The Horse

by Wendy Williams

  Leonard Radinsky was one of her intellectual offspring. At the American Museum of Natural History, he followed up on Edinger’s work and discovered that, interestingly, some of the details were wrong. Edinger had described the brain of the little dawn horse as being extremely primitive. Indeed, according to Radinsky, she had written that the early horse brain was “strikingly similar” to that of a marsupial mammal, the opossum. Radinsky found that Edinger had misidentified the brain she was studying. She was looking at the brain not of an early horse, but of a different animal.

  In fact, Radinsky found that the brain of the dawn horse was “considerably advanced.” Moreover, the dawn horse was showing some clues as to its future direction. The horse’s olfactory bulbs, organs in the forebrain dedicated to the detection of smells, were already well developed. The neocortex, involved in what we call intelligence, was proportionally much smaller than it would be tens of millions of years later, but compared to other animals of the time, the horse had a “larger, advanced brain,” the paleontologist Richard Hulbert told me. “This advanced brain could have conveyed competitive advantages over other Eocene herbivores such as more complex social behavior and better recognition and evasion of predators.”

  Radinsky compared the brain of the dawn horse with that of Mesohippus. He found that although this horse was only a bit larger than the dawn horse, he had changed in an important way. He had a substantially expanded frontal lobe. Among the new experiences Mesohippus would be able to enjoy because of this expanded brain, Radinsky wrote, was an enhanced sensitivity of the lips and the mouth.

  So it seems that one result of the colder period that closed out the Eocene was that the lives of horses became sensually and intellectually enriched. The environmental challenge facing first Mesohippus and then Miohippus wrought a smarter horse with a greater ability to perceive and use information in the more open world in which he lived.

  Some primates also seem to have started down a different evolutionary pathway following the arrival of the cold and the spread of grasslands. This is when, according to some researchers, some primate lines first evolved the three cones in the eye that allow us to see more colors than other mammals (including horses)—a change driven in part by the fact that as fruits became limited, primates needed to forage with greater focus to detect protein-rich young leaves, which are often red.

  Our modern horse’s ability to interface with the world via supersensitive lips, muzzle, mouth, and nose is an evolutionary gift resulting from the deep cold of the late Eocene, from La Grande Coupure, and from the transition into the harsh Oligocene.

  “A cow could never play Mr. Ed,” the paleontologist Donald Prothero once wrote, as a way of extolling the nimble lips of horses.

  The path that led to Whisper’s labial dexterity—his ability to use his lips the way I use my hands, to manipulate the latch on his stall door and pasture gate, to break into the grain bin, and to make a cup with his lower lip in order to drink out of my outdoor faucet—has some pretty impressive roots in deep time, and is associated with the sudden appearance of the Antarctic Circumpolar Current and the development of the Antarctic ice cap.

  * * *

  North America remained the center of horse evolution. Mesohippus eventually died out, leaving only Miohippus, the three-toed browser with slightly longer legs who roamed North America from about 32 million years ago to about 25 million years ago. Some descendants of Miohippus escaped to the Old World, where they evolved into strange forms like Sinohippus, a three-toed animal that looked something like a cross between a modern cow and a modern donkey. Anchitherium, a narrow-muzzled three-toed browser with a long neck that could reach up to eat the leaves of deciduous trees, evolved in North America and then spread to the Old World, including Africa.

  Nevertheless, the real action in horse evolution continued to be in North America, where the Rocky Mountains continued to rise, where the continent’s interior was drying, and where grass was spreading prolifically. A different kind of grass had evolved, one that could flourish in the most challenging of environments and could spread boldly into regions where the earlier grasses could not go. Horses, with their ability to run quickly over treeless grasslands and with their teeth that could, by now, grind up the toughest of fodder, also spread.

