The Story of the Horse

Wild horses in Wyoming

At the CANA Foundation we are committed to saving the wild horses of the American West through advocacy and education. In this section we outline how our scientists are studying the evolutionary history of the horse using fossil and modern evidence.  

By: Ross MacPhee, PhD

Background: The earliest identifiable members of the horse family, known as Equidae, arose some 13 million years after the demise of the dinosaurs 66 million years ago. That places them among the oldest families of mammals that have survived into modern times. Equids have lived in North America for 53 million years, in an unbroken stretch from the early Eocene until the end of the Pleistocene 11,500 years ago.

Fossil records indicate that at the end of this interval Equus populations were living not only throughout North America, from Alaska to Central America, from California to the Atlantic coast, but also in South America, from the llanos of Venezuela to the pampas of Argentina. Much earlier they had entered Eurasia over the Bering landbridge. Everywhere horses went, they prospered. 

But then something happened. In one of the greatest natural history mysteries of all time, horses, along with dozens of other large (or megafaunal) species, died out in the Americas 10,000-11,000 years ago. For most of them, this was the end of their evolutionary road. In a few cases, species that had migrated during their evolutionary history managed to carry on elsewhere. In even fewer cases, the immigrants came back home to the places in which they originated. The horse is such a one.

Beringia, the intercontinental land bridge, and the Equus diaspora. Long before equids disappeared in the New World, they repeatedly dispersed over the Beringian landbridge, which links northeastern Asia with Alaska. Unlike the Panamanian land bridge, which formed comparatively recently, the Beringian land bridge has been up and down multiple times over the past 66 million years. North American equids crossed over the Beringian landbridge on several occasions, allowing them to find new lands and opportunities in Eurasia and Africa.

Various lineages within Equidae eventually differentiated into some of the species we know today, including asses, kiangs, zebras, and others. Thanks to their ability to successfully disperse, the horses we have today are among these survivors, despite the fact they completely died out in their original homeland, North America.

Or did they? According to various Native American oral traditions, not all horses died out at the end of the Pleistocene. Some survived; later on, their descendants were domesticated by Indians long before the Spanish showed up—with domesticated European horses–in the early 16th century.

Whether these traditions have a factual basis is currently difficult to determine from a scientific perspective, as there are no horse fossils from the New World which confirmably fall within the critical interval between roughly 10,000 and 500 years ago. At CANA we are intrigued by the possibility of late survival, and our affiliated scientists are actively seeking specimens that might document this.

What does domestication mean biologically? We are all familiar with domesticated mammal species, especially companion animals like dogs and cats. The process of domestication is multifaceted, involving not only changes in physical characteristics, but also modifications in behavior. But such changes are due to artificial selection, that is, breeding for characteristics determined by humans, not nature.

There are many obvious examples. In the case of horses, think of the differences between a Belgian (bred for great size and strength) and a Shetland (bred for small size and behavioral docility). Yet despite their physical differences these breeds share a very recent common ancestry. Their joint ancestors lived just a few centuries ago.

How can such massive changes, normally taking hundreds of thousands and millions of years, be explained? Artificial (manmade) selection works on a faster time scale than natural selection, because humans are usually interested in enhancing only a limited range of traits in their domesticants. By restricting breeding to animals that express acceptable versions of such traits, within a few generations so-called “pure-breeding lines” can be created, ones that express the features desired and are capable of passing them on to their progeny.

There is nothing pure about pure-breeding lines. If you ignore the traits that define a particular breed and focus instead on features that were not subjected to artificial selection, you’ll see that they often vary freely, just as they would in natural populations. For an overwhelming number of traits, horse breeds do not significantly differ from one another (for example, non-conformational characteristics, facial features, organ size).

How did domestication occur? According to the archeological record, efforts at horse domestication began at least 6000 years ago in Central Asia, on several different occasions. One of the first things that humans probably did as part of the domestication process was to select for behavioral characteristics like docility and responsiveness to human commands.

Ancient horse breeders realized that one key to successful domestication was to select for animals that could be trained to accept human dominance. They probably did that by observing the roles of dominant mares in herd organization. Interestingly, although there were efforts in the 19th century to domesticate zebras, they were unsuccessful in part because zebras aren’t like our horses. Dominant females do not play an overarching role in zebra herd structure; no domination, no effective domestication.

Domestication comes at a price—for the animals concerned. Controlling breeding for features that might appeal to humans, but have no useful role otherwise, may result in creating lineages that are, in adaptational terms, total disasters. Such lineages are only kept from rapidly dying out because we can make it so. Particularly common in many breeds of domestic animals is the high frequency of recessive traits that would normally be edited out if natural selection were in full force.

No surprise: we know that the same thing happens in human groups where excessive inbreeding is allowed. In short, domestication is not and never was a natural process, whatever its perceived benefits to ourselves.

What is the biological status of domesticated lineages compared to their wild ancestors? Although there is disagreement among scientists as to how such lineages should be classified, at CANA we take a position that is grounded in evolutionary biology.

Read more of this story.

Leave a Reply

Your email address will not be published. Required fields are marked *