It has long been known that horses shed their toes in favour of hooves 5 million years ago in an effort to gallop faster.
Now experts believe traces of those long-lost digits still exist.
Scientists studying the feet of modern equineshave discovered ‘buds’ of five digits in both horses developing in the womb and in adults of the species.
It suggests that horses still carry the the remnants of the four toes they appear to have lost, complete with nerves and blood vessels.
Early horse species progressively reduced the number of toes in their feet. Hyracotherium had four toes on each front foot, with three on each hind foot, while Mesohippus had two small side toes and one large middle toe on each foot
Researchers from the New York Institute of Technology (NYIT) looked at two late-stage horse embryos in the lab.
Under the microscope they found that their hooves are formed of five distinctly toe-like lumps in the womb.
These gradually narrow into a claw-like tip that is shed after birth.
They also found traces of these bumps in adult horses, on the metacarpal bones.
There they discovered irregular protrusions rising from the smooth splint bones around the central digit.
The team also discovered 14 clusters of nerves, arteries and veins, in five structures similar to fingers or toes.
Speaking to The Times, lead researcher Nikos Solounias, professor of anatomy, said: ‘I think what has happened with the horse is that in the embryo the limbs form all five digits but they are arrested during formation.
‘It’s like a flower bud that starts to form but then never expresses itself fully.
‘All the rudiments of the digits are there and then it just doesn’t form all the way.’
This image shows the proposed positions of the five digits in the distal forelimb of modern horses and their Mesohippus ancestors. Digit one is in red, two is in green, three is dominant and uncoloured, four is in yellow and five is in blue
Researchers from the New York Institute of Technology (NYIT) looked at two late-stage horse embryos in the lab. Under the microscope they found that their hooves are formed of five distinctly toe-like lumps in the womb
The story of how the horse lost its toes begins millions of years ago when they moved from living in protected forests to open grassland, according to Harvard University researchers.
At that time an ancient horse would have moved relatively slowly with a small body, short legs, three toes on its front feet and four toes on its back legs.
But its new exposed environment may have forced the creatures to develop longer legs to run from predators and become larger to make them harder to eat.
It is the loss of toes which may have enabled horses to support this larger weight and move faster on their longer legs.
A single hoof better supports a horse’s weight and allows it to swing its legs more efficiently to gallop at a much greater speed, Harvard found by scanning 12 fossils.
Horses are the only creature in the animal kingdom to have a single toe, the hoof, which fully evolved around five million years ago.
Their side toes first shrunk in size, it appears, before disappearing altogether.
It happened as horses evolved to become larger with legs allowing them to travel faster and further.
Harvard scientists modeled the reasons behind hooves using 3D models of fossils from 12 kinds of extinct horse, measuring the pressure on their legs at trotting speed and while galloping and jumping.
The results show that, for many early horse species, side toes played a critical role, helping to bear some of the animal’s weight to avoid it breaking a bone.
But as the horse’s mass increased, these extra toes used too much energy because they were too far from the source of movement – the horse’s leg.
This slowed the animal down, reducing the number of strides it could make per minute.
The full findings of the NYIT study were published in the journal Royal Society Open Science.