  By the Miocene, horses filled so many different nooks and crannies that nobody knows how many species of horses roamed the world. We know of at least twenty different horse species in North America at this time. There were others in Asia, in the Middle East, in Europe, and even in Africa, where we primates were at that time also undergoing some pretty dramatic changes. There, during the Miocene, the first apes appeared, and then the first humanlike primates.

  The diminutive dawn horses of the Eocene, so well-preserved in the Messel strata, had long since disappeared. But following the near-extinction crisis of Epihippus, horses once again became one of the world’s dominant mammals. In fact, the Miocene, which extends from about 23 million years ago to a little more than 5 million years ago, was the epoch of the horse. The continued rising of mountain ranges all over the world—the Andes, the Himalayas, the Rocky Mountains—lifted the crust of Earth, changed global wind patterns, dried out continents.

  “This was a major episode in the spread of grasslands,” Mike Voorhies told me when I visited him. “You had a relatively new habitat and a family of mammals—horses—that could exploit that habitat in ways that other animals could not.”

  And as the interior of North America dried, and as grasslands, Candace Savage’s “drought specialists,” spread, horses adapted. At the beginning of the Miocene, horses with three toes were all there were. By the end of the Miocene, the three-toed horses, who had roamed Earth for more than 50 million years, had begun to disappear, although the process would take a while. Many of those had found their way from the center of North America all the way to Africa, via a route through Asia. Strangely, it would turn out that Africa, of all places, would be one of their final refuges.

  4

  THE TRIUMPH OF HIPPARION

  The history of the horse family is still one of the clearest and most convincing for showing that organisms really have evolved, for demonstrating that, so to speak, an onion can turn into a lily.

  —GEORGE GAYLORD SIMPSON, Horses

  On the edge of northern Tanzania’s grassy Serengeti, a tiny mare and her rambunctious foal hurried along a path that ran not far from an active volcano. The ominous cone, lurking high above the open plain, spouted irregular plumes of gas and ash much as a modern whale spouts water.

  Most likely, the rumbling of the earth did not alarm the horses. It was about 3.6 million years ago, and where the dam and foal lived, light ashfalls were routine. The uneasy plain was dotted with such volcanoes.

  The little mare, Hipparion, weighed much less than a modern horse, probably just a bit more than one hundred pounds. We would have recognized her as kin to the modern horse, but we wouldn’t have mistaken her for one: She had three toes on each of her four feet. But by now, more than 50 million years after the dawn horses first appeared, the two side toes were very small, so small that nineteenth-century paleontologists, finding Hipparion fossils, thought the “extra” toes were “useless” vestiges left over from evolutionarily older species.

  As she traveled, prudence was the order of the day. She had to pick her way lightly. The slippery surface under her hoofs was covered with loose ash—about as easy to negotiate as ice-covered ground topped by a thin layer of new snow. No modern horse would voluntarily choose to trot over such a complicated surface and risk a torn ligament or broken leg. Neither did this ancient mare. Instead, she adopted a four-beat running walk—a stabilizing gait that allowed her to move quickly but also have three feet on the ground at any one time. She was being cautious.

  Her foal, however, was not. Perhaps too inexperienced to recognize the danger of falling—or perhaps just behaving in the brash way that babies sometimes do—the youngster wove back and forth from one si
de of his dam to the other. His path was haphazard. The mare seems to have felt no sense of urgency, no need to move with anything but deliberate speed. The same cannot be said for her baby.

  At one point, the clueless foal ran right in front of the mare. The mare responded by glissading for an instant. Next, she steadied herself with her side toes. They may have been considerably smaller than those of her ancestors, but those side toes were still useful. Planted in the ash, they created a firm tripod that helped her stay upright. Having regained her footing, she traveled on.

  * * *

  Around the same time, a small band of our own ancestral relatives, with brains about one-third the size of ours, ambled along a pathway that, for a few footsteps, almost paralleled the horses’ tracks. Standing up straight, with only two feet to worry about, these early hominids seem to have had an easier time navigating the terrain. Their footprints show no evidence of slipping and falling.

