The Bering Land Bridge was episodically open throughout the Pleistocene until about 13,000 years ago. A corridor was created by falling sea levels that provided an opportunity for Asian species including mammoths, bison, muskoxen, caribou, lions, brown bears, and wolves to move into North America. One American species, the horse, dispersed westward across the land bridge to Asia. One iconic Pleistocene species, the woolly rhino, never made the journey east into Alaska, while short-faced bears never ventured west to Siberia.
Pleistocene Megafauna and the Bering Land Bridge
Interestingly, DNA evidence suggests that despite its vast area, the land bridge was not a busy highway with populations moving back and forth between the continents. While the distribution of woolly mammoths extended from Europe, across Siberia into North America, the genetic evidence suggests after colonizing North America, there was minimal gene flow back west across the land bridge (Chang et al. 2017). It is possible that some eastern Beringian males dispersed westward long distances, just as modern male elephants do, but female mammoths mostly stayed on just one side or the other of the land bridge. Pleistocene bison also had a wide distribution and had two major dispersals into North America from Asia. But DNA analysis shows a fair amount of genetic separation of Pleistocene bison from east and west Beringia suggesting there was only limited movement of bison back west over the land bridge (Shapiro et al. 2004, Froese et al. 2017).
Since the Land Bridge is now flooded by the Bering Strait, we have no way of knowing if megafauna actually inhabited that region and what the landscape was like there during the Pleistocene. The Bridge would have been low-lying land and closer to the ocean than the continental Mammoth Steppe. There is some evidence from the edges of the land bridge that when exposed, it was dominated by shrub vegetation and had a mesic (wet) climate (Elias and Crocker 2008). These characteristics would have made it a barrier to the grazing species adapted to well-drained grasslands. Movements of megafauna across the land bridge may have been limited to narrow windows of time when the conditions were more favorable.
Chang, D., M. Knapp, J. Enk, S. Lippold, M. Kircher, A. Lister, R. D. MacPhee, C. Widga, P. Czechowski, and R. Sommer. 2017.
The evolutionary and phylogeographic history of woolly mammoths: a comprehensive mitogenomic analysis. Sci Rep 7: 44585.
Elias, S. A. and B. Crocker. 2008.
The Bering Land Bridge: a moisture barrier to the dispersal of steppe-tundra biota? Quaternary Science Review 27: 2473-2483.
Froese, D., M. Stiller, P. D. Heintzman, A. V. Reyes, G. D. Zazula, A. E. R. Soares, M. Meyer, E. Hall, B. J. L. Jensen, L. J. Arnold, R. D. E. MacPhee, and B. Shapiro. 2017.
Fossil and genomic evidence constrains the timing of bison arrival in North America. Proc Natl Acad Sci USA.
Shapiro, B., A. J. Drummond, A. Rambaut, M. C. Wilson, P. E. Matheus, A. V. Sher, O. G. Pybus, M. T. P. Gilbert, I. Barnes, J. Binladen, E. Willerslev, A. J. Hansen, G. F. Baryshnikov, J. A. Burns, S. Davydov, J. C. Driver, D. G. Froese, C. R. Harington, G. Keddie, P. Kosintsev, M. L. Kunz, L. D. Martin, R. O. Stephenson, J. Storer, R. Tedford, S. Zimov, and A. Cooper. 2004.
Rise and fall of the Beringian steppe bison. Science 306: 1561-1565.