In a corner of northwestern Wyoming the long chain of the Rocky Mountains splits into two separate stands of peaks that corral a broad stretch of desert country called the Bighorn Basin. Almost no one lives there. The basin is roughly the size of West Virginia, but its sherbet-colored badlands have less than one percent of West Virginia’s population. The land in most places looks like it always did: chalky, dry mesas fenced in by distant blue peaks. There are only two real breaks in the hills, cut by the deep, green Bighorn River, which flows in through a slot canyon in the south and out through another canyon in the north.
The Bighorn Basin exists in what people in the West call a “rain shadow”—a patch of perpetual drought cast by the mountains. It is created by prevailing winds pushing weather from west to east. Moist air cools as it rises over the twelve-thousand-foot peaks of the Absaroka Range before reaching the basin. The cold wrings out most of the moisture, which falls on the thick mountain forests as rain or snow. The air passing over the mountains and down into the basin warms and expands. Expanding air acts almost like a sponge that, once squeezed, absorbs more water. And so it almost never rains. The basin receives only about six inches of rainfall a year—less than in the cactus-studded deserts of Arizona.
Where water flows out of the mountains in swift, emerald rivers, farmers have created a lush corridor of green. Waving fields of malt barley stretch out in neat irrigated rows above the muddy banks. Swallows cut through the moist air, snatching fat mayflies on the wing. Grebes and mergansers paddle the lazy currents. After a long, dry drive through the sage flats of Wyoming, the feel of water in the air along the river is so thick and abrupt that you can taste it. But walk away from the river, through the fields and across the two-lane highway that traces the Bighorn’s banks through the basin, and your feet are soon kicking up chalky gray dust. The river and its irrigated fields on either side make only a thin line through the basin. Then bare shale terraces rise up and away from the fields, their red, gray, and green bands slumping together like slowly melting ice cream. The roads and farmhouses cease. On a map, the shale sometimes runs twenty miles without interruption in crumples and jags so erratic that it seems as if the contour lines were plugged into an electric socket.
The rain shadow has left the hills bare except for sparse polka dots of greasewood and black sage. There is no shade. It is boiling in the summer and arctic in the winter. There are creeks that were marked on the map long ago by cartographers with either a naive sense of hope or a cruel sense of humor, but there is almost never water. It is classic badlands: desiccated, defiant, devoid.
Naturally, it is Wild Horse Country.
I drove into the basin on a hot July afternoon to spend a few days out on the shale benches far from the paved road, looking for wild horses. I turned off the highway at the farming town of Worland (one of the biggest towns in the basin, population 5,487) and sped west, kicking up a plume of fine dust on a narrow dirt track.
As I drove, I scanned the empty horizon. I wasn’t looking for mustangs—not live ones, at least. I was looking for the camp of Dr. Ken Rose, a longtime summer resident of the basin and professor of anatomy at Johns Hopkins University, who happened to be one of the leading experts on the first horses ever to walk the earth.
The Bighorn Basin may not be a very hospitable place for farming, or ranching, or really anything else, but it holds one of the greatest troves of early mammal fossils in the world. Scientists from around the globe, Rose included, have been making pilgrimages to the site for more than a century to search for old bones. I was making my own pilgrimage to the basin because it is one of the best places to study where the wild horse began. I wanted to see it because I had heard over and over from government employees, ranchers, big-game managers, and others who want to limit wild horse numbers, that wild horses aren’t really wild—they are just escaped livestock that are now infesting the native ecosystem like so much kudzu. As a nonnative invader, the argument goes, mustangs represent an imbalance to the native ecosystem. They are a usurper, a contagion, and because of this, they must be rounded up to protect the natural balance.
The assertion that horses are not native made perfect sense to me at first. Europeans brought horses to the Americas. That is clear. There are a few fringe wild horse advocates who will argue that horses were actually here before Columbus arrived, but there is not good evidence. No explorers who made first contact with native tribes ever documented horses. If Europeans brought them, the argument goes, then they are an invasive species. If they are an invasive species, we must control them.
But as I kept turning the idea over in my mind, the main points started to dull by knocking into competing arguments. The first was that the wild horse is not invasive. I had some vague memory from a high school textbook of an illustration of prehistoric horses roaming North American among woolly mammoths and giant sloths millions of years before humans arrived. Second, I began to wonder what “native” really meant. Of course the definition seems obvious: A plant or animal is native when it exists in its natural range or ecosystem, as opposed to a species introduced by man. But it actually isn’t so simple. That natural range of a species is often thought of as static. In North America, we generally define it as wherever it was found by the first European observers. Any subsequent changes often are portrayed as unnatural. But animals and ecosystems have migrated thousands of miles over the millennia, gaining new ground and losing old. Oceans have risen and fallen. Whole continents have been covered in ice. Landmasses have drifted from the equator to the poles, burying jungles in ice. Mountains have become seas and seas, mountains. Isolated continents have slammed into each other, unleashing their own invasive species like a geologic D-day landing. The writer John McPhee summed up the constant change on Earth with one observation: “The summit of Mt. Everest is marine limestone.”
In North America, we generally use native to mean any critter that was here before Christopher Columbus hit land. In recent decades, we have given these native animals special rights, trying to protect them while working to eradicate the invaders. We spend millions of dollars trying to save a black-footed ferret or a sage hen in the West, and millions more building electric fences to keep the Asian carp out of the Great Lakes or kill Asian pythons in the Everglades. Wildlife laws protect the bobcat, but not the feral house cat. We want to preserve an old balance in a world where we have shifted the balance dramatically. I share this desire. I want protection for the natives. As I write this, a thumb-size broad-tailed hummingbird is hovering at a feeder in an old pine above my keyboard. I love seeing such a precise distillation of life so much that I would do nearly anything to protect its annual migration over hundreds of miles along the Rockies.
But what is really native? How long do migrants like the wild horse have to be here before they count? Certainly more than a hundred years, we would all likely agree. But what about a thousand? What about ten thousand? At some point, the label of being invasive must wear off. What is the process through which an animal becomes native? And how do we decide? Those were questions I wanted to start answering with a trip to Bighorn Basin.