  Two of these beings, members of a species we call Australopithecus afarensis, appear to have been walking side by side. They left footprints resembling those we make in beach sand. Their toes were pointed fairly straight ahead and there even seems to have been a bit of an arch that may have encouraged a rocking motion from heel to toe. The footprints of one of these side-by-side hominids measure almost twice that of the other set of footprints, so that we might in the twenty-first century wonder wistfully: Were they parent and child? Was the parent trying to lead the child to safety? It’s possible.

  But perhaps they were not walking together at all. One may have been following the other. Maybe it’s only a coincidence that the footprints left in the ash made them appear to be companions. The precise details elude us. We can only see so much through the thick veils of time. We do know, though, that if all the A. afarensis prints on the scene were left at the same time, the pair was not alone: following them was at least one more two-legged walker. There is at least one more set of footprints in the ash.

  What were they doing there? Were they out hunting? Again, it’s possible. By this time, the tiny proto-thumb found in the Messel primates had developed into an opposable digit something like our modern thumb, so that these two-legged creatures would easily have been able to grasp and manipulate a naturally sharp stone as a tool, just as modern chimpanzees can. Or perhaps they weren’t “hunting” in the modern sense of the word, but were instead out “power-scavenging”—forcing predators like hyenas to give up the prey they had just taken down.

  After these horses and hominids passed by, more ash fell. The heat of the African sun baked the prints into the ash and then, over the ages, soil and other detritus covered the evidence. A natural time capsule, this fossilized record of horses and hominids lay buried for several million years. Then the wind and the rain washed away the protective layers.

  Slowly, the prints began to reappear: proto-human and proto-horse, striding along under a sky full of volcanic ash. This behavior was not evidence of any earth-shattering event. There’s nothing in the tracks showing that the animals were in a mad dash. It’s not like Pompeii. Indeed, the day seems to have been rather mundane. There were no disasters in the offing, no mass extinctions just around the corner. It was rather quotidian. Just a routine scene under the African sun. A moment caught in time. Like family photos at the beach.

  Nevertheless, the tracks, despite their mundanity, are compelling. We can see here, in these fossilized prints, our own continuity and our own futures and the coming era when horse and primate, finally reunited after their expulsion from the Garden of Eden Eocene, would partner up to depend on each other for food and survival. The day of H. sapiens was nearing. And so was the day of the one-toed horse. Indeed, on the other side of the Atlantic on the North American plain, the modern horse Equus had already arrived.

  On that day 3.6 million years ago, did the proto-horses and proto-humans encounter each other? I’m betting against it. After nearly tripping up the mare, the foal traveled on heedlessly. We can see the baby’s fossilized hoofprints next crossing the fossilized footprints of the A. afarensis group. The foal’s tracks show no hesitation during that crossing. Then, as today, it’s unlikely that a callow baby would have blithely met up with these proto-people without at least one surprised side step. The foal’s tracks show no such startle response. We have to remain content that these two species shared the same plain, but that, at that instant at least, they probably missed each other like the proverbial two ships in the night.

  Hipparion and A. afarensis were not alone in this landscape. The site is covered with at least sixteen thousand fossilized tracks of ancient mammals and other animals, all probably made within days or weeks of each other. Scientists have uncovered tracks of large cats, giraffes, elephants, hyenas, ostriches, and so much more. There’s even a fossilized insect trail.

  * * *

  The story of the tracks is a superb tale of how serendipity and science sometimes work together. Local people had known there were ancient tracks in the area, but no one had really “noticed” them in a significant way. They didn’t fully grasp how much information the tracks held. We routinely see amazing things in the world but too often have tunnel vision and don’t pay attention to what we’re seeing, and so it was with these tracks. No one had taken the time to stop and think about the volumes of ancient behavior recorded there.

