Bison originated in South Asia and China, with the earliest species appearing somewhere between 3.6 and 2.6 million years ago. From there, they spread across Eurasia as the steppe bison, eventually crossing into North America over a land bridge connecting Siberia to Alaska. The American bison grazing on plains today are the smaller, modern descendants of enormous Ice Age ancestors that arrived in at least two separate waves of migration.
Early Origins in Asia
The genus Bison first appeared during the Late Pliocene or Early Pleistocene in what is now South Asia and China. These early bovines were part of a larger family tree that includes modern cattle, and the evolutionary relationship between the two groups is tangled. Genetic studies have revealed ancient hybridization events between the bison lineage and the ancestors of cattle, making the exact branching point difficult to pin down.
From this Asian starting point, bison spread westward across Europe and eastward into the grasslands of Siberia. The species that came to dominate this vast range was the steppe bison, known scientifically as Bison priscus. These animals were substantially larger than any bison alive today, with massive horns and a heavy build suited to the cold, open grasslands of the Pleistocene. They were so prominent in the landscape that early humans painted them on cave walls hundreds of times. Roughly 820 known cave art depictions show bison, making up about 21% of all identified cave ornamentation from the Paleolithic era.
Two Waves Across the Bering Land Bridge
During the ice ages, so much of Earth’s water was locked in continental glaciers that sea levels dropped dramatically. The shallow seabed between Siberia and Alaska became exposed, forming a wide stretch of dry land called Beringia. Bison were among many large mammals, including mammoths, that walked across this land bridge into North America.
Genomic analysis of ancient bison bones has identified two distinct waves of migration. The first occurred roughly 195,000 to 135,000 years ago, bringing the earliest bison onto the continent. These pioneers preceded the explosion of different bison forms that would later roam North America. A second wave arrived between 45,000 and 21,000 years ago, during the last ice age when the land bridge was exposed again. Researchers confirmed these two pulses by comparing mutation rates in ancient DNA with the known periods when sea levels were low enough for overland crossing.
Ice Age Giants in North America
Once in North America, bison diversified into several species, some of which were enormous. The largest was Bison latifrons, the long-horned bison. It stood about 2.3 meters (roughly 8 feet) at the shoulder, stretched nearly 4.8 meters (over 15 feet) in body length, and weighed around 1,024 kilograms (2,257 pounds). Its most striking feature was its horns: the bony cores alone spanned 1.4 to 2.2 meters (up to 7.3 feet) from tip to tip.
Bison antiquus, sometimes called the ancient bison, was slightly smaller but still dwarfed modern bison. Its horn cores spanned about 1 meter. The steppe bison that had crossed from Asia also persisted in North America alongside these newer species. A specimen found in Soda Springs, Idaho, preserved in an ancient hot spring deposit dated to about 13,700 years ago, had a partial horn measuring 370 millimeters, larger than the maximum horn length recorded for any other North American bison species. The steppe bison went extinct at the end of the last ice age, roughly 10,000 years ago.
How Ancient Bison Became Modern Bison
Modern American bison didn’t simply replace their Ice Age ancestors. They evolved from them. Genetic evidence shows that Bison antiquus did not go extinct in the traditional sense. Instead, it gradually transitioned into the modern species through an intermediate form called Bison occidentalis. This shift was driven by adaptation to dramatic climate and environmental changes as the ice sheets retreated.
The most visible change was a steady decrease in body size. Between about 12,500 and 9,250 years ago, bison shrank considerably, losing both height and mass. Mitochondrial DNA from Bison antiquus specimens dated to around 12,100 to 12,200 years ago sits at the base of the genetic lineage that leads directly to living bison, confirming the continuity between the ancient and modern forms. What we see on the plains today is essentially a downsized, environmentally adapted version of a Pleistocene giant.
A Different Path in Europe
While American bison descended from the steppe bison through a straightforward (if size-reducing) evolutionary line, the European bison, called the wisent, took a more complicated route. Genetic research published in Nature Communications revealed that the wisent is actually a hybrid, the product of ancient crossbreeding between the steppe bison and the aurochs, the wild ancestor of domestic cattle.
This hybrid origin left traces not only in DNA but also in art. Cave paintings older than about 22,000 years show a bison form with long horns and a pronounced hump, resembling the steppe bison and modern American bison. But paintings from the Magdalenian period, roughly 17,000 to 12,000 years ago, depict a different animal: thinner double-curved horns, a smaller hump, and more balanced proportions that closely match the wisent. Ancient DNA from bison remains in European caves lines up with this artistic transition, with a steppe bison genotype found in a 19,000-year-old specimen from southern France and a wisent genotype in a 16,000-year-old specimen from northern Italy.
From Millions to Near Extinction and Back
Bison are one of the few large Pleistocene mammals that survived the wave of megafauna extinctions at the end of the last ice age. While mammoths, giant ground sloths, and saber-toothed cats all disappeared, bison persisted and thrived. By the time European settlers arrived in North America, an estimated 30 to 60 million bison roamed the continent. Industrial-scale hunting in the 19th century drove them to fewer than a thousand individuals by the late 1800s.
Conservation efforts pulled them back from the brink, though the bottleneck left a genetic mark. Historical crossbreeding with domestic cattle during early conservation attempts introduced cattle DNA into some bison herds, adding yet another layer of hybridization to an already complex evolutionary story. Today’s bison carry the genetic signatures of millions of years of migration, adaptation, and survival, tracing an unbroken line from the grasslands of ancient Asia to the plains of modern North America.