The forebears of the modern domestic horse used the crucial land bridge between Eurasia and North America during at least two time periods, dispersing their genes in both directions, researchers have found.
The international study team determined that the ancestors of modern horses, including the Przewalski’s horse, moved between Eurasia (comprising the Russian Urals, Siberia, Chukotka, and eastern China) and North America (comprising Yukon, Alaska and the continental United States) at least twice.
It happened during the Late Pleistocene (2.5 million years ago to 11.7 thousand years ago).
“We found out that the Beringian Land Bridge, or the area known as Beringia, influenced genetic diversity within horses and beyond,” said Dmitry Gimranov, a senior researcher at the Ural Branch of the Russian Academy of Sciences and the Ural Federal University.
The Bering Land Bridge was located at what is now the Bering Strait, which separates the extreme northwest of America and the extreme northeast of Asia.
The physical and geographical region of Beringia stretched from the Lena River in Russia to the Mackenzie River in Canada. It consisted of both land and sea components.
The land area of Beringia changed during the Pleistocene, along with changes in the size of continental ice and its effect on sea level.
Gimranov said the formation of the land bridge meant that the flow of genes among mammoths, bison, and wolves could occur regularly.
“And if 1 million to 800,000 years ago, horses from North America were not yet widespread in Eurasia, then in the periods of 950 to 450 and 200 to 50 thousand years ago, there was a bidirectional spread of genes over long distances.”
In other words, horses migrated between continents not only in one direction but also vice versa. The first wave of migration was predominantly from North America to Eurasia. The second migration was dominated by the movement from Eurasia to North America.
The researchers concluded that most animals used the Beringian Land Bridge only once, while horses used it several times.
This could have significantly affected the genetic structure of horse populations and made them very interesting objects of research for paleogeneticists, the researchers said.
To determine the area of settlement of horses, molecular biologists studied horses’ DNA from both continents.
From 262 samples of bones and teeth, they selected 78 with sufficient DNA. Researchers conducted radiocarbon dating and genetic analysis at laboratories in Denmark and the United States. In addition, they looked at the research data from 112 samples.
“The data shows that horses returned to North America from Eurasia across Beringia at about the same time as bison, brown bears, and lions,” Gimranov said. “That is, in the last ‘days’ of the late Pleistocene, when the territory was not covered by water and it was like a bridge for the movement of many groups of animals.
“With the beginning of climate warming (the beginning of the Holocene, or 11.7 thousand years ago) and the last disappearance of Beringia at the end of the Pleistocene, the biogeographic significance of this ecological corridor radically changed the history of terrestrial animal species on both continents.”
Although the North American horse population eventually became extinct in the early Holocene, horses became widespread on both continents through domestication and are now found far beyond their historical range.
The researchers’ findings have been published in the journal Molecular Ecology.
The abstract of the study, “Ancient horse genomes reveal the timing and extent of dispersals across the Bering Land Bridge,” can be read here.