  It took an accident and a lightbulb flash of insight from a young scientist for the phenomenon to win the attention it deserved. In 1976, a thirty-year-old researcher, Andrew Hill, now an anthropologist at Yale University, drove down from Nairobi with friends to visit the site, near the present-day village of Laetoli, at the invitation of Mary Leakey, a well-known scientist. Leakey was focused on finding fossilized hominid bones and skulls. She was not thinking about animal trackways.

  Neither was Hill. But one day Hill and some buddies, engaged in their own particular form of horseplay, began throwing elephant dung at one another. Hill slipped and fell, he told me. At ground level, he noticed a few of the fossilized tracks from a new angle.

  In a gratifying eureka moment, he associated those tracks on that African plain with an illustration of fossilized raindrops he’d seen in a book about geology by the influential nineteenth-century scientist Charles Lyell. In that book, Lyell discussed the scientific value of these kinds of records: Something as simple as fossilized impressions of raindrops, Lyell wrote, can—if paid attention to—tell us a great deal about what life was like in prehistoric times. For example, Lyell wrote, take note not just of the raindrops themselves, but of the size of the raindrops.

  Lying there on the hot African plain, Hill remembered Lyell’s admonition and realized that the animal trackways he was looking at that day could well contain a whole library of information. When researchers began a full-scale investigation of the footprints, they found that many, many animals had passed through the same region during that time, and that many of them had left evidence of their presence and their behavior in the ash.

  The finding of footprints left by ancient members of the human family made headlines worldwide. It settled (to the degree that anything is ever settled in paleoanthropology) a long and sometimes rancorous debate: Were our ancestral relatives beings who walked on all four limbs? Or were they able to stand upright? How early did bipedalism appear? Laetoli shows that A. afarensis, thought by many researchers to be intermediate between apes and modern humans, not only walked upright—but had a heel, a big toe, and even an arch.

  By 1987, a two-volume set of papers appeared that included a pile of maps with the trackways of all the animals found to date. I unfolded the maps on my dining room table one afternoon, piecing them, jigsaw-puzzle-like, together, until they covered the table. You could see almost the whole of the animal world that then inhabited this region of Africa—a cornucopia.

  For anyone who loves the detail you find in maps, these sheets of paper were dazzling. There were the footprints of the ancient hominids right in front of me. I could imagine the three-toed horses, wander
ing under the African sun, steadfastly enduring that light rain of volcanic ash. Looking at the maps, I felt in my dining room the way I’d felt at Polecat Bench and at Messel: as though I’d crossed some sort of fourth-dimension time barrier and entered forbidden territory.

  Seeing the footprints right there on my table of so many animals who had walked the Serengeti millions of years ago was a metaphysical experience, but the tracks, it would turn out, also revealed important information.

  Eadweard Muybridge’s sequence photographs of the horse’s gallop and walk

  For example, in that same volume I found an article addressing an issue that has long bothered horse aficionados worldwide: Do horses come by the four-beat single-foot gait naturally? Or is special breeding necessary? Horses today have three basic gaits: the walk, the trot (or pace, for some horses), and the canter, or lope. (The gallop is a faster version of the canter.)

  But some horses can also do a four-beat gait that’s faster than the walk and is sometimes called the single-foot or running walk. Because it’s both comfortable and fast, riders treasure this gait. Since horses tire less using this gait than using a canter or gallop, it allows horse and rider to cover a lot of ground in a day without the horse becoming overly tired. But only certain breeds of horses, like the Tennessee Walking Horse or the Icelandic horse, do this naturally. Do they have this ability because it was bred into them, or did at least some horses always have this ability?

  The Dutch scientist and avid horse enthusiast Elise Renders decided to use the Laetoli tracks to try to answer that question. When she first read about the Hipparion tracks, she, like me, was fascinated. A record of the specific behavior of two individual, now long-extinct horses made her wonder if she could discover how these horses actually moved. She also wanted to know if the two tiny side toes on each foot were really superfluous. Or did they have a function?