These tiny, prehistoric ancestors not only looked different from their modern descendants, they ran differently too. The key contrast? It’s all in the back, says a new study conducted by Dr. Katrina Jones, a post-doctoral researcher at Harvard’s Museum of Comparative Zoology.

By studying the fossil record for 16 species of early horses dating from 50 million years ago, Jones found that the animals evolved different vertebrae joints over time. The earliest ‘dawn horses’, or Hyracotherium, had more mobility in the middle and lower regions of the back, indicating that these animals moved their spines as they ran. It’s a characteristic that’s common among many four-legged mammals today—but not horses. The modern horse restricts the motion of its lumbar spine to one joint, located near the rump.

By Nachosan (Own work) Palaeotherium_hassiacum-Messel_via Wikimedia Commons.

By Nachosan (Own work) Palaeotherium hassiacum-Messel (via Wikimedia Commons).

As these now-extinct horses grew in size over the millennia, Jones hypothesizes that more and more energy was needed to propel them forward and that more restrictive back joints helped to minimize trunk motions, thereby improving mobile efficiency. For more than 100 years, scientists have studied the fossilized feet of horses in an effort to explain how they evolved specialized features for running. Jones says relatively little is yet known, however, about the role of the spine in this process.

You can read Science Daily’s full story here.