Fifty million years ago, early horses roamed the hot, damp jungles of North America and Europe. Named Hyracotherium, or ‘the Dawn Horse’, they were small, no bigger than a dog.

And they had four toes.

Fourteen to be exact, four on each front leg and three on the back. Each toe was enclosed in a thick keratinous hoof, with an underlying footpad between them all – similar to the modern tapir – to help them traverse the swampy ground underfoot.

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However, as habitats dried out to become grassy plains and horses evolved, those four toes eventually became one, until they were eventually galloping around on the equivalent of a human’s middle finger and toe. 

What happened to the others has puzzled scientists for decades, but now an international team of scientists think they have the answer.

‘In later fossil horses there were only three toes front and back,’ said co-author Professor Christine Janis, from the University of Bristol.

‘The extra toes, known as side toes, in these horses were smaller and shorter than in a tapir, and likely did not touch the ground under normal circumstances, but they may have provided support in exceptional situations, such as sliding or forceful impact.

‘The upper portions – the remains of the additional hand and foot bones – remain as ‘splint bones’ fused with the remaining central one, but where are the fingers and toes?’

In 2018, a paper proposed the missing toes had evolved to form part of the frog – a spongey v-shaped structure on the sole of the hoof.

The authors argued that, given two of the bones higher up the horse’s leg, the splint bones either side of the cannon bone – just below the knee – are known to be leftover remnants of the second and fourth toes, the lower part of them could have been absorbed into the formation of the frog.

Evidence to support this was the apparent lack of a frog in three-toed horses part way along the evolutionary chain.

However, further investigation has since found the clear presence of a frog in the fossilised hoofprints of three-toed horses, suggesting the frog evolved independently of the digits.

In short, they have simply been lost over time.

‘Although it does seem that remainders of the proximal [upper portions] of the side digits have been retained in modern horses, as the earlier 2018 paper claimed, the distal [lower portions, or toes] have simply been lost,’ said lead author Professor Alan Vincelette, of St John’s Seminary, California.

‘The frog of the horse’s hoof evolved independently of the side toes as a unique structure providing shock absorption and traction during locomotion.’

Further evidence the extra toes haven’t been folded into the frog is the rare occurrence of vestigial toes seen in modern horses – essentially extra hooves growing out from the lower leg.

‘While the notion that modern horses have retained all of their original toes as within-hoof remnants is a novel one, and so rather appealing, it can be shown to be incorrect,’ added co-author Professor Christine Janis, from the University of Bristol.

The study is published in Royal Society Open Science.

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