As I looked for Professor Rose’s camp, I drove through old beds of sandstone and shale laid down millions of years ago, when this part of Wyoming was a flat, muddy coast that alternated among thick jungle, tropical swamp, and shallow sea. A postcard sent during the era might have looked a lot like the coast of South Carolina. Over millions of years, the area was a depository for silt and muck washed down from the mountains to the west. The layers of sediment made a great Cenozoic layer cake, with bands of hard white sandstone and bands of crumbly shale in colors ranging from milk to gray to dusty mint to clay-pot red. They eventually piled up more than twenty-five hundred feet thick.
Then the whole area was slowly pushed up, between eighty million and thirty-five million years ago. The sea receded to the east. Layers of muck that had piled up and become stone were exposed to the elements as they rose. Eons of wind and rain have whittled them into buttes, mesas, and rumpled shale hills. This ancient, hardened layer cake of jungle muck is called the Willwood Formation. It makes up every hill and draw for miles around. Crush the shale between your fingers and it turns velvety smooth. Set a hard Wyoming wind on it and it fills the air, covering everything with fine dust. Get it wet and it turns into a slippery, sticky, shoe-sucking glop that the paleontologists who have been traipsing these hills for generations call gumbo.
I spotted Rose’s camp about five miles from the highway. It was off in a low draw where occasional moisture in the sand had nurtured a few stunted cottonwoods, which offered a little shade and a buffer from the wind. It was near sunset when I parked on the edge of the camp, and I could see graduate students milling around a dozen tents tucked among giant sage bushes that grew five feet high. One big cooking tent stood in the middle.
The students—sunburned, weary, but otherwise cheerful—were splashing the dust off their faces and arms in plastic basins after a long day in the field. Most of them planned to spend the whole summer here, searching the hills for fossils by day and bringing back their finds at night.
When I got out of my car, Rose came out of the cook tent at a fast walk, hand extended to greet me. He wore a blue Oxford button-down shirt and wire-frame glasses, and had a distinguished gray professorial beard. But this East Coast University look was offset by a wildly battered cowboy hat crowned with a dark band of dirt and sweat and punctuated by a long magpie tail feather that poked from the band. It was a unique mix of starch and grit that only field paleontologists seem to have.
“To most people, this is badlands,” he said, after shaking my hand and ushering me to the edge of the camp where we could see the bare shale hills beyond. “To paleontologists, it is the most valuable land in the world.”
The Willwood has been a destination for researchers for more than a century because its many layers, gently laid down during a period between fifty-five and fifty-two million years ago, record a turning point in history, right after the dinosaurs disappeared, when mammals began to flourish. The period when it formed is called the Eocene Epoch, from the ancient Greek word eos, meaning dawn, because it was the dawn of the modern era. The Bighorn Basin is one of the few places in the world where a large slice of the Eocene is preserved. And it is perhaps the best place to see the beginning of horses.
“Everything is just right here for fossils,” Rose said. Conditions were right millions of years ago to gently bury plants and animals in wet, flat, sediment-rich plains. After that, conditions were just right to bury those plains under more sediment so they could be pressed to make stone and be preserved. More recently, conditions were just right when the tectonic plate that makes up North America began to buckle, slowly uplifting the land. And conditions were just right when erosion removed thousands of feet of newer rock that covered the Willwood. On top of all that, the rain shadow was just right to keep almost all of the ground from being covered by grass and trees, letting wandering paleontologists find their quarry.
“You could not ask for better conditions,” Rose said. Just then, a gust came up and took off his hat, which went bounding through the sage. He added, “Well, I guess a little less wind would be nice.”
Rose grew up in New Jersey, far from the vast Eocene beds of the West. But, as a boy, he had developed an insatiable thirst for collecting—primarily trilobites, sharks’ teeth, and other fossils from the ancient shallow seabed that today makes up much of New Jersey.
“Other kids played baseball,” he said as we sat down for dinner at the camp’s single table. “I had a museum in my basement.”
He first came out to the Bighorn Basin when he was an undergraduate in 1968 and has come back nearly every summer since, earning an MA degree from Harvard University and a PhD from the University of Michigan along the way.
I had contacted him because I wanted to get as close an understanding as I could of the first horses. I wanted to try to see what the first horses looked like and imagine their journey from the dawn of mammals to the present day. But I also wanted to understand how this long history has shaped the recent history of wild horses. We talked a few times by phone, and then he said, “If you really want to get a feel for it, you should just come out to the field.”
Before we turned in for the night, he unrolled a large topographical map to decide where his team would focus the next day. The decades-old sheet was peppered with neat pencil notes showing the dates he had collected in each spot. Some showed two or three passes. Every decade or two, he said, he returns to see if erosion has turned up anything new. His map also acts as the unpublished modern history of the Willwood. Here and there, a few words recorded the dates and places of notable occurrences observed over the decades that broke the normally timeless silence of the badlands: the scorpion bite, the nudist, the madman at the sheep camp.
Several of the pencil inscriptions prompted stories he told the students by the light of the camp lantern. I pointed to one note: The day of 427 jaws. I asked what it meant.
“That was maybe the greatest collecting day we’ve ever had,” Rose said.
Jaws, it turns out, are what Rose’s crew is after. Often, they are all that is left of a mammal after fifty million years. Dying is a pretty brutal process. Even if an animal dies peacefully, it is almost always picked apart by scavengers, the remains scattered and broken and nibbled into oblivion. It is almost astronomically unlikely that a carcass is left intact to be buried. And even then, to be discovered fifty million years later, it has to survive the slow, uncertain process of fossilization, then the uplift of the formation, then erosion. Almost nothing does. But jaws, it turns out, are sturdy, and that is a very lucky thing. More than any other part of the skeleton, jaws carry a wealth of clues about ancient mammals: size, diet, evolutionary lineage, even social structure.
“Full skeletons are pretty to look at but not scientifically valuable,” Rose said. “They don’t tell us much. Jaws tell us a lot.”
Rose now has tens of thousands of jaws in his collection. Over the years, his teams of students have collected so many that they can see how the population changed over millennia and start to describe not just what was in the Bighorn fifty million years ago but also how twenty or so species changed over three million years as the Earth rapidly warmed.
“We can’t do that for animals today,” Rose said that night, right before he turned off the camp lantern. “In some ways we know more about these ancient horses than we know about the animal today.”
