A new study published in the peer-reviewed journal Science on February 10, 2017 is challenging long-held ideas about how horses evolved. Paleontologist Juan Cantalapiedra and team compiled decades of previous research into an evolutionary tree of 138 horse species (seven of which exist today), spanning roughly 18 million years. This new work revealed three major bursts of horse evolution, in which new species emerged. But, as ancient horse species diversified, the horses showed very little change in their teeth or body size.
This result is contrary to long-proposed evolutionary theory.