Researchers have long wondered how horses evolved from a
five-toed ancestor to the single-toed animal we know today. While it is
largely believed that they simply evolved with fewer digits, Professor
Nikos Solounias from the New York Institute of Technology College of
Osteopathic Medicine pose a new theory that suggests remnants of all
five toes are still present within the hooves of the horse.
Horses. Image credit: Toni X. Jesse.
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,
Professor Solounias
and colleagues 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, researchers 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 this explanation of the second and fourth digits is viable, it
is incomplete and fails to account for the animal’s first and fifth
digits,” the paleontologists said.
“The horse is not truly monodactyl, that is, one-toed. Fragments of
the ‘missing’ digits can be found in the form of ridges on the backside
of the splints.”
According to the team, 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,”
Professor Solounias said.
“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.”
Silhouettes show Mesohippus primigenium,
an early ancestor of the modern horse that lived 40 million years ago
and was previously believed to have three toes, and the modern horse.
Photographs of both animals’ hand bones appear alongside renderings of
the researchers’ proposed digit identities. The researchers argue that
missing digits one and five are partially expressed on the surfaces of
the side toes (shown in red/blue). While the horse is described as being
monodactyl, with only one complete digit, the researchers demonstrate
that digits two and four are expressed as the splint bones and frog
(padding of the foot), as shown in yellow/green. Missing digits one and
five are expressed as ridges on the splint bones and as the hoof
cartilages, as shown in the lower red/blue areas. Image credit: New York
Institute of Technology College of Osteopathic Medicine.
Professor Solounias first considered this theory in 1999 while
studying fossil evidence from an 8-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, the scientist 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,” Professor Solounias said.
“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,” said co-author Melinda Danowitz, also from the New York
Institute of Technology College of Osteopathic Medicine.
“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 team 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 said.
“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.”
The
research appears in the journal
Royal Society Open Science.
_____
Nikos Solounias et al. 2018. The evolution and anatomy of the horse manus with an emphasis on digit reduction. R. Soc. open sci 5: 171782; doi: 10.1098/rsos.171782
