Testing of ancient DNA has revealed fresh insights into the evolutionary history of the horse, with findings challenging some common views.
The rich fossil record of horses has made them a classic example of evolutionary processes.
However, while the overall picture of equid evolution is well known, the details are surprisingly poorly understood, especially for the later Pliocene and Pleistocene, from around 3 million years ago to about 10,000 years ago.
This is especially so in the Americas, Dr Jaco Weinstock and his colleagues noted in the open-access journal PLOS Biology.
“Nowhere is the later evolution of horses more problematic than in the Americas, where more than 50 species of Pleistocene equids have been named, most of them during the 19th and early 20th centuries,” they wrote.
Weinstock, a lecturer in zooarchaeology at the University of Oxford, and his fellow researchers observed that while evidence from more recent studies suggest this number should be drastically revised, there is no consensus on the number of equid species, or even the number of lineages that existed in these continents.
“Likewise, the origin of the endemic South American genus Hippidion is unresolved, as is the evolutionary position of the ‘stilt-legged’ horses of North America.
Using analysis of ancient DNA sequences, the researchers have shown that, in contrast to current models based on body form and a recent genetic study, Hippidion was, in evolutionary terms, close to the caballine (true) horses, with origins considerably more recent than the currently accepted date of around 10 million years ago.
Their analysis also revealed that stilt-legged horses, commonly regarded as Old World migrants related to the hemionid asses of Asia, were in fact an endemic North American lineage.
The DNA evidence also suggests that there were far fewer horse species in late Pleistocene North America than have been named on the basis of body shape.
“Both caballine and stilt-legged lineages may each have comprised a single, wide-ranging species,” they reported.
The study team worked on ancient mitochondrial DNA from fossil equid remains from North and South America and Eurasia, ranging in age from about 53,000 years ago to historical times. The DNA was extracted from cortical bone using ancient-DNA techniques.
The authors said the close evolutionary relationship between Hippidion and caballine horses is in direct contrast to current paleontological models of hippidiform origins.
“Nevertheless, we are confident that these sequences are those of Hippidion rather than the South American caballine form E. (Amerhippus), which dispersed into South America later than Hippidion.”
Hippidion, they said, should not be seen as a descendant from the Miocene pliohippines; instead, its origins appear to be much more recent, probably during the last stages of the Pliocene, around 3 million years ago.
The origins of the New World stilt-legged horse probably lie south of the Pleistocene ice sheets, they said, where Mid-Pleistocene remain from about 500,000 years ago, with similar limb characteristics, have been found. Late Pleistocene specimens occur in high frequencies.
North of the ice sheets, remains of this horse are relatively rare and appear to have been there for only a short time.
“In spite of their presence in eastern Beringia (unglaciated Alaska/Yukon), the stilt-legged horses apparently failed to disperse through the Bering land-bridge into western Beringia (northeast Siberia).”
In addition, the extinction of this horse north and south of the ice sheets clearly took place at quite different times.
Populations in eastern Beringia disappeared around 31,000 years ago, while it persisted south of the ice sheets until at least 13,000 years ago, close to the date of the other megafaunal extinctions in North America.
In another significant finding, the researchers concluded that all caballine horses from western Europe to eastern Beringia — including the domestic horse — are a single species that ranged across the majority of habitats found throughout the northern continents of the world.
This species, they said, was highly mobile and adaptable.
The authors noted that size has been used as one of the main criteria for defining species of Pleistocene equids. The body size of the Late Pleistocene North American caballines sampled certainly exhibited marked regional variability, they acknowledged.
“The DNA evidence strongly suggests, however, that all of these large and small North American caballine samples belong to a single species.
“Thus, our results indicate that only two lineages — a caballine and a stilt-legged — may have been present in North America during the Late Pleistocene, each comprising perhaps only a single species with temporal and regional variation in body size and morphology (form).
“This model would greatly simplify the systematics of North American Pleistocene horses and could open the way to the analysis of morphological variation in terms of adaptation to different environments.
“The study of this variation, in combination with paleoclimatic and paleoenvironmental data, should dramatically improve our understanding of the biogeography, evolution, and extinction of horses in this continent.”
The study team, from a range of tertiary institutions around the world, comprises Weinstock, Eske Willerslev, Andrei Sher, Wenfei Tong, Dan Rubenstein, John Storer, James Burns, Larry Martin, Claudio Bravi, Alfredo Prieto, Duane Froese, Eric Scott, Lai Xulong and Alan Cooper
Weinstock J, Willerslev E, Sher A, Tong W, Ho SY, Rubenstein D, et al. (2005) Evolution, Systematics, and Phylogeography of Pleistocene Horses in the New World: A Molecular Perspective. PLoS Biol 3(8): e241. https://doi.org/10.1371/journal.pbio.0030241