At dawn, Rose’s students gathered for coffee and a quick breakfast of fruit and granola, seeking shelter in the cook tent from the sun, which felt searing on the skin at 6 a.m. Amid the clatter of spoons and bowls, Rose, who likes to begin each day by reading a passage from a notable paleontologist, opened a marked page in a book by George Gaylord Simpson, one of the greatest evolutionary biologists of the twentieth century. Rose handed the open book, which detailed Simpson’s explorations of Patagonia, to a student, who began to read:
A 50 MILLION-YEAR-OLD FRAGMENT OF JAW FROM THE FIRST HORSE, HYRACOTHERIUM, IN BIG HORN BASIN, WYOMING.
Fossil hunting is far the most fascinating of all sports. I speak for myself, although I do not see how any true sportsman could fail to agree with me if he had tried bone digging. It has some danger, enough to give it zest and probably about as much as in the average modern engineered big-game hunt, and the danger is wholly to the hunter. It has uncertainty and excitement and all the thrills of gambling with none of its vicious features. The hunter never knows what his bag may be, perhaps nothing, perhaps a creature never before seen by human eyes. Over the next hill may lie a great discovery! It requires knowledge, skill, and some degree of hardihood. And its results are so much more important, more worthwhile, and more enduring than those of any other sport! The fossil hunter does not kill; he resurrects. And the result of his sport is to add to the sum of human pleasure and to the treasures of human knowledge.”1
“Hallelujah, well said!” Rose applauded.
With that, the crew climbed into a mud-splattered 1990 Chevrolet Suburban that had the fossil of an early ancestor of the modern deer, called Diacodexis, stenciled on the side. Rose stomped on the gas pedal and took off with a lurch through the badlands, speeding like someone who had just robbed a bank. The old Suburban bounced and bounded at sixty miles an hour down a narrow dirt track through the desert. As he sped up over a rise, Rose pushed a cassette into the old tape deck in the dash and the speakers blared the theme song for Indiana Jones—Dun da dun-dun, dun da-duuuunnnn!
I gripped the edge of the vinyl seat as the truck flew over rolls and swayed through turns. Rose barely held the loose steering wheel as he flew along. He seemed much more focused on pointing out places where he had gathered fossils than he did on the road. I realized only later, when we stopped at the site where we were going to collect that day, why the distinguished professor drove like such a madman. After more than forty years collecting here, he knew every dip and roll of this country, and knew how much he could push the needle. Also, hunting for Eocene jaws is an agonizingly slow process, requiring sauntering inspection for hours on end. You might cover a mile in a day. Probably less. Do that all day without a little thrill and it could be a very long summer. As George Gaylord Simpson said, fossil hunting should have enough danger to give it zest.
An avid hunter like Rose would probably add that the faster you drive, the sooner you are searching for fossils.
“Horses!” he shouted. He jammed on the brakes. The Suburban skidded to a stop and was enveloped a second later in the gray cloud of dust that had been trailing off the back. As the dust cleared, he pointed to six wild horses on the horizon. We watched them silhouetted in the morning sun. They stood a few seconds, their heads up and ears pricked. Then the lead mare wheeled and disappeared over the horizon. The others followed, dissolving into the badlands.
“Have you ever seen them here before?” I asked. I had no idea wild horses still wandered the Bighorn Basin.
“Oh, lots of them,” Rose said. He had seen horses in varying numbers since he first started coming to the basin in the late 1960s. No one quite knows how far back these horses go. The BLM’s first record of horses here dates back to 1938, when the bureau’s predecessor, the United States Grazing Service, tried unsuccessfully to remove all of them. In the 1940s, one local cowboy claimed to have removed twenty thousand mustangs from the basin with the help of a small plane. But the presence likely goes back much farther, possibly to the Crow and the Shoshone tribes. It was near this area that a few Shoshone became especially skilled at hunting with horses, split from their old tribe, headed east, and became the Comanche. Lewis and Clark had lost some horses not far north of here. It could be their descendants we saw running wild.
The horses we spotted that morning weren’t technically supposed to be there. As I mentioned before, the BLM divides wild horse areas into two types: Herd Management Areas and Herd Areas. Herd Management Areas are places where horses are known to live and have the BLM’s blessing. Local district offices are supposed to manage those areas primarily for the horses. Herd Areas, on the other hand, are places where horses are known to live, but the BLM has decided they should not be there. Those horses are slated for eventual removal. The horses we saw were in a third situation. They were not in an HMA or an HA. They were completely off the map, not known to exist. Truly wild. Maybe they had wandered through the unfenced badlands from the nearest HMA, a dozen miles away. Maybe they had been here for centuries, unnoticed or unnoted by BLM managers. The Bighorn Basin is a rough, lonely place. A lot can hide out in the Willwood.
After the horses galloped away, we drove on, finally stopping at a broad amphitheater of shale and sandstone, completely bare of vegetation. “This is my empire,” Rose said. “Every outcrop in miles I have collected.”
We split up to look for fossils. The graduate students went off in pairs, and I teamed up with Rose. He slipped on his sweat-stained cowboy hat and we started walking. Or, rather, we started ambling. Walking suggests a little too much speed. We shambled, shuffled, dallied, and perused, covering maybe ten feet per minute as our eyes scanned the ground.
We were looking for the telltale glint of teeth. In the fossilization process, white teeth turn nearly black. In a museum, they can look like little bits of onyx, but in the sun of Wyoming they have a glint like the brown glass from a broken beer bottle.
“You find that glint,” Rose said. “And chances are you have found something good.”
The only problem with looking for something that looks like a broken piece of beer bottle in Wyoming is that cowboys have been coming out to the middle of nowhere to shoot beer bottles here for a very long time. I spent a lot of the day bending down, only to find the remains of a distant Saturday night.
If we had been sashaying through the Bighorn Basin fifty-five million years ago, the scene would have looked fantastically different. It might have looked a lot like the coast of Laos or Brazil. The basin, now at about four thousand feet, then sat at sea level next to a shallow sea that once covered much of the Great Plains. Though the latitude was about the same as it is now, the climate was tropical. Paleontologists refer to this time as “greenhouse Earth.” There was no ice, even at the poles. Palm trees and crocodiles extended above the Arctic Circle. Forests dominated the planet. Grass, which in modern times came to cover most of the Earth, was relegated to a few small clearings. The prickly brush of windy Wyoming was then steamy jungle with canopies of ginkgoes, sycamore, hickory, and medicine wood rising over a soggy understory of thick green. Turtles of all sizes paddled the slow streams, squirrel-size primates skittered up rough forest trunks.
