Ancient Horse DNA Reveals Gene Flow Between Eurasian and North American Horses
New findings reveal fascinating connections between ancient horse populations in North America, where horses originally evolved, and Eurasia, where they were domesticated. A recent study of ancient DNA from horse fossils found in both regions shows that these populations remained connected through the Bering Land Bridge, facilitating movement and interbreeding over hundreds of thousands of years.
The study demonstrates the genetic continuity between horses that became extinct in North America at the end of the last ice age and those domesticated in Eurasia, later reintroduced to North America by Europeans. Published in the journal Molecular Ecology, the study is currently accessible online.
“The results of this paper show that DNA flowed readily between Asia and North America during the ice ages, maintaining physical and evolutionary connectivity between horse populations across the Northern Hemisphere,” said Beth Shapiro, professor of ecology and evolutionary biology at UC Santa Cruz and a Howard Hughes Medical Institute investigator.
The Bering Land Bridge served as an ecological corridor for large animals during the Pleistocene, when massive ice sheets formed during glacial periods. Lower sea levels exposed Beringia, a vast land area with extensive grasslands supporting horses, mammoths, bison, and other fauna.
Paleontologists have long known that horses evolved in North America. One lineage, the caballine horses, dispersed into Eurasia over the Bering Land Bridge about 1 million years ago, diverging genetically from those remaining in North America.
The study reveals at least two periods of movement and interbreeding between the continents, allowing North American horses to acquire Eurasian DNA and vice versa.
“This is the first comprehensive look at the genetics of ancient horse populations across both continents,” said Alisa Vershinina, a postdoctoral scholar at UC Santa Cruz. “With data from mitochondrial and nuclear genomes, we saw horses dispersing and exchanging genes.”
Mitochondrial DNA, inherited from the mother, is useful for studying evolutionary relationships due to its steady mutation rate and abundance in cells. The nuclear genome, however, offers richer evolutionary information.
Researchers sequenced 78 new mitochondrial genomes from ancient horses across Eurasia and North America, combining them with 112 previously published genomes to reconstruct a phylogenetic tree, tracking the movements of ancient horse lineages.
“We found Eurasian horse lineages in North America and vice versa, suggesting cross-continental movements,” Vershinina explained.
The analysis identified two dispersal periods, both coinciding with the Bering Land Bridge's accessibility. In the Middle Pleistocene, movement was mostly east to west, while the Late Pleistocene saw movement in both directions, primarily west to east. Limited sampling may have missed other events.
The team also sequenced two new nuclear genomes from well-preserved horse fossils in Yukon Territory, Canada, combining them with seven previously published genomes to quantify gene flow between Eurasian and North American populations.
“The usual view was that horses differentiated into separate species in Asia, but these results show continuity between populations,” said Ross MacPhee, a paleontologist at the American Museum of Natural History. “They interbred freely, as seen in the genomes of fossils from both sides.”
The findings fuel the debate over managing wild horses in the U.S., descendants of domestic horses brought by Europeans. Some view them as invasive, while others see them as native fauna.
“Horses persisted in North America for a long time, occupying an ecological niche,” Vershinina said. “They died out about 11,000 years ago, but that’s not much time in evolutionary terms. Present-day wild North American horses could be considered reintroduced, rather than invasive.”
Grant Zazula, a paleontologist with the Government of Yukon, said the findings help reframe why horses disappeared from North America. “It was a regional population loss, not an extinction,” he said. “Conditions in North America were dramatically different at the end of the last ice age. If horses hadn’t crossed to Asia, we would have lost them globally.”
This project was a large international collaboration involving researchers from multiple institutions, including the University of Toulouse, France, the Arctic University of Norway, and others in the U.S., Canada, Sweden, Denmark, Germany, Russia, and China. Supported by the U.S. National Science Foundation, Gordon & Betty Moore Foundation, and the American Wild Horse Conservation (formerly American Wild Horse Campaign).
Originally posted by Zephyr Net