The Horse

by Wendy Williams

  First she thought about the gait the Hipparions were using. She couldn’t tell just by looking, so she made a cast of the original Laetoli tracks. Then she made similar copies of modern horse tracks. She had these horses walk, trot, and canter. She compared the modern tracks with the ancient tracks and was surprised to find that none of the modern tracks matched the Hipparion tracks. Apparently the mare and foal were not using the walk, trot, or canter.

  Then she began looking at modern horses using the running walk, a four-beat gait. She hit pay dirt. She measured the distance between the front foot and the hind foot, and then measured how far the hind foot stepped over the front foot. She found that both the little Hipparion mare and her foal were moving in a four-beat gait.

  “This mare and foal had a natural traveling gait,” she told me.

  In a regular walk, a horse moves one front foot, then the opposite hind foot, then the other front foot, and then the opposite hind foot. In the running walk, the order is different. The horse moves first one front foot, then the hind foot on the same side, then the other front foot and then the hind foot on that side. The gait is also sometimes called the “broken pace.”

  The Hipparion had adopted this gait, which allowed the mare to move faster than a normal walk but still have three feet on the ground. It’s a very secure gait, one that allows the horse to move faster but also helps the horse not slip.

  “Everybody in the horse world today is familiar with the walk, trot, and gallop,” she told me, explaining why she looked at those gaits first. “We have the belief that the running walk is an artificial gait or a show gait,” she explained, adding that when she realized that was what she was seeing in the Hipparion footprints, “it just seemed to be the most natural thing, especially when you take the slippery soil into account. A horse that can do a running walk won’t trot when the soil is not good enough. With my horse, as soon as the soil is uneven or slippery or as soon as she has some kind of stress, she will go into a running walk.”

  The ability of the horse to use a running walk has long been known to be a heritable trait, but it was thought to be a trait that had been bred into domesticated horses. Until Renders’s research, few people thought the gait was natural not just to a few breeds of modern horses, but to at least one mare and foal 3.6 million years ago.

  Renders also found that the Hipparion mare had an added measure of security to keep her from slipping—those tiny side toes. At first glance, the side toes don’t seem to make sense: we assume that horses “should” have one toe and that horses “improved” when they got rid of their extra digits. We tend to look at the “extra” toes of earlier horses as vestiges, excess clay that the sculptor evolution needed to remove.

  But Renders discovered that horses actually used those side toes. They weren’t simply relics from a bygone time. In the case of the Laetoli mare, those toes served her well on that particular day. A modern horse traveling over that terrain might have fallen and broken a leg (this happens to many modern horses trying to cross ice, for example), whereas the Hipparion mare didn’t break her stride. Instead, the evidence shows that when the mare slipped, she planted her side toes on the surface. The three toes—one main hoof and two supporting toes—made a triangle in the ash and kept her upright. You could think of them as similar to training wheels on a child’s bicycle.

  * * *

  Because of the toes and because of this horse’s natural four-beat gait, and for many other reasons, Hipparion horses were enormously successful. They first appeared in North America roughly 17 million years ago and quickly multiplied into countless species. Eventually, they spread all the way to Africa.

  The several earliest Hipparion species were larger than dawn horses, but much smaller than the mare and foal traipsing the Laetoli plain. But while the dawn horses were evolutionarily conservative, Hipparions excelled at evolutionary experimentation. This meant that as the natural world changed gradually through the Miocene epoch there always seemed to be at least one new type of Hipparion able to take advantage of new opportunities. Some Hipparions evolved into larger animals with larger teeth. Some stayed small. One group even evolved teeth that would literally grow continuously until the age of about five.

  The large Hipparion group, with many different genera, had a talent for adapting to whatever natural system they encountered. Because of this malleability, they spread across the world. Their fossils have been found in the soggy areas of Florida, a state with so many fossilized horse bones that you may well find one as you walk the Gulf Coast beaches. They turn up in the dry areas of Asia, in the Middle East, in Greece, on several Mediterranean islands, in Europe, and even north of the Arctic Circle on Ellesmere Island.