In the undergrowth, a giant, flightless bird called Gastornis stood more than six feet tall on ostrichlike legs. It had a sicklelike beak fifteen inches thick, which might have been an oversize nutcracker for an omnivore or might have been a lethal neck cracker for a predator, used to grab and crush smaller animals. If it was a neck cracker, its typical meal might have been the first-ever horses.
The first horse was not much bigger than a cat and as slender as a deer, with a back hunched a bit like a rabbit’s and dainty legs like those of a deer. It was called Hyracotherium (pronounced “heer-a-co-theer-ee-um”). It was likely a bounder, not a runner. It had four small toes on its front feet and three on its back. Each toe ended in a nimble hoof, like the trotter of a piglet. Its delicate muzzle was made to nibble tender leaves and berries near the forest floor. A lot remains unknown about this ancient animal, but one thing Rose knows for sure: There were a heck of a lot of them. He finds more Hyracotherium jaws in the Willwood than any other fossil. It dominated the forest.
The first fossil of Hyracotherium wasn’t found anywhere near Bighorn Basin. If it had been, it likely would not be called Hyracotherium, a name that George Gaylord Simpson, who spent decades hunting fossil horses, described as “a jawbreaker that is not likely to win so many friends.”
The name doesn’t have anything to do with horses. It means hyraxlike beast. It got the name in an unlikely fashion, a long way from Wyoming. In 1838, a brickmaker digging in the clay near an estuary of the Thames River in England uncovered a tiny fossilized jaw. It found its way to the leading paleontologist of the time, a man named Sir Richard Owen, who was known, among other things, for being such a rabid student of anatomy that he arranged to have right of first refusal on any animal that died at the London Zoo. (His wife is said to have complained about the arrangement after coming home and finding a dead rhinoceros in her front hall.) Owen, who had classified hundreds of species of extinct creatures, examined the primitive jaw. It had pointed, low-crowned teeth that looked nothing like those of a modern horse. He first decided it must be the jaw of a prehistoric monkey. Later, on closer inspection, he decided its teeth looked more like those of a pig. Finally, he decided the teeth resembled those of an obscure rock-dwelling critter from the Middle East called a hyrax, which looks like a groundhog but is more closely related to elephants. In 1841, he published a paper describing a new order, using the teeth as evidence. He called his new find Hyracotherium—the hyraxlike beast.
AN EARLY TWENTIETH-CENTURY RENDERING OF HYRACOTHERIUM IN THE LUSH FORESTS.
Owen went on to catalogue uncounted thousands of other fossils, including all of the fossils Charles Darwin brought home from his voyage on the Beagle. He named hundreds of new species, coined the term dinosaur, and founded London’s Natural History Museum, but he never did figure out that Hyracotherium was not a hyrax. That happened decades later and thousands of miles away in the United States.
In 1866, a dour-looking young man named Othniel Marsh, with a very straight, prominent brow often accentuated with a straw boater hat, was named the first professor of paleontology at Yale University. He got the job primarily because his fabulously rich uncle, the industrialist George Peabody, built a paleontology museum for Yale. Marsh was a passionate, and fairly talented, cataloguer of bones. And his timing was excellent. Some of the greatest fossil beds in the world were about to become accessible through railroads opening the American West.
In 1868, Marsh bought a ticket on one of the first trains to the frontier. Passing through what is now western Nebraska, at a whistle stop called Antelope Station, he chatted with some workers who told him they had encountered unusual bones while digging a well. They thought the bones were from ancient humans or maybe even tigers. Hopping off the train, Marsh examined them hungrily and, as he later wrote, “soon found many fragments and a number of entire bones, not of man, but of horses, diminutive indeed, but true equine ancestors.” With the train waiting, Marsh threw the bones in a sack and went on, but later, on closer examination, found fossils of four separate horse species, including a small, odd-looking horse with not one toe, but multiple toes. Describing it, he wrote, “During life, he was scarcely a yard in height, and each of his slender legs was terminated in three toes.”
Marsh went back to the West repeatedly in the next few years to search for bones, often in dangerous and remote regions where native tribes and the US Cavalry clashed. For safety, his researchers carried both rock hammers and Colt revolvers. On one early expedition near Bighorn Basin, he employed a young man named William “Buffalo Bill” Cody as a scout, until Cody had to ride off to respond to some trouble with Pawnee warriors. Within a few years, Marsh had so many crates of fossilized bones stored in the attic of Yale’s Peabody Museum that he had to prop up the ceiling with extra beams.
The collection catalogued more than thirty species of horse ancestors that had lived in North America. At Yale, Marsh began to piece together a horse lineage. He started with horses from the relatively recent past. They had a single hoof that marked them distinctly as relatives of the modern horse. These animals, he found, were clearly related to somewhat older, smaller animals, which had a main hoof with two smaller hooves on either side. Those appeared to be related to even older animals that had three fully developed hoofed toes on each foot. Before this, many scientists had given little thought to how the strange beasts they found in fossil deposits were related to the present. But Marsh had found evidence of a lineage that had changed over time. It became a foundational argument for evolution.
In 1876, the world’s leading proponent of evolution, a largely self-taught English biologist with broad, drooping sideburns, named Thomas Huxley, sailed to the United States to deliver a series of lectures on the controversial new theory, formally proposed by Charles Darwin twenty years earlier. Evolution had a lot of prominent backers, but it did not have much hard evidence to support it. Huxley argued that evidence could be found in the fossil record, and he focused on fossil horses from Europe that seemed to show a lineage of change over time. But his evidence was thin: three species so distantly related that drawing a line between them was at best sketchy, and not very good for the lecture circuit.
