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Palaeobiologists from the University of Bristol and Howard University have uncovered new evidence that suggests that horses' legs have adapted over time to be optimised for endurance travel, rather than speed.
The ancestors of horses (including asses and zebras) had three toes on each foot. Because only single-toed (monodactyl) forms survive today this anatomy has been perceived as a superior evolutionary outcome, enabling horses to outrun predators.
Interpretation - Evolution - History - Horses - Performance
But our interpretation of equine evolution may be biased by our own history with horses: performance at the racetrack has been less important for human history than the endurance of horses at slower speeds, and such endurance may have been the critical factor in horse evolution.
The research team combined evidence from the fossil record with existing studies on horse locomotion and propose that the adaptive significance of single-toed limbs was for trotting during roaming for food and water, rather than for galloping to avoid carnivores.
Forward - Horse - Foot - Anatomy - Loss
The real evolutionary 'step forward' in horse foot anatomy was not the loss of additional toes, but the evolution of the 'spring foot'.
This pogo-stick type of foot anatomy evolved in the three-toed distant ancestors of modern horses, which sported an enlarged central toe but retained small 'side toes', which likely prevented the foot from over-extending during extreme locomotor performance.
Foot - Storage - Energy - Limb - Tendons
The 'spring foot' enables the storage of elastic energy in the limb tendons during locomotion, and its evolution coincided with the spread of grasslands around 20 million years ago in North America (the original home of horse evolution).
The spring-footed horses radiated extensively and were as diverse during their time as antelopes in Africa today.
11-million-years - Eurasia - Africa - Forms - Horse
By around 11-million-years ago they also spread into Eurasia and Africa, where they eventually included forms larger than a modern horse. But only the lineage leading to modern horses, one amongst many, showed any tendency to reduce the number of...
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