  Hipparions must have been able to eat an astonishing range of forage. With a light body, complicated teeth, a deeply grooved astragalus, and a useful three-toed foot, Hipparion was “one of the great animal travelers,” wrote George Gaylord Simpson, the mid-twentieth-century expert on horse evolution who was based at the American Museum of Natural History. As with the dawn horses, the first appearance of Hipparion in mid-Miocene North America correlates closely with a time of warmer and wetter weather. It was not as hot as the Eocene, but the global climate was warm enough that the ice in Antarctica melted a bit, allowing a variety of plants, including small trees, to grow even on the edges of that continent.

  We know this because Sarah Feakins, a molecular stratigrapher, has studied leaf waxes of plants growing in Antarctica from this period. She found that just when horse populations were exploding in North America, the temperature in Antarctica was about 11 degrees Celsius above what it is today.

  Curious about how 20-million-year-old leaf waxes could provide such information, I called her. “What,” I asked, “do leaf waxes have to do with global temperatures?”

  “Look at the waxy coatings on your house plants,” she said. “You’re seeing very resilient hydrocarbons, evolved to protect the leaf. They’re very resilient molecules. When the plant dies, these molecules don’t get eaten [by bacteria]. These waxes wind up in the ocean.”

  Which is where they stay, in marine sediments, until researchers come along and take deep-time seabed cores that Feakins and others can open up and study. By looking at the isotopes of carbon atoms and calibrating them to tables that scientists have spent decades working out, they can determine environmental conditions. Feakins found that in the throes of a general worldwide warming trend, even Antarctica had become less icy. Scientists suspect that these relaxed temperatures in the Antarctic began a cascade of events, from shifting marine currents to changes in rainfall, that accelerated this period of global warmth. Of course, we don’t know exactly which came first—the warmer Antarctica or the warmer planet itself. But we do know that during this period of 5 million years, a multitude of horse species evolved in North America.

  And yet—there were, at this time, no horse species living in Europe. Relatives of horses like tapirs and rhinos were common—but not horses. The absence of horses in Europe and Asia ended about 12 million years ago. One single Hipparion species escaped from North America to colonize Asia by traveling over Beringia, the swath of land connecting Alaska and Siberia. In the Old World, this Hipparion struck it rich. The Asian steppes were full of grazing opportunities. And horses, with their ability to eat so many different kinds of food, took full advantage of the forage around them. One scientist wrote enthusiastically about Hipparion “galloping” from Alaska to Spain.

  * * *

  Which brings us back to further discussion of Charles Darwin’s stress headache. The sudden and prolific appearance of little Hipparion in Europe was one of his life’s major conundrums. To Darwin, the little horses with the two tiny side toes seemed to materialize out of thin air in the Old World. And yet, Darwin, as we said, was not comfortable with this kind of change. Although life clearly evolved—such change should not stand out in such an apparently gross manner.

  Hipparion made evolution seem like a magic trick. Rememb
er: while Darwin was struggling with the question, scientists still believed that horse evolution occurred not in the New World, but in the Old, so that the “sudden” evolution of a tremendously prolific type of horse seemed almost like a kind of alchemy.

  One of Darwin’s British critics had accused him of claiming that life forms appeared on Earth in “higgledy-piggledy” fashion. This bothered Darwin a great deal, because he was anything but anarchic—and because Hipparion’s sudden appearance in Europe did, at least at first glance, seem to be higgledy-piggledy. Until Huxley visited O. C. Marsh at Yale, Darwin couldn’t reconcile the sudden appearance of Hipparion with his understanding of how life changes over time: “That many species have been evolved in an extremely gradual manner, there can hardly be a doubt,” he asserted in The Origin of Species.