Before lecturing in New York, Huxley made his way to Marsh’s museum for what he described in a letter to his wife as a quick “canter over the fossils.” He ended up staying for weeks. The two bone hunters hit it off fabulously, studying at the museum by day and going for carriage rides in the evening. “His collection of fossils is the most wonderful thing I ever saw,” Huxley wrote to his wife. A biography written by his son described how Huxley would ask if Marsh had a specimen to exemplify each transition. “Marsh would simply turn to his assistant and bid him fetch box number so and so, until Huxley turned upon him and said, ‘I believe you are a magician; whatever I want, you just conjure it up.’ ” 2
Marsh had dozens of examples of horse ancestors that showed a gradual evolution from three toes down to one hoof. As the two delved through the specimen boxes, Huxley posited that somewhere in the distant past, there must have been an original member of the horse family with four toes. He called this theoretical beast Eohippus—the Dawn Horse.
THOMAS HUXLEY’S 1876 DRAWING OF THE FIRST HORSE AND FIRST MAN.
He drew a cartoon for Marsh of a human ancestor riding on a multitoed horse. Yale still has the doodle in its special collections. Months later, after Huxley had sailed back to England, Marsh wrote to tell him that he had found the four-toed Eohippus. In fact, it had been at the museum during Huxley’s visit, but Marsh had so many crates of fossils from out West that he had not yet had time to clean the mud off all of them.
Unbeknownst to either, the creature Marsh called Eohippus was the same one Sir Richard Owen had called Hyracotherium thirty-five years earlier. The name Eohippus reigned for decades, until 1932, when an English researcher compared Eohippus and Hyracotherium and concluded they were the same. Since zoological naming rules give priority to the earliest description, Eohippus was scrapped. Hyracotherium is considered the proper name today. In the United States, you still hear a lot of ancient-horse lovers refer to the first horse by its American name, which has a nicer ring. George Gaylord Simpson, who had an even finer knowledge of horse fossils than Marsh, got around the problem by referring to the first horse as eohippus with a lowercase E—a sly way of noting that, while it was not the proper name, it was the right one.
I trailed Rose up through a jumble of boulders and out onto a bare shale flat. He moseyed with his arms loosely tucked behind his back, like a man strolling through an art museum but with his eyes trained down, as if all the paintings were screwed to the floor. In one hand he gripped an ice pick, ready to pry a jaw out of the hard ground. To outside observers, it must have looked maddeningly dull to stand out in the heat and wind, slowly scrutinizing each inch of the middle of nowhere, but each step presented a challenge—thousands of new shapes in the ground for the brain to quickly classify. Tan rock, not a fossil. Broken turtle shell, too common, not worth collecting. Sheep poop, not worth collecting. White-colored rock, oblong, requires further inspection. Nope, not a fossil.
The process continued like a mantra with each step, and took considerable concentration. It insisted that in order to better understand the distant past, we remain fixated on the present. After an hour, I began to wonder whether we would ever find a jaw. Then Rose suddenly bent down and bobbed up holding three pieces of beige stone. He fit them together loosely in his hands to form the spreading spade of a scapula bone.
“What is it?” I asked, hopefully.
“Not what I’m looking for,” he said. He tossed them back in the dirt. “I’m after the little things. They are much more important. Easier to carry, too.”
As he ambled on, I pressed him about the fossil he had just tossed aside. It was a Coryphodon, the largest mammal of the Eocene, he said. About three feet high, maybe a thousand pounds—unrelated to the hippopotamus but with a similar heft and love of aquatic plants.
“Smallest brain-to-body ratio of any mammal you find here,” he said—an Eocene lummox, and an evolutionary dead end.
We ambled on. I turned my eyes back to the ground, trying to sort through the galaxy of pebbles for something useful.
“Jaw!” Rose yelled. He bent down, then popped up grinning with a fragment of fossil jaw. It was about an inch long, with a back molar still embedded in the bone, glinting in the sun. The small, narrow tooth had four simple peaks for chewing. It did not look like a scale version of modern horse molars, which have a wide maze of ridges used to grind grass with a sideways motion. Hyracotherium’s teeth looked more like plain old teeth—a few crests and valleys for chewing and that’s about it.
“These are very primitive teeth, not much specialization in the direction of modern horses,” Rose said, looking down at the fossil in his palm. “The story of the horse is really the story of its teeth. They have changed so much you wouldn’t be able to recognize the species if you couldn’t connect them over time.”
The sun rose higher and the shadows disappeared. Rose found about twenty more jaws. I found none. Noon is the best time to find jaws because the light glints off the teeth. But it is also the most brutal time to be in the badlands. The searing sun was amplified as it bounced off the light-colored rock. We shambled and sweated. We sweated and shambled. We stopped to drink water. We sweated more. The moisture evaporated instantly in the dry wind, leaving a crust of salt that felt like fine dust. Or maybe it was dust. There was plenty of that around, too. I could feel it infiltrating my shoes and grinding between my toes. My neck and ears were crisping. I was wearing an old baseball cap and envied Rose’s big straw brim—the only shade in miles. It gave me an appreciation for the modern horses that live out here. There isn’t much to eat. There is less to drink, and there is no cover from the sun or wind. I barely made it through a day, and they had been doing it for as long as anyone could remember.
I started to wonder why anyone would want to gather fossils all day, every day, every summer, for a career. I was so hot I wanted to call it quits, speed down to the cool, green Bighorn River and jump in. But it was strangely addicting. Every time I decided to give up and try to find a small rock outcrop that might offer shade, I would see a small shelf or ravine that looked promising. And I would think, Maybe there are jaws there.
Then I saw one. A perfect outline of a jaw, the little black teeth glinting just like a broken beer bottle. I picked it up, cupped it in my palm, and felt my heart beat faster. In my hand was the first horse, which didn’t look like a horse or eat like a horse, but even so, it was key to understanding the wild horse today—why it developed in the West, and how it was able to do so well here.
The evolutionary saga of the horse is a kind of cast-out-of-Eden story. The horse started in a lush paradise that it lost as the global climate changed. Since then, it has had to find its way over dry, barren ground, where it has managed not just to survive but also to thrive. That evolution helps explain why the animal made such a tight bond with humans, and why it was able to spread all over the world in the modern era when so many other animals disappeared.
The story starts with little bands of Hyracotherium. The first of them appear to have evolved on what is now India, when it was still an island in the Indian Ocean. When India slammed into the southern coast of Asia, Hyracotherium spread out and feasted on tender leaves and berries all over greenhouse Earth’s forests, eventually reaching to Wyoming and beyond.
