This revolution, which was clearly under way 100 million years ago but which may have begun tens of millions of years earlier, was one of the most important upheavals ever to occur in the history of our planet—bar none. It was certainly much more important than the fall of the asteroid, for example. Before flowers, Eiseley wrote, “wherever one might have looked, from the poles to the equator, one would have seen only the cold dark monotonous green of a world whose plant life possessed no other color.” After the revolution, flowers were everywhere, color covered the planet, and the world would never be the same.
In fact, without flower power, the Age of Mammals itself might never have occurred. We mammals might have stayed small, inconsequential, and rather boring, just as we were for most of our 200-million-year history. Let’s face it: for much of our time on Earth, we just weren’t that interesting. For one thing, we mammals stayed hidden—the best defense we could come up with, given the mass of dinosaur flesh stomping around on the green Earth during that time. We probably ate mostly insects and grubs and worms, and we probably only came out at night. We didn’t eat fruits or grains, because there weren’t any. We did not enjoy a glamorous lifestyle.
Most likely, we just tried to stay out of the way. We probably defended ourselves by becoming what the paleontologist Christine Janis once called “artful dodgers,” animals who scurried around in the underbrush. While the dinosaurs reigned, mammals were little more than also-rans, tiny little ratlike creatures with runty snouts and four legs and five digits at the end of each leg. Sure, a few of us, like a wolverine-size Repenomamus, may have made it a habit to eat small dinosaurs (one fossil of this animal has been found with a young dinosaur in its stomach), but for the most part we were all potential and not much self-actualization. Our self-esteem was probably pretty low, and we probably would have greatly benefited from the help of a New Age guru—had any been around.
Then the non-avian dinosaurs died. So did many of our mammal cousins. Along with the dinosaurs, an estimated two-thirds of the then extant thirty-five mammal families became extinct. These mammalian extinctions, though, were not spread across the globe evenly. (Extinctions rarely are.) Rather than being global, they were somewhat glob-like. A bunch here. A bunch there. In North America’s northern interior, including the area around Polecat Bench, extinctions were highly glob-like: as many as nine-tenths of mammal families may have disappeared.
But, for better or for worse, one creature’s misfortune is another’s opportunity. A huge hole appeared in the planet’s tightly knit web of life. The end of so many life forms opened the way for the dance of communication between horses and humans to take its first baby step forward. Correlated with the 66-million-year-old disappearance of most dinosaurs was the fall of a large asteroid into what we now call the Gulf of Mexico. So without the asteroid impact that cleared the stage for a great evolutionary leap, horses and humans might never have appeared. Was the asteroid’s fall the cause of the extinction? Or was the timing merely coincidental? Were there larger forces at work, like tectonic movements and shifting ocean currents?
Most likely, paleontologists suggest, the truth behind the extinction involves many factors. When the asteroid fell, the world was already changing. The great supercontinent of Pangaea had broken up and North and South America were slowly migrating west, creating an ever-widening Atlantic Ocean—an ocean that would become a major player in the appearance of humans and in the evolution of horses and in the flight paths of birds and in the pulsations of ice and rain and drought for the coming tens of millions of years.
These long-term events, the results of our always-convulsive, seething-with-energy planet, were probably more influential in the appearance of horses and humans than the onetime crash of a mere mega-asteroid. So although paleontologists do not debate that the impact occurred, its role remains a perpetual, often contentious source of disagreement. A 2010 paper in Science authored by forty-one researchers, mostly non-paleontologists, asserted that the asteroid was the extinction’s “sole” cause. This in turn caused a great deal of snorting when paleontologists met for lunch breaks. Speaking for many others, the Yale University paleontologist Chris Norris called the emphasis on disaster as a major evolutionary force “asteroid porn.” Popular descriptions of the effects of the asteroid impact, he explained, “have a feverish quality to them that verges on the unsavory.” His point is well-taken: the worldwide climate had been changing for 10 million years before the asteroid fell. The dinosaurs were no more enjoying a steady-state world before the asteroid impact than we are today.
