By Dr. Becker
Millions of years ago, there were many different types of horses, some only slightly bigger than small dogs.1 The oldest known horses lived some 55 million years ago. Unlike modern-day horses that graze on grasslands, these ancient dog-sized horses lived in forests, eating mostly leaves.
Horses stayed small and primarily in forests for millions of years, but around 20 million years ago many new species of horses rapidly emerged. It was long thought that an increase in the length of horses’ teeth was pivotal to this new species emergence, as it would have allowed the horses to eat grass.
As paleontologist Bruce MacFadden, Ph.D., of the University of Florida in Gainesville told Science News, “You can’t live on a grassland as a grazer and have short teeth,” says MacFadden, an expert in horse evolution. “You’ll wear your teeth down and that’s not a recipe for success as a species.”2
At this point, it was thought horses diversified to take advantage of this ecological niche, a process scientifically known as adaptive radiation. New research suggests otherwise, however, and reveals the formation of new and distinct horse species may not have been the result of changes in teeth.
Teeth and Body Size Didn't Change Much as Horses Rapidly Diversified
The study, published in the journal Science,3 created an evolutionary tree of 131 extinct horse species and seven still living today.
While it was expected that significant changes in teeth and body size — larger body size would help the animals transition from forests to grasslands — would be seen during periods of rapid diversification, the rates of change were similar to periods with low diversification.
In fact, body size evolution remained fairly consistent during times of low and high speciation (or the formation of new species), but periods with high speciation actually were associated with lower rates of teeth changes.
Instead, it’s thought that external environmental factors and patterns of migration may have been the driving forces behind the rapid diversification into new horse species. As Science News reported:4
“[The researchers] speculate that during the periods of rapid speciation, the environment was so expansive and productive that there just wasn’t a lot of competition to drive the evolution of adaptive traits.
Perhaps, for example, North American grasslands were so rich and dense that there was enough energy for various species to evolve without having to develop traits that gave them an edge.”
Energetic Constraints Have Been Shown to Prevent Diversification
MacFadden noted the study’s important role in showing that rapid diversification of new species does not always come along with “morphological diversification.” It’s not unheard of for evolutionary theories to be challenged or turned on end, however.
Earlier this year, for instance, researchers from the University of Wisconsin-Madison wondered why, with trees in such plentiful supply, more animals haven’t diversified to take advantage of forests’ bounty. In other words, why aren’t there tree-dwelling animals everywhere?
By studying sloths, an example of a rare arboreal folivore, animals that live in trees and eat leaves, they revealed it may be due to the “energetic constraints of a leafy diet,” which “seem to prevent adaptive radiation.”5
How Were Ancient Horses Different From Horses Today?
Getting back to horses, ancient horses had multiple toes — four, then three and most species eventually lost their side toes and ended up with one single large hoof. The early forest-dwelling horses also had shorter legs than the later, grassland-dwelling species.
The longer legs helped the horses to run faster, a useful ability when you live out in the open plains. The American Museum of Natural History (AMNH) reported that changing climates also played a role.
When the earliest horses roamed the Earth about 55 million years ago, there was a rise in temperature (by 9 to 18 degrees F) that left North America with a rainforest-like climate, perfect for the small forest-dwelling horses of the time. The museum continued:6
“Then, about 35 million years ago, global temperatures dropped, creating a climate more similar to today's.
Thereafter, dry grasslands replaced much of the North American forest, leading to rapid evolution among horses. By about nine million years ago, most forest browsers had disappeared, leaving primarily grass-eating grazers like those alive today.”
Also interesting, while most horse species evolved in North America, they went extinct in this region and in South America about 10,000 years ago.
It’s speculated that this was due to disease or humans, who may have hunted the animals for food. Horses didn’t return to this area until Spanish explorers reintroduced them in the 16th century.7
Can You Guess Who Horses' Closest Living Relatives Are?
Only one branch of the horse family is still around today, the genus Equus, which also includes zebras, asses and donkeys. Outside of this family, horses’ closest relatives may surprise you, but you can take a guess by counting their toes. As noted by AMNH:8
“Horses belong to a group of mammals with an odd number of toes. That rules out mammals with two toes, or ‘cloven hooves,’ like goats, pigs, cows, deer and camels.
So who are the other odd-toed, plant-eating animals? Most members of this group, known as perissodactyls, are extinct. But several species survive at present. They include rhinoceroses and tapirs, the horse's closest living relatives.”