A few tantalizing fossil finds suggest that then, as now, horses were social. In 1952, George Gaylord Simpson, who at the time was a curator at the American Museum of Natural History in New York, found an array of twenty-six Hyracotherium skeletons in southern Colorado. Later analysis showed the group had far more females than males, and the males were bigger, suggesting bands with one dominant male and a harem of females.
That same band structure is still around today wherever horses roam free. Each band numbers anywhere from four to maybe twelve animals. It consists of one dominant stallion, his mares, and their offspring. The stallion acts as security, watching for both competing stallions and for predators. You will often find the stallion standing on a rise overlooking his band. Despite his size, though, he is not in charge. There is always a dominant mare that leads the band. She decides where they go, when they eat, and when they drink. If chased, the mare leads the way, the band follows, and the stallion herds from the rear, keeping his band together while holding a position where he can fight off predators.
There is not enough fossil evidence to know whether Hyracotherium bands worked in the same way, but the record suggests that horses have been living in these stallion-dominated family bands for a very long time.
Back in the Eocene, bands of tiny horses nibbled at lush forest growth. One well-preserved fossil from Germany showed a digestive system filled with the preserved leaves and pits from wild grapes. Just as Hyracotherium was getting established, about fifty million years ago, Earth began to steadily dry and cool. One of the leading theories is that the cooling was driven by an explosion in aquatic ferns that sucked so much carbon dioxide out of the atmosphere that it turned down the whole planet’s thermostat. Whatever the cause, the forests began to thin. An obscure plant family called grass, which was once relegated to marshes and riverbanks, started to spread. It now covers almost a third of our planet.
If the spread of grass doesn’t seem like a big deal, that’s because you’ve likely never tried to eat it. And for good reason. It’s harsh stuff. As a defense mechanism, grass draws abrasive silica from the ground and stores it in its cells. This sand built into the plant wears down herbivores’ teeth to nubs. An herbivore with no more teeth is a dead herbivore.
Fossils suggest that, as grass spread, the horse at first stayed in the retreating forests. But then, in the Oligocene Epoch, about thirty-two million years ago, Hyracotherium split into two distinct lines in North America: Miohippus and Mesohippus.
Mesohippus (middle horse) basically carried on the Hyracotherium way of life, though there were some notable changes. The fourth toe on its front feet had withered to a bony nodule along the ankle, leaving just three toes on each foot. It was slightly larger, about two feet tall at the shoulder, with longer legs and a longer face. Its back had lost some of Hyracotherium’s rabbitlike hunch. It had developed an extra pair of molars for grinding. But its low-crowned teeth showed it continued to browse on lush forest foliage. Fossil beds formed from swampy areas, like the Big Badlands of South Dakota, are full of Mesohippus fossils, suggesting it lived in moist forests along rivers, going on as if global cooling had never happened.
Its cousin, Miohippus (small horse), struck out on a different path. Like Mesohippus, it had three toes, it was bigger than Hyracotherium and more horselike, but its upper molars were bigger and wider. Paleontologists see this as evidence it had started eating grass. The difference may seem minor, but it’s not. Mesohippus is gone. It was an evolutionary dead end. Miohippus passed on its genes to modern horses that now cover the earth.
Miohippus’s evolution can be explained in part by what it was trying to eat, and in part by what was trying to eat it. By thirty-two million years ago, predators had proliferated across North America. Thousand-pound Hyaenodons, doglike predators with muscle-piled shoulders and back legs like springs, prowled the continent. Nimravidae, early ancestors of the big cats, waited in the trees to pounce. The forest became dangerous. On the grasslands, at least you could see predators coming.
With the constant pressure of predators, Miohippus started to show characteristics now inseparable with horses. Its eyes moved farther back on its head, putting it in a better position to spot predators while lowering its head to graze or drink. Its legs became longer, stronger, and faster. Its brain became rapidly larger, perhaps because it needed to navigate new social behavior and make quick decisions about potential threats on the open plains.
The global trend in cooling and drying continued to spread. Forests continued to retreat. Grasslands and tundra continued to expand. Changes occurred gradually until about twenty-three million years ago, when something really big happened—George Gaylord Simpson called it “The Great Transformation.” To watch it happen, the “Great Transformation” wouldn’t have seemed all that great. But a crucial change allowed horses to increase around the planet and dominate grasslands and savannas. The transformation was this: Miohippus had gradually evolved through a series of limbs and branches into a distinct descendant called Parahippus (near horse). The big revolution in Parahippus was its molars. They were broad and complex—a series of ridges and valleys that could cut grass like a threshing machine. These back teeth also became much, much longer than typical teeth. Most of their length remained below the gums, housed in the jaw. The new teeth acted a bit like a mechanical pencil. As a steady diet of grass wore down the crown of the tooth, more rose out of the socket to replace it. Biologists call this trait hypsodonty—long-toothedness.
The new teeth could grind grass for decades before wearing out, allowing horses to eat a harsh diet of grass and still thrive and reproduce. Modern horse teeth can be up to five inches long and take thirty years to wear down. This, by the way, is where we get the sayings “He is long in the tooth” and “Don’t look a gift horse in the mouth.” A person could gauge a horse’s useful life expectancy by how much of its back teeth it had left.
A similarly crucial transformation happened around the same time in the horse’s digestive tract. Most herbivores are ruminants that ferment plant matter in their stomachs. Horses developed a specialized fermentation chamber near the end of their digestive tract, called the cecum. Elephants and rhinoceroses also have a cecum, or “hindgut,” which is located between the intestine and the colon.
Hindgut digestion is less efficient than the digestion of most other herbivores, including cows and deer. A horse can extract only 45 percent of nutrients, while a ruminant can extract 60 percent. But a hindgut also gives horses a big advantage: They can eat low-quality food that ruminants can’t. Ruminants must selectively graze tender grasses to maintain the slow process of bacterial breakdown in their stomachs. Hindgut herbivores don’t have the same limitation. They can eat harsh grasses and shrubs a cow wouldn’t touch. Forage passes through a hindgut system relatively quickly, so even though horses get less energy from every bite, they can take a lot more bites, so they end up getting more nutrients. This allows them to survive on forage that is too rough for most other animals.
