The Wisent, or European bison, is a look-alike of the American bison, but its genetics tell a different story.
As the story goes, once upon a time between ice ages, steppe bison wandered the grasslands of Europe, Asia, and North America, traversing the Bering land bridge during glaciated times. Cut off from Asia when sea levels rose, the steppe bison in North America underwent moderate changes to become the buffalo we recognize today, but then suffered huge declines in numbers when railroads through the central plains brought hunters that killed off huge numbers of them.
The steppe bison was also hunted extensively, and may have gone extinct from overhunting about 11,000 years ago. But aurochs (European progenitor of cattle) and Steppe bison matings in Europe produced hybrids (now known as Wisent) that survived the hunting pressure, perhaps by retreating deep into the primeval forest.
Wisent numbers also declined precipitously with settlement and expansion of agriculture in Europe, but a few remained in the small forest fragments, like the one near Bielowieza.
Hybrids usually are less fit than their ancestors, largely because they are less fertile (e.g., mules), but in the case of the European bison hybrid, they appear to have survived both hunting pressure and the extreme cold of the interglacial periods better than their steppe bison ancestors, and retreated to the forest for protection.
How do we know this? A fascinating study compared animals in cave art paintings with DNA fragments from bison remains of 15-50,000 years ago and found transitions in the DNA that coincided with animals represented in the cave art. The animals represented during the coldest periods were the short horned, less humped at the shoulder, Wisent.**
Captive breeding of Wisent at the Bison reserve just outside Bielowieza attempts to track genetic ancestry and propagate animals that could be released to the wild, but wild Wisent exist in small herds through the forest-grassland spaces in eastern Poland. They are mighty hard to spot — judging from our early morning explorations of the area.