertebrate fossils have been uncovered at CRH11, including woolly mammoth, horse, and caribou, but bison fossils have been lacking. The CRH11 assemblage falls below the tephra, suggesting that the fossils found there are older than the last interglacial period. A direct single-grain optically stimulated luminescence dating analysis gave a mean age of 208,000 years for this site.

Bison fossils are present, however, at the younger Ch'ijee's Bluff site, which is located above the tephra layer. Based on this, the researchers estimated the bison fossils there to be about 130,000 years old.

Shapiro and her colleagues sequenced mitochondrial DNA obtained from a partial bison metacarpal found at Ch'ijee's Bluff and a 120,000-year-old humerus previously found near Snowmass, Colorado. Based on their size and location of origin, the researchers noted that the Ch'ijee's Bluff sample came from a steppe bison, B. priscus, while the Snowmass sample came from a giant long-horned bison, B. latifrons.

They generated a complete genome with 159X coverage for the Ch'ijee's Bluff sample and a near-complete, 6.6X coverage genome for the Snowmass sample.

They compared these mitochondrial genomes to 44 other bison mitogenomes, including six Siberian and 26 North American bison between 400 and 45,000 years old, 10 modern-day bison (B. bison), and one ancient Siberian bison, to construct a phylogeny.

The researchers reported that the Ch'ijee's Bluff and Snowmass samples fell near the base of the tree, suggesting that they were both descendants of the first bison that came to North America. In addition, they reported that the North American bison share a common maternal ancestor dating back to between 195,000 years and 135,000 years ago.

This close genetic relationship suggested to Shapiro and her colleagues that bison quickly expanded across North America and underwent rapid phenotypic change from the steppe bison found in Siberia and the Yukon to becoming the giant long-horned bison found in the continental United States.

The researchers also noted a second wave of bison migration to North America that took place during the Late Pleistocene, between 45,000 years and 21,000 years ago.