I remember being out in a wind-whipped stretch of Nevada desert with a rancher who was explaining to me how he had to take his cattle out of a pasture because wild horses had eaten all the grass. If the grass was largely gone, I asked, how were the horses still surviving? “Shoot,” he said, “wild horses can get fat in a place where a cow would starve.”
About twelve million years ago, a volcano erupted in Idaho and began spewing thick plumes of ash that the wind carried eastward almost a thousand miles, where the ash rained down over what is now eastern Nebraska. By the time it was all over, the fine, silvery ash was a foot deep across the state and ten feet thick where it drifted. As the ash sifted down, large mammals—choking on the dust—made for the nearby water hole, where hundreds collapsed of lung failure and were covered over with more dust.
The ash became rock, preserving that moment in time. Then, starting in the 1970s, it was excavated by paleontologists. You can now visit the stunning jumble of stricken skeletons at Ashfall Fossil Beds State Park: camels, giant tortoises, pint-size North American rhinoceroses, and five species of horses that ranged from a gazellelike three-toed horse called Protohippus (first horse), to a medium-size three-toed group called Hipparion (ponylike horse), to the ancestor of the modern horse, called Pliohippus (horse from the Pliocene Epoch), which stood on one central hoof, its other toes little more than calluses on its ankles.
The epoch when the volcano blew, known as the Miocene, lasted from about twenty-three to five million years ago. It was the boom time for horses. North America boasted at least nineteen species. Like the ungulates of today’s African savanna, they fanned into different niches. Some were forest dwellers, some ate grass. Some were almost as big as modern horses, others were small and light like antelope.
One of the most widespread species was Dinohippus (terrible horse). It was larger than previous horses, though still pony-size by today’s standards—about fifty inches at the shoulder. More important than size, though, were its innovations. Dinohippus lived on the open grasslands where predators like dire wolves and saber-toothed cats prowled. Dinohippus evolved under the twin pressures of a grass diet and lurking meat eaters. Its mouth became packed with broad, long molars and it had a strong, thick jaw for grinding grass. Its face grew longer, with eyes set farther up on the head so that it could graze while still keeping watch above the grass for predators. These changes to the head made Dinohippus look very much like a modern horse.
The leg bones that allow many other mammals, including humans, to rotate their ankles, changed, too. They became fused to allow only forward and backward motions. This made horses stronger and more efficient. Dinohippus could stand with leg bones locked in place while grazing to conserve energy, and could run with increased speed.
In terms of evolutionary success, this new design was a hit. During the Miocene, the horse family, in all its variations, spread from North America to the rest of the world. They covered Florida, Oklahoma, and Texas in great herds. They crossed the Bering land bridge—which then was not a narrow bridge, but a plain hundreds of miles wide—and spread all the way to the ends of Europe and the cape in Africa.
“You just find them everywhere,” said Rose. “There are places where there are just piles of them in Asia.”
Spend any time looking at the history of life in the last fifty million years and what becomes astounding is not only the innovations but the constant beat of catastrophe: volcanic explosions that blotted out the sun, meteor impacts that hit like a nuclear Armageddon, repeated ice ages. Continental collision. Climatic upheaval. Droughts and storms that likely eclipse what we have ever witnessed. And yet horses thrived.
This is significant because an awful lot of other life didn’t. Giant sloths, gone. Car-size armadillos, gone. Mammoths and mastodons and any number of other strange pachyderms, all gone. The Hyaenodons and Gastornis that preyed on horses, gone. Taking the long view, the story of life on Earth is mostly a story of death. Only one out of every thousand species that ever existed is still alive today. Bad things happen all the time. Five million years ago, North and South America reconnected for the first time since dinosaurs roamed the earth. Eight-foot-tall flightless birds called Phorusrhacidae, known informally as “terror birds,” invaded North American savannas. For two million years, they stalked small horses, hunting with axelike beaks and top speeds estimated at over fifty miles an hour. No doubt they ate their share of horsemeat. But horses are still here, and terror birds are not.
A threat even more fearsome than terror birds popped up around the same time that the two Americas joined. It was a new kind of grass. As the steamy jungles of the Eocene slowly ceded to open savannas, the grass that took over was almost all of one type called C3 grass, named for the type of photosynthesis it used. Starting about seven million years ago, a type of grass known as C4 started to spread. C4 had a more efficient recipe for photosynthesis, so it could grow faster, especially in hot, dry areas where growing seasons were short. But it had another advantage. C4 grasses could store five times as much abrasive silica as C3. It was a built-in defense mechanism because it was too gritty for most animals to chew. But not horses. As the abundance of C4 grasses increased, so did the length of the horses’ teeth. Paleontologists have found that horses were some of the first large herbivores to be able to stomach this new food. Herbivores that couldn’t adapt either died out or evolved to find sources of C3 grasses and shrubs.
It was with this last environmental change that many of the horse ancestors disappeared and the modern horse genus, Equus, emerged in North America, somewhere between three and six million years ago. What did Equus look like? A number of modern examples are still around us: zebras, wild asses in Africa, and a donkeylike resident of the Mongolian steppes called the onager. As for the true wild horse before it was domesticated, forget the image of the lithe Thoroughbred. The best indication we have of their shape and size is from paintings in the caves of southern France. Here, between thirty and fifteen thousand years ago, Paleolithic hunters recorded the shapes and colors of the animals they saw. They were stocky with big heads, short legs, and big, barrel-shaped bodies. They had short manes like those on zebras. They were mostly dun-colored, though some had stripes at the base of the neck and some had spots. For a long time, most paleontologists thought the paintings on the cave walls were just doodles that didn’t show what ancient horses looked like, but a 2011 study examining ancient horse DNA found that the paintings were probably right. Ancient horses, like horses today, varied in color.3
Some wild horses today still have remnants of stripes on their legs and necks.
At the end of our day collecting fossils in Bighorn Basin, Professor Rose had found dozens of jaws, and I had managed to spot three. Our finds were all small enough to fit in Rose’s pocket. “That’s the nice thing about Eocene mammal bones,” he said as we walked back to the Suburban. “Not much to carry.”
Back at camp, I washed off the dust and sweat in a small, plastic tub. After dinner, we gathered at the table in an old RV that Rose uses as his field laboratory, and the students dumped out what Rose called their “goodies”—the day’s haul of fossils.