  Hipparion wasn’t the only animal that worried Darwin. The horses came along with a whole host of other animals that seemed to scientists to have flooded Europe, almost as though there was a second Grande Coupure. In fact, there was indeed a boundary line in the rock record, just as there was in the 34-million-year-old event.

  This clear line in the rock was so obvious that nineteenth-century scientists could easily recognize it and, since the newly arrived Hipparion were so plentiful at the time, the event was called the Hipparion Datum. (For paleontologists, this is akin to the iridium layer, the result of the asteroid impact, that demarcates the end of the Age of Dinosaurs and the beginning of the Age of Mammals.) The Hipparion Datum helps paleontologists working in Asia and Europe understand the age of the rocks they are looking at: if Hipparion bones are present, the rocks must be less than 12 million years old.

  But why did Hipparion appear in the rock record as if these three-toed horses had traveled almost instantaneously from the farthest reaches of Asia all the way to Spain? The answer to that mystery has only recently come to light: the horse was following the grass. The triumph of Hipparion was also the triumph of grass.

  * * *

  We underestimate grass. The ten thousand or so varieties of grasses that cover Earth today take up an estimated 30 percent of our planet’s land surface. We see grass all around us, but what we see aboveground is not where the action is—not the part of the plant that conquered the world. About 80 percent of a grass plant—the most important part of the grass plant—lives under the soil surface, in roots that can intertwine in networks so thick that early European settlers sometimes needed teams of twenty or even thirty horses to plow the prairie sod for the first time.

  Grasses store away so much carbon in their massive underground root systems that the paleontologist Gregory Retallack and many others suggest that grasses ultimately became as important an evolutionary force as tectonics. When he told me that, I was skeptical. But the more I read, the more I realized that many people agreed with him. My skepticism was probably no more than a typical case of familiarity breeding contempt: having mown a suburban lawn, I thought I knew what grass was. I was wrong. Very wrong.

  Grasses are too modest for their own good. They don’t get respect. They’re often easily trod upon or weeded out of our gardens. Because they seem simple, we assume that they appeared early in evolution. Nothing could be further from the truth. “In fact,” wrote Candace Savage in Prairie, “they are highly evolved organisms, especially adapted to cope with extreme climatic uncertainties, including frequent drought.” For example, the aboveground blades of grasses can sometimes curl themselves up during droughts to conserve as much water as possible. In other words, when trees die out—grasses live on.

  Of course, most of the grasslands we see today are just pitiful remnants of what was here prior to the expansion of farming. Before that happened, a rider on horseback could travel through some expanses of tallgrass prairie and not see over the tips of the tallest grasses. I had read about this, but, to be honest, didn’t quite believe the authors. I suspected them of hyperbole. So I decided to see for myself. This isn’t as easy as it sounds, as almost all the original tallgrass prairies are gone. But in Illinois, of all places, at the Midewin National Tallgrass Prairie, just a bit southwest of Chicago, scientists and volunteers have worked hard to rehabilitate some of the land. Some of the stems of big bluestem, a species of grass, rose to nearly ten feet.

  These are not your city park grasses. To ride through a field of big bluestem must have been like riding through a forest, but one perhaps much more threatening than a forest of trees, since you wouldn’t have been able to see beyond the next few blades. We think of prairies as being places of wide vistas, but if the tall grasses were healthy, visibility at times might have been almost nil. Anything could be hiding in those grasses, only a few feet away, and you might not know until it was too late. A healthy grassland can create its own kind of junglelike environment. No wonder horses are so easily startled. Of course, when I was in Illinois, I didn’t have to worry about saber-toothed cats or packs of dire wolves creeping up through the grasses behind me, but I did get a whole new understanding of why horses are so cautious and always listening, alert for even the smallest rustling.

