Darwin's evolutionary theories got a big boost when Othniel Charles Marsh assembled his North American horse fossils in the 1870s. They showed the gradual (presumed linear) evolution of a small, multi-toed
Since then, the validity, origin, and relationships of these classifications have been revised, especially for the species of the Pliocene (5 Ma to 1.8 Ma ago) and Pleistocene epochs. In a new paper, Jaco Weinstock, Alan Cooper, and their colleagues use ancient DNA to upset conventional models describing the origins of two extinct forms: the North American “stilt-legged” horses and the stocky South American genus,
New World “stilt-legged” horses often appear in fossil deposits along with a second equid form that shares morphological characteristics with Eurasian caballines—a group that includes the domestic horse and the nearly extinct Przewalskii horse of Mongolia. Because stilt-legged horses had builds similar to the onager and kiang, rare wild asses that live in Asia (Asian hemionids), it's been suggested that stilt-leggeds migrated from Asia across the Bering Strait.
To analyze the horse fossils, Weinstock et al. used a well-established genetic technique based on mitochondrial DNA (mtDNA). Mitochondria provide most of a cell's energy needs, but it is their genome that interests evolutionary biologists—specifically, a stretch of mtDNA sequence called the control region. This region mutates at a high rate, but the patterns of mutations remain stable over thousands of generations, providing a tool for inferring evolutionary relationships.
The authors first extracted mtDNA from horse bones (mostly toes) from Eurasia and North and South America dating back to 53,000 years ago. Their genetic analysis showed that
A final analysis of horse specimens from the Pleistocene, historic, and recent caballines—which have been grouped as separate species based on their diverse size—suggests that all North American caballines may belong to the same species. Altogether, the results suggest that just two horse lineages—caballine and stilt-legged—may have lived in North America during the Late Pleistocene. Both lineages showed regional and temporal variations in size and form. Though these variations have been taken to represent many different species, the authors propose that the two lineages are more likely just two species whose variations reflect adaptations to different environments. If true, this model could provide a tool for exploring how environmental adaptations give rise to morphological variation.