American researchers suggest the remnants of all five toes are still present within horse hooves.
Their theory goes some way toward answering the riddle around how the horse evolved from an ancestor with five toes to the animal we know today.
While it is largely believed that horses simply evolved with fewer digits, researchers at the New York Institute of Technology College of Osteopathic Medicine have offered an alternative explanation.
Humans and horses are descendants of a common ancestor with five digits.
As horses evolved to live on open grassland, their anatomy required a more compact design to enable movement across the hard plains.
Until now, scientists believed horses adapted to these conditions by gradually evolving with fewer digits than its five-toed ancestor, with the first horse retaining only four digits, its later descendant reduced to three, and today’s horse retaining just the central digit known as the metacarpal, the long bone above the hoof.
For the first time, reported this week in the journal Royal Society Open Science, a paleontologist and anatomy professor at the institute, Nikos Solounias, and a team of researchers propose that the reduction in the number of digits is not a matter of simple attrition.
Instead, they believe that all five digits have merged to form the compacted forelimbs with hooves that we know today.
Currently, scientists accept that splints – small bones found along the outer sides of the metacarpal in modern horses – are partially formed remnants of second and fourth digits. Tapering mid-way down the metacarpal, these fragments were inherited from an earlier ancestor, but ceased to develop into fully formed digits in modern horses.
While the researchers note that this explanation of the second and fourth digits is viable, they argue that it is incomplete and fails to account for the animal’s first and fifth digits.
Arguing that the horse is not truly monodactyl – that is, one-toed – they contend that fragments of the “missing” digits can be found in the form of ridges on the backside of the splints.
According to the researchers, this demonstrates that the first and fifth digits were not simply lost to evolution, but attached to their neighboring second and fourth digits.
“With a distinct surface from the metacarpal, we know the splints on today’s horses to be the remnants of the second and fourth digits,” explains Solounias.
“However, these partially formed digits also appear to contain their own elevated surfaces which hold additional evolutionary clues. We find these ridges, located on the posterior of each splint, to be the partially formed remains of the first and fifth digits, which were once connected to the cartilages of the hoof.”
Solounias first considered this theory in 1999 while studying fossil evidence from an eight-million-year-old horse known as Hipparion primigenium. The famous Laetoli footprints in Tanzania demonstrate Hipparion walked alongside early humans, and was believed to have had three digits.
However, Solounias noticed that the bottom surface of Hipparion’s fossilized forelimb appeared to be divided in five sections, as though small toes had bonded together.
After further studying images of the Laetoli footprints, he confirmed his finding in several of the impressions, and considered that Hipparion not only had five compacted toes, but likely passed this trait on to its descendants.
“Interestingly, we not only find hints of the missing digits on the modern horse, but also its ancestors, such as Hipparion and Mesohippus, two species believed to have three toes,” Solounias said.
Melinda Danowitz, a recent graduate from the institute and Solounias’ co-investigator in the study added: “While the horse’s lineage is classically described as having evolved from four to three toes, and eventually one single toe, we show that its extinct ancestors exhibit the reduced toes both at the wrist and at the hoof.
“These findings show that today’s horse is not truly monodactyl, and earlier ancestors were not in fact tridactyl or tetradactyl, that is, three-toed or four-toed.”
The researchers have also discovered neurovascular evidence in support of the five-digit theory, with dissections of modern equine fetus forelimbs revealing a greater number of arteries and nerves than would be expected in a single digit.
“If today’s horse does indeed have one digit per forelimb, we would expect each forelimb to have a total of two veins, two arteries, and two nerve bundles,” Danowitz explains.
“However, our dissections found between five and seven neurovascular bundles per forelimb, suggesting that additional toes begin to develop, but do not become fully differentiated.”
Following the study, the researchers anticipate that geneticists will use cellular DNA samples to test the theory.