  Both bison and horses love to graze on big bluestem, but it took quite a while for the grass to evolve. Grasses, of course, are a type of flowering plant, so they had to wait for the Cretaceous Terrestrial Revolution. When they first appeared (no one knows exactly when that was), they were simple and slow on the uptake. They certainly did not conquer the world by storm. While the world was warm and wet, grasses could grow along the edges of the forests and in glades and clearings. But when the Eocene ended and the world became drier and the trees slowly died back, the grasses—plants that could protect themselves from drying out by growing deep, thick root systems—began to spread.

  They were still somewhat handicapped, though, in that they did not do well in some conditions, like strong sunlight. Before they were truly able to take over the world, grasses needed to come up with an evolutionary innovation. It was after that innovation that Hipparion began spreading worldwide. A new type of grass evolved that could endure some truly harsh conditions, such as very high temperatures and drought.

  This innovation changed the world. The marriage of these two types of grass—cool-season grass and warm-season grass—created a power couple that the whole planet would embrace. If you think about this, it’s pretty obvious: cool-season grasses grow better during the cool season, and warm-season grasses grow better during the warm season. When you get down to individual species, it’s more complicated than this, but for our purposes, all we need to know is that neither type of grass is “better” than the other. Each has its own advantages and peculiarities, but together—what a marriage they make.

  Sometimes the two grasses grow in one area, and when one type dies back, the other thrives. You can see this phenomenon by looking at a typical suburban lawn. Many have at least small amounts of both types of grasses. Some green up in the spring, then become brown in the summer, and then green up again in the fall. Some green up during the summer. If you see patches of brown in a green summer lawn, you may well think that the brown grasses are dead, but they’re not. They’re just resting until cooler weather returns.

  In wild lands, something similar occurs. Cool-season and warm-season grasses may both grow in the same general region, so that horses could, for the first time, count on having plenty of green food for much of the year. When one type of grass was brown, often the other type was green. All the horses had to do was know where to find which type of grass during certain seasons.

  Additionally, the proportion of the two types of grass in an area could change over time. Weather conditions during one decade might favor cool-season grasses. But the next decade the weather might change to favor warm-season grasses. This made a grassland system that was much hardier in the long run.

  The initial spread of the two types of grasses correlates closely in Africa with the triumph of Hipparion. Sarah Feakins has also studied cores of deep-sea sediments from the Gulf of Aden, just off the East African coast. Her
research shows that in East Africa, just after the 12-million-year-old Hipparion Datum, both kinds of grasses ebbed and flowed over the landscape in response to shifting patterns of rainfall and temperature.

  Feakins’s colleague Kevin Uno has studied the ways in which some African animals changed in response to the new grasslands by examining the teeth of horses, rhinos, and other species from about 10 million years ago. After studying the wear patterns on the fossil teeth, Uno found that horses were uniquely flexible in their eating habits. Within less than a half million years after the new grasses—called C4 grasses by scientists—first appeared in Africa, horses were eating them. And the horses who were eating these grasses turned out to have different teeth.

  To us, of course, a half million years seems like a long time.

  “That’s a startlingly fast shift over geological time scales,” Uno told me.

  He studied the same type of tooth from the same horse species and found that before the C4 grasses spread, Hipparions had tooth surfaces that contained sharp, knifelike serrations, indicating that the animals were eating food that didn’t need much grinding. About a half million years later, when the grasslands spread, the Hipparion teeth had flatter surfaces. The horses living in the same area of Africa had to grind their food a whole lot more before swallowing. Their teeth showed the results.

  Horses were once again exhibiting their exceptional flexibility. Uno found no other animal that showed this kind of quick adaptation.

  “There are two strategies when the environment changes,” paleontologist Richard Hulbert once told me. “You can find the few places left in the world that don’t change. Or, you can adapt.”

  Horses adapted.

  * * *

  Even as Hipparions spread through the Old World, they were already being replaced in North America, where the spread of the new grasses encouraged the evolution of a larger, faster horse. We know something about this transition because of yet another volcanic eruption, but one that was much more violent than the one that left the tracks at Laetoli.