The Survival Journey of Horses: From North America to Global Existence
Horses have a rich history that traces back to North America, where they evolved and eventually spread to other continents. This journey was crucial for their survival, as they faced extinction in their native land. Recent studies have shed light on the genetic connections between horses in North America and Eurasia, offering insights into their migration patterns and the ongoing debate about their place in modern ecosystems.
Ancient Origins and Migration
The first ancient horses lived in forests 55 million years ago, both in Europe and America, and were the size of dogs. Horses resembling those we know today evolved in North America and spread to Asia and Europe between one million and 800,000 years ago, according to a new genetic study published in the journal Molecular Ecology. If these ancient horses hadn’t migrated to Eurasia, there might not be any horses today.
About 10,000 years ago, horses disappeared from North America, where they had thrived for over 50 million years. Researchers have investigated whether horses in North America and Europe had contact after the first dispersal. Peter Heintzman, an associate professor at UiT Norway's Arctic University, participated in this study.
Intercontinental Connections
The researchers analyzed ancient genetic material and found that horses from Eurasia and North America intermingled following the first dispersal. This was possible when the ocean level was low, creating a land bridge at the Bering Strait, connecting Siberia and Alaska. Heintzman explains, “We found out that the populations of horses traveled across the land bridge when it existed. They intermingled as well, indicating that North American and Eurasian horses were basically the same species.”
The land bridge existed during ice ages and cold periods over the last million years. While the exact timing of its accessibility is debated, genetic approaches could help hypothesize when it was accessible.
The Mustang Debate
Domestic horses originated from wild horses in Europe and Asia, domesticated about 6,000 years ago. The wild horses are gone, except for Przewalski’s horse from Central Asia. However, wild horses in North America today were reintroduced by the Spaniards in the 16th century. There is a debate in America about whether wild mustangs are an invasive species or a reintroduced native species. Heintzman suggests that biologically, it’s easier to view them as reintroduced rather than invasive.
The American Wild Horse Conservation (formerly American Wild Horse Campaign) supports studies that highlight the historical presence of horses in North America. Recently, another study showed that wild horses and donkeys dig water holes in dry areas, potentially improving water access for other animals.
Genetic Insights
The new study reveals at least two periods in the last million years where horses migrated between continents after the first dispersal. Researchers examined mitochondrial DNA, which tracks maternal lineage, and nuclear DNA, which provides insights into intermingling. Despite limited ancient DNA samples, the study indicates some degree of interbreeding.
Appearance of Ancient Horses
A million years ago, American horses were as large as Shire horses, the world's largest breed. Between 25,000 and 15,000 years ago, they shrank to resemble Icelandic horses. Another genus, Haringtonhippus, also existed in North America but died out at the end of the last ice age.
Przewalski’s horse, once considered the last wild horse, has a single color variant and is short and stocky. Genetic evidence suggests that ice age horses had varied coat colors, as depicted in cave paintings. A 2018 study indicates that Przewalski’s horse descended from early domesticated horses and later became wild.
Wild Horses in Scandinavia?
Heintzman speculates that wild horses might have existed in Norway, but evidence is scarce due to glacial coverage during the last ice age. However, 120,000 years ago, Scandinavia was ice-free, and megafauna likely roamed the region.