Has A Mystery Of Horse Evolution Finally Been Solved?
Humans and horses share a common genetic ancestor – a five-toed mammal. Scientists previously had competing theories about how, when and why horses’ digits diminished, but new science suggests that modern horses actually do still have five relic toes in their hooves.
The general scientific consensus, before the introduction of this new evidence, was that horses only have one central digit in their feet, the metacarpal. However, the new study, published in Royal Society Open Science, proposes that a horse’s forelimb is composed of five fused digits.
One of the researchers who authored the study, Nikos Solounias, also examined the “Laetoil footprints,” ancient footprints that show humans walking alongside the Hipparion primigenium, an early horse that was previously thought to have three digits. However, Solounias believes that the print shows five, and not three, sections. The Hipparion’s smaller toes likely fused together.
Additionally, modern horses have a higher density of nerves and blood vessels in their forelimbs than would make sense if they really only had one metacarpal.
Melinda Danowitz, a researcher on the study, elaborates, “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. 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.”
If you want to take a deep dive into horse evolution, check out this photographic timeline.