“Don’t get me wrong,” David Archibald, an expert in pre- and post-impact mammals, told me. “It wasn’t just another bad day for the planet. It was the bad day.” But, he added, trouble had been brewing in paradise long before then.
In any case, after the asteroid fell, a prehistoric landgrab began. A whole lot of unoccupied space was suddenly there for the taking. All you had to do was figure out how to fit in. We mammals were really good at that task. Those of us who remained alive quickly morphed into all kinds of new shapes and sizes. It must have been somewhat like the early days of the Internet, when the future was wide open and anything could—and often did—happen.
We don’t know a lot about how mammals revved up to meet these new conditions, but we do know that during these 10 million years, plant life changed dramatically. The evergreen forests that covered much of Earth died back and deciduous, broad-leafed trees began very slowly to spread, according to ecologist Benjamin Blonder. The expansion of these plants, with their delicious and highly browsable leaves provided an abundance of easily digestible food.
This, in turn, encouraged mammalian innovation. A new type of mammal arose—a mammal that could feast not on insects but on flowers, fruits, shrubs, and even, to a limited extent, on the very few grasses that were able to take root. Lots of these mammal experiments would turn out to be failed ventures, evolutionary dead ends that evolved and died out in a paleontological blink of an eye.
But a few succeeded spectacularly. Horses and primates ranked high among the triumphant. (We probably shouldn’t feel too proud of our achievement, though. In the words of Christine Janis, we were “victors by default.”)
* * *
Preator and Kelley and I walked around Polecat Bench, wishing we could find some of those cool ancient fossils, but before long I gave up. Philip Gingerich, a world-renowned paleontologist and a fossil-horse expert, once told me that when he first came to the bench as a young paleontologist in the 1970s, the fossils were just lying there, right on the surface, but those easy finds are long gone. Preator hunted for tepee rings, circles of stones outlining the circumferences of tents pitched by pre-Europeans who loved to camp up here and take advantage of the great view, just as modern local folk do on summer evenings.
Kelley looked for landmarks up in the distant mountains—the natural signs her own ancestors used a century earlier to orient themselves in place and time when they first arrived in the wilderness to run sheep.
“There’s the Horse’s Head,” she said, pointing to a large, easily seen snowpack up in the Ishawooa drainage of the Absaroka mountain range. “The reins and the nose are melting. That means the high water’s done and you can take your livestock up into the mountains and cross the rivers safely.”
I saw that she was right. Just below one of the tallest mountain peaks there was still, even in early July, a large amount of snow tucked into a ravine, and that snow did indeed look quite like a horse’s head.
“I guess,” I said, “horses just belong here.”
It’s hard to imagine this Wyoming land without horses. One way or another, people in the region have for centuries timed their own yearly rhythms to the rhythms of horses. Without horses, life would have been very challenging. When the Spanish brought domesticated horses to this region, Native Americans immediately recognized their value and became some of the world’s best horsemen. And when Kelley’s great-grandfather, a Mormon, came here at
the end of the nineteenth century, horses meant the difference between surviving and dying. The outlaw Butch Cassidy, who lived in the area, was sent to jail for the first time as a horse thief. Given the life-or-death importance of horses, he was lucky they didn’t hang him.
Even when Preator’s father brought her to Wyoming as a child in the 1950s, horses were still essential. Sure, there were cars by then, but cars needed roads, and there just weren’t a lot of roads in the Wyoming wilderness. For a while, her dad trained horses for a living. Then he became a wildlife ranger and spent his life in the saddle patrolling the Wyoming backcountry, where cars could not—and still cannot—go. Preator often rode along at his side. She and her pony covered a lot of ground. I suspect that nearly every inch of territory in these mountains has been visited, trod upon, sat upon, or at least looked upon by Preator at one time or another. To her, there’s something quite satisfying in knowing that horses have lived for 56 million years on the land where Peaks and Pryor horses live today.