“A lot of what we find doesn’t have much significance on its own,” Rose said. “It’s only when we get it back in the lab and really compare it with everything else we’ve collected that we can see how things started to change.”
I was so spent from a day in the dry heat that I couldn’t bear thinking of the team going out day after day, but the slow shuffle in search of the past had left an impression on me. The transformations and tribulations the horse went through, from the jaw I had held in my hand to the wild mustangs we saw on the horizon, were so revolutionary that the change from wild to domestic and then back again seemed minor.
After what I had learned, I felt that the horse deserved to be counted as a native in North America. It had developed here and lived here for more than about fifty-five million years. It had disappeared ten thousand years ago, but then it had returned to its home range when Europeans arrived. Since then, it has thrived, sometimes despite our best efforts.
If the fifty-five-million-year history of horses in North America were condensed into a day, horses would be a native species right up until they became regionally extinct at 11:59:43 p.m. When they returned in the last second of the day, why did they no longer belong?
There was one more part of the story I wanted to understand: the disappearance. Why, after fifty-five million years, had the horse gone AWOL, and what effect did that have on any claims to being a native species?
To try to answer that, I headed to the University of Colorado in Boulder, where a drawer in the offices of anthropologist Douglas Bamforth held some promising evidence.
The office, in a castlelike, 130-year-old stone science building that is one of the oldest on campus, had grand oak specimen drawers and tall windows to let in natural light. In that setting, I half expected Bamforth to arrive with a herringbone jacket and a pipe clenched in his teeth. But Boulder has become a casual place, and he arrived in jeans and a Patagonia pullover, having just come from the gym.
As the local anthropologist, he told me, he gets a lot of calls from the public. “Pretty regularly anyone who finds a triangular rock will come in and ask if it’s an arrowhead,” he said with a long-suffering sigh.
One of those calls came in on an afternoon in 2008 from a doctor who was having a koi pond built in his backyard at the edge of town. “I went up there to check it out, a bit reluctantly,” Bamforth said. “And what I found was really astonishing.”
Workers digging a shallow pit had found more than eighty stone tools—scrapers, cutters, and spear points that dated to about thirteen thousand years ago. Some of them were nearly the size of a dinner plate and exquisitely crafted to almond-shaped points with delicate serrated edges made by carefully flaking both sides with precise hits from a stone. These were not just arrowheads—they were rare Clovis points dating back to the period when the first humans arrived in North America. This was also the time when the horses disappeared.
The name Clovis comes from an archaeological site near Clovis, New Mexico, where the first points from the era were found amid mammoth bones in 1929. Since then, Clovis sites have been uncovered in all corners of the Americas, and they are almost always associated with the hunting of huge mammals. Though much about the Clovis people is still unknown, the leading theory is that they came across the land bridge from Asia about fifteen thousand years ago, just as the glaciers from the last ice age were retreating. Everywhere they went, they left large, exquisitely crafted, stone spear points. Within a thousand years, these small bands of hunters had reached the tip of South America, a spread that one anthropologist described as “an unprecedented rate of diffusion not seen again until the invention of the hula-hoop.”4
All evidence suggests Clovis hunters were an even more disastrous introduction for North American fauna than the South American terror birds had been. Within a few thousand years of their arrival, more than thirty large animal species had disappeared from North America, an event known as the Quaternary Extinction. Mammoths, giant sloths, giant beavers, saber-toothed cats—all disappeared. There has been a great deal of debate among scientists as to whether the extinction was caused by hunters, climate change, or both. There is still no consensus, but most think Clovis played a big role.
Professor Bamforth picked up the biggest, most striking stone tool from the drawer, a broad, almond-shaped point so big it filled both of his hands.
It was made of a honey-colored stone with the soft shine of hard toffee, and expertly flaked edges still looked sharp, more than 10,000 years later.
“This stone isn’t from around here,” he said. “None of this stone is. It comes from hundreds of miles away. What that shows us is that these hunters were highly mobile, highly skilled, and knew the land well.”
In 2009, with the permission of the doctor who uncovered the points, Bamforth sent the stone tools to a lab in California to see if any residue containing DNA could be recovered. It was a long shot—the tools had been buried for hundreds of generations—but it was worth a try.
The results came back positive. Three of the tools had ancient DNA. One was of camel, one was of sheep, and one was of horse. Horse DNA has been found at numerous other Clovis sites since then. Horse bones have been found piled next to Clovis hearths. There is no evidence Clovis people dined on horses alone, but evidence does suggest they dined on horses a lot.
So, does that change whether the horse is a native species? If we classify nonnatives as species introduced through the interference of man, how do we adjust when a species may have been wiped out by man? In modern times, we have reintroduced countless species that have been regionally wiped out by men—elk, bighorn sheep, buffalo, mountain goats, condors, falcons. Does it matter if their absence is measured in millennia instead of decades?
Maybe it does matter. If an animal has been gone so long that the rest of the ecosystem has moved on, and left no room, then perhaps the case can be made that a species is no longer a native. I asked Bamforth about this. Are the grasslands of the West much different today than they were ten thousand years ago? “It was cooler then, and drier,” he said. “But no, the species are basically the same.”
Evolution can have a long memory. The North American pronghorn evolved to outrun a North American cheetahlike cat that was wiped out when Clovis points showed up. The pronghorn can still run at sixty miles per hour, even though it hasn’t needed to for millennia. Maybe the horse and the western grass are the same way: After developing together for so long, they still remember in their DNA what it was like to live together.
The Bureau of Land Management, many ranchers, and even some conservationists argue that because the wild horse was introduced, it does not fit in the ecosystem here. It is therefore a damaging invasive and must be tightly controlled. That is in part why roundups have continued for decades. That is why we have fifty thousand horses in storage.
Any argument against counting the horse as native is biologically naive. Spend time with paleontologists and you learn to take a longer view. They don’t use the word native. Nor do they talk about a delicate balance in the ecosystem. All they see is upheaval and change. Nothing lasts. From this point of view, it’s hard to give a concept like being “native” much value. Everything that was native to the horse has been lost and remade repeatedly in its evolution, including the horse itself. What matters is what lasts. Through its own roundabout history, the horse is still here.