Lions and brown bears colonized North America in multiple synchronous waves of dispersal across the Bering Land Bridge

Abstract

The Bering Land Bridge connecting North America and Eurasia was periodically exposed and inundated by oscillating sea levels during the Pleistocene glacial cycles. This land connection allowed the intermittent dispersal of animals, including humans, between Western Beringia (far north-east Asia) and Eastern Beringia (north-west North America), changing the faunal community composition of both continents. The Pleistocene glacial cycles also had profound impacts on temperature, precipitation, and vegetation, impacting faunal community structure and demography. While these paleoenvironmental impacts have been studied in many large herbivores from Beringia (e.g., bison, mammoths, horses), the Pleistocene population dynamics of the diverse guild of carnivorans present in the region are less well understood, due to their lower abundances. In this study, we analyze mitochondrial genome data from ancient brown bears (Ursus arctos; n = 103) and lions (Panthera spp.; n = 39), two megafaunal carnivorans that dispersed into North America during the Pleistocene. Our results reveal striking synchronicity in the population dynamics of Beringian lions and brown bears, with multiple waves of dispersal across the Bering Land Bridge coinciding with glacial periods of low sea levels, as well as synchronous local extinctions in Eastern Beringia during Marine Isotope Stage 3. The evolutionary histories of these two taxa underscore the crucial biogeographic role of the Bering Land Bridge in the distribution, turnover, and maintenance of megafaunal populations in North America.We would like to thank the following institutions for allowing access to specimens in their collections:

Insights into bear evolution from a Pleistocene polar bear genome

“…Although several studies demonstrate a consistent signal of gene flow between the polar bear and brown bear lineage, we find that previous use of the f 4 -ratio estimation to infer gene flow direction between polar and brown bears ( 8 ) has been inadequate, and even inappropriate by its required use of unadmixed populations. Instead, we find that admixture graph fitting, using the methods of admixturegraph and TreeMix analyses, favors predominant gene flow into the polar bear lineage from ancestors of Alexander Archipelago brown bears, whose matriline likely was once more geographically widespread, at least ranging also to Haida Gwaii and interior Alaska ( 62 ). This admixture would have occurred before the split between the ∼120,000-y-old polar bear and MPB, an inference that is supported by negative f 3 values observed for multiple polar bear individuals, including APB, but no definitive evidence for admixed brown bear individuals.…”

Insights into bear evolution from a Pleistocene polar bear genome

“…Although several studies demonstrate a consistent signal of gene flow between the polar bear and brown bear lineage, we find that previous use of the f 4 -ratio estimation to infer gene flow direction between polar and brown bears (8) has been inadequate, and even inappropriate by its required use of unadmixed populations. Instead, we find that admixture graph fitting, using the methods of admixturegraph and TreeMix analyses, favors predominant gene flow into the polar bear lineage from ancestors of Alexander Archipelago brown bears, whose matriline likely was once more geographically widespread, at least ranging also to Haida Gwaii and interior Alaska (61). This admixture would have occurred before the split between the ~120,000-year-old polar bear and modern polar bear, an inference that is supported by negative f 3 values observed for multiple polar bear individuals, including APB, but no definitive evidence for admixed brown bear individuals.…”

Demography and evolutionary history of grey wolf populations around the Bering Strait

“…The same pattern is also being suggested for the early lineages of domesticated dogs, which diverged from the Eastern Siberian lineage c. 16 ka (da Silva Coelho et al, 2021). Other large carnivores, such as brown bears ( U. arctos ), could also have used this coastal route to reach the SEAk region c. 17 ka (Salis et al, 2021). Based on the similarity between our estimated divergence time for inland and coastal NNA populations and those previously described for other species, and the concordance with geomorphological evidence of a viable coastal route, wolves may also have taken such a route when expanding out of Beringia towards continental North America.…”