Would you rather have one super strong toe or multiple weaker toes? Horses made the seemingly odd move of a single toe on each foot, but it’s worked out in their favor, as they can run super fast.

Of course, this hypothetical game of Evolutionary Would You Rather belies the complex process of evolution. Natural selection is not a conscious choice but a long-term process of ecological pressures and biological changes. Now, scientists have shed a little more light on how this process played out in horses.

In a study published Wednesday in the journal Proceedings of the Royal Society B, researchers used statistical techniques to demonstrate how horses evolved over time to have one large toe on each foot. The study’s authors say this evidence supports the historical hypotheses that horses’ increasing body mass created selective pressure for a single toe, and that longer limbs made the increased inertia created by multiple toes inefficient for running.

Lead author Brianna McHorse (yes, you read that right) has been horse crazy since she was a child but has put her riding on hold a bit to dig into the evolutionary history of horses and their prehistoric ancestors. A Ph.D. candidate in the Department of Organismic and Evolutionary Biology at Harvard University, McHorse has leveraged her personal passion for horses and professional interest in computational science to explore the biomechanics of how horses evolved to have only one toe on each foot.

Eohippus, an early relative of the horse, had a couple more toes -- and a smaller body -- than the modern horse.

To do this, she took measurements of internal bone geometry from 13 different fossil horse genera, ranging from the extinct Hyracotherium all the way up to the modern Equus. McHorse and her co-authors modeled horse movement to shed light on how body mass changed the anatomy of horses’ legs and to test the importance of side digits in bearing weight. They found evidence that central bones in horses’ legs and feet (metapodial bones) increased in diameter over time to accommodate increased body weight, and that side digits bore weight even as the central metapodials increased in diameter throughout horse evolution.

This image depicts the evolution of digit reduction, from four toes to three toes to a single toe in Eohippus, Mesohippus, Merychippus, and Equus.

“I was surprised by how clearly our results showed the side digits needing to bear some of the load,” McHorse tells Inverse. “It was very striking to see how the safety factor was so consistent across the whole swath of the evolutionary tree that we studied, once you account for those side toes.”

This is McHorse’s first foray into using statistical models to look at the specific ways in which changes in anatomy affect how horses move, but in the near future, she aims to take a broader view, examining more factors that influenced horse evolution.

“I’ll be looking at the whole pattern of horse evolution, which at its maximum diversity had something like 80 species,” says McHorse. “I’ll be tracking how digits are reduced across their whole history, and testing whether that matches up with changes in speciation or extinction rates, changes in other traits like their diet, and changes in environment, such as temperature or habitat type.”

Abstract: Digit reduction is a major trend that characterizes horse evolution, but its causes and consequences have rarely been quantitatively tested. Using beam analysis on fossilized centre metapodials, we tested how locomotor bone stresses changed with digit reduction and increasing body size across the horse lineage. Internal bone geometry was captured from 13 fossil horse genera that covered the breadth of the equid phylogeny and the spectrum of digit reduction and body sizes, from Hyracotherium to Equus. To account for the load-bearing role of side digits, a novel, continuous measure of digit reduction was also established—toe reduction index (TRI). Our results show that without accounting for side digits, three-toed horses as late as Parahippus would have experienced physiologically untenable bone stresses. Conversely, when side digits are modelled as load-bearing, species at the base of the horse radiation through Equus probably maintained a similar safety factor to fracture stress. We conclude that the centre metapodial compensated for evolutionary digit reduction and body mass increases by becoming more resistant to bending through substantial positive allometry in internal geometry. These results lend support to two historical hypotheses: that increasing body mass selected for a single, robust metapodial rather than several smaller ones; and that, as horse limbs became elongated, the cost of inertia from the side toes outweighed their utility for stabilization or load-bearing.

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