Story of modern horse's evolution intertwines humans and the ubiquitous plant we call grass
Horses evolved here in North America. Based on fossils collected from the western United States in the 1800s, the orderly progression from a small, dog-like, five toed, leaf-browser (Eohippus, the “Dawn Horse”, now known as Hyracothere) to modern-day Equus has been touted as an example of evolution occurring over thousands to millions of years.
Image courtesy of Wikimedia
It now appears that horses are survivors of a complex family tree with many more side branches and extinct relatives than previously envisioned (note that this link is not for the faint-hearted).
Instead of an orderly sequence of gradual improvements, the horse family lived, changed and died in response to random genetic variation, natural selection, changes in our fickle earthly environment and the co-evolution of plants and people.
Over the past 55 million years, there have been some 30 to 40 branches in the horse family, with a few hundred extinct species along the way.
The five-toed ancestor of the modern horse came to rely on speed to evade its predators in an increasingly dry and open landscape. The middle toe’s fingernail, or claw, developed into a hoof that became bigger and better able to provide traction for fast get-aways. The first and fifth toes became smaller and disappeared.
The horse's strategy of fast-breaking speed led to the single toe and highly specialized hoof.
Studying horses’ teeth is an important way in which researchers trace their evolution. Teeth provide clues to what was being eaten and what was growing there and then.
It seems the early equines dined on leaves and fruit typical of a tropical environment. As their world dried out and grasses became available, horses’ teeth grew longer, with adaptations for breaking off and crushing the tougher grass.
Kathy Lundberg | Contributor
Wide grasslands were perfect for long, swift-legged herbivores. Large herds developed and returned the favor to the grasses by eating and destroying trees, providing space and sunshine for the grasses to thrive.
Interestingly, the evolution of the horse is tied to the evolution of grass.
Grass is considered a flowering plant and came along just before the disappearance of dinosaurs. Some 55 million to 65 million years ago, just as little Hydracothere was creeping around, grass was taking hold. Grass is pollinated by wind, freeing its life cycle from the vagaries of insect pollination.
Grass grows fast, can colonize bare ground and tolerates being stepped on or eaten, as its leaves grow from its base. Grass also developed a revolutionary form of photosynthesis, which uses less water, helping survival in drier conditions.
Grass now includes some 10,000 species in its family, providing about 20 percent of the vegetative cover of the world.
Human evolution is also closely tied to the evolution of grasses. Grasses such as wheat, rice, and corn are the most important calorie source for mankind. In fact, the domestication of grass, about 10,000 years ago, marks the start of civilization as we know it. Of course, the domestication of horses 6,000 years ago gave us humans much assistance in growing our grasses and populations.
From the burgeoning grass family tree came an expanded horse family tree, both through adaptive radiation.
The grass family continues to prosper — did you eat your grass today (wheat, rice or corn)?
Of the horse family, only three groups remain on one surviving branch (genus): domesticated horse, zebras and donkeys. Domestic horses split off from zebras and donkeys about 4 million years ago in North America. From here, they spread across the Bering Strait and populated the Old World.
About 14,000 years ago, a capable, clever, two-legged species crossed that same land going in the opposite direction, spreading into the New World. As a result of multiple factors, probably including climate change, possibly including hunting by these early humans or infection by diseases they brought with them to the New World, or even due to a large meteor detonating in the atmosphere, Equus became extinct in the Americas.
Many large animals and marine life also died out at this time (Pliocene Mass Extinction).
As recently as the blink of an evolutionary eye, we humans have re-introduced the sound of hoof beats to the Americas. The story of horses, people and grass continues to unfold.
Kathy Lundberg is a regular contributor on AnnArbor.com's pet section where she addresses topics regarding horses. She is the owner of Scio Church Stables. She welcomes your contact via email.