Of course, it’s important to keep in mind that when the dawn horses lived here, Wyoming looked quite different. It was wet—so wet that it was covered with tropical foliage. There were no cold-weather trees. Instead of dust-filled air and dry, hard ground covered with sagebrush, there was a lot of mud and swampland.
It was hot. For a brief period, it was very hot, much hotter than when I visited. In fact, it was as though there was a sudden explosion of heat, as remarkable in its own way as the fall of the asteroid had been 10 million years earlier. Curiously, this explosion of heat also marks the appearance of Polecat Bench’s horses and primates. This was a time when temperatures in some places shot up by 6 or 8 degrees Celsius in a very short time period, lingered at those heights, then, almost as suddenly, dropped back down. The cause of this heat spike remains elusive, but it may have been due to large bursts of methane that bubbled up from the deep ocean.
On temperature charts that track the rise and fall of heat throughout our planet’s history, the heat spike looks to me like the outline of the Eiffel Tower. The anomaly is officially called the Paleocene-Eocene Thermal Maximum, PETM for short, but I prefer to think of it as the Eiffel Tower of Heat, with its sharp lines of ascent and descent that mimic so closely the graceful lines of the Parisian landmark. It’s a weird event.
And it’s doubly weird that both horses and primates may owe their existence, in part, to its existence: the spike marks the beginning of the Eocene, when not just horses and primates, but most modern mammal groups finally came into their own. Many of our major mammal groups trace their first appearances to this puzzling heat spike. It’s as though the whole world had become a giant petri dish brought to a boil by a colossal Bunsen burner. And voilà! A world that was drowning in post-asteroid misery suddenly experienced a global spring.
Opinions vary as to whether horses and primates originated on Polecat Bench or traveled there from somewhere else. Some paleontologists believe that proto-horses migrated over from Asia at the beginning of the very hot time. One very early fossil of an animal that might have been an ancestor of the dawn horses has turned up in China. Others say horses originated in Europe and traveled west. Philip Gingerich suggests that horses may well have originated right there, right in pre-Wyoming Wyoming, just east of the Rockies, right in the same region where Kelley and Preator and I were standing.
There’s even more disagreement over the origin of primates. Some researchers suggest we originated in North America, while others say Asia, and still others, Europe. In what I think of as the Huck Finn theory, another group proposes primates evolved in Africa, then rode the continental plate of India as it separated from Africa and traveled north to collide with Asia, where they dismounted and spread across the Northern Hemisphere while it was still a pretty warm place to be.
Theories abound, but current evidence shows that the earliest dawn horses and the earliest true primates made their grand entrance onto life’s stage as a duo, one species living in the trees, the other browsing below. And if we weren’t yet a matched pair on Polecat Bench, if we hadn’t yet partnered up, we certainly lived as close companions.
They were strange little things, these dawn horses. They certainly don’t look like animals fated to run the Kentucky Derby 56 million years later. But, as I said earlier, if you know what you’re looking for, you can see in these early fossils some of the basic characteristics that say “horse” to us today.
* * *
I, of course, did not know what I was looking for. Since childhood, I had seen fossils of dawn horses in museums and never really understood why scientists thought of these strange little beings as “horses.” What was it in these skeletons that said “horse”? I knew a little about some of the bones we share with horses, like the calcaneus and the patella. But I wanted to know specifics. When I looked at a fossil of a dawn horse in a museum, it looked to my uneducated eyes rather like a dog.
Why isn’t it a dog?
I called up Phil Gingerich. He told me to check out the astragalus bone, present in dawn horses as well as in modern horses. The horse’s astragalus bone is unique. It’s the horse’s equivalent to the talus bone in the center of the human ankle, which lets us rotate our foot in a circle. But the horse astragalus is shaped differently. Our talus, the second-largest bone in the human foot, allows us to easily change the angle of our feet in relation to our legs. It is the reason why we can use our feet to climb trees.
Remarkably, by 56 million years ago, the horse’s astragalus bone had already become distinctively shaped. It’s in front of the hock bone, or the calcaneus, and by the time the earliest known horse appeared, the horse astragalus was already shaped differently from the primate astragalus, which would become our talus. Even at this early date, the horse astragalus was deeply grooved and committed to limiting the movement of the horse’s hock to forward and backward instead of in a circle. Thus the horse, from his earliest days, could not climb trees.
This was not necessarily a handicap for the horse. Instead of being able to climb trees to escape predators, he opted for speed as an escape strategy. By becoming deeply grooved, the astragalus bone limited the movement of the leg below the hock to only one plane, giving the horse a slight advantage—the ability to move forward faster than other animals. Thus did speed become, from the horse’s earliest days, his primary defense.
Of course, speed is relative. You have to think of the horse’s speed in the context of the time. Dawn horses couldn’t gallop, but at least they could hop, sort of, when most predators could not. “To get away from trouble,” the paleontologist Mike Voorhies once told me, “a horse wants to run.” He compared that strategy to the strategy of a bison, who, with his huge head and curved horns, wants to fight: “I think it’s quite important that, from the beginning, there’s never been a time when horses had any cranial ornaments.” So, because of the Polecat Bench fossils, we know that from his very first days, the horse was better equipped to choose flight over fight when danger appeared.
Horses weren’t the only early Eocene mammals on their way to becoming cursorial (committed to running), but something else important set horses apart: on their hind feet, they were odd-toed. It was very odd to be odd-toed. Most ungulates were (and still are) even-toed. As early as the Eocene, plant-browsing cursorial runners had divided into two groups: the odd-toed perissodactyls (horses and their close kin) and the even-toed (cloven-hoofed) artiodactyls. This is how scientists talk about them, but it’s not the number of toes per se that’s important. Rather, in horses, it’s how the toes bear the animal’s weight.
Even at this early date, even on Polecat Bench, the middle toe of the horse’s hind foot carried more than its fair share of the animal’s weight. You can see this if you look closely enough: the middle toe of the three on the hind foot (the toe that would eventually become the hoof of the modern horse) was just a little bit bigger than the two outside toes. The even-toed artiodactyls had already taken a pathway that would prohibit them, forever, from evol
ving a foot with only one toe. The dawn horses kept that option open.
This large middle toe was key to the horse’s survival. Over time, horses placed more and more weight on this middle toe, until the other ones became useless and disappeared. Ultimately, the result of this 56-million-year-old evolutionary decision would allow Whisper to throw his weight back over his hindquarters and take a graceful leap over my pasture fence in search of water. Quarter horses would be able to spin around after a cow. Dressage horses would be able to execute a levade or courbette. When the Vogelherd horse coiled his hindquarters and raised his back and neck, or when the Pryor Mountain stallions confronted each other, they owed their powerful abilities to this early evolutionary choice, traceable to the Eiffel Tower of Heat, to the early Eocene, and perhaps to the very spot on Polecat Bench where Preator and Kelley and I stood.
Diagram by the paleontologist O. C. Marsh of the evolution of horse feet and teeth over time
I know this now, but still, when I see the dawn horses, my heart aches for them just as it did when I was a child. They don’t look comfortable. They had high arched backs that remind me of my border collie, who can twist up and down and to the right and the left with equal agility. The Polecat Bench dawn horses lack the stable backbone and high withers and straight cannon bones that allow a modern horse like Duke to stand so commandingly. I couldn’t imagine what gaits these early horses would have had. Long before I visited Polecat Bench, I’d asked the paleontologist Margery Coombs, who was showing me dawn horse fossils in the paleontology museum of Amherst College, what she thought their fastest gait might have been.
She replied: “The scamper.”
The Horse Page 6