And yet, of all these gathered bones, almost none come from Epihippus, the mysterious horse of the late-middle Eocene, the horse that could have marked not the end of the beginning of horses—but the end of horses forever.
Epihippus fossils are as rare as early Eocene horse fossils are plentiful. I picked up a tooth, only a few millimeters in size. I mulled over what Mihlbachler had said. Had Epihippus become extinct, there would have been no Vogelherd horse, no horses to carry people over the North American sea of grass, no horses to bring Genghis Khan and his men from the far end of Asia to the gates of Vienna, no warhorses, no plow horses, no cow ponies or wild horses. We humans would be living a much lonelier life. Not to mention what might have happened to civilization itself.
“So this was it? Hanging on by a thread?” I asked.
Little Epihippus lived from about 46 million to 38 million years ago in North America. He turns up in the American West in a rock stratum called the Uinta Formation. Despite the countless numbers of horses who had covered the land during the early Eocene, toward the end of Epihippus’s time on Earth, horses seem to have become rare.
“We have drawer upon drawer of Uinta fossils in this museum, but that’s all we have of Epihippus,” Mihlbachler said. “It’s hard to know why. They may have been superabundant in Ohio, but we don’t have fossils in Ohio. They may have been supersuccessful somewhere else, but at least where we have fossils, Epihippus is just not common.”
I had read about Epihippus once before, in Tim Flannery’s wonderful epic of North America, The Eternal Frontier. “One wonders,” the author had written, “how different human history would be if Epihippus had not made its knife-edge escape from extinction.”
The basic problem appears to lie in the fact that early horses had not yet developed their evolutionary facility for adaptation. They were well suited to their heavenly Eocene hothouse. The foliage was lush. Food was abundant and available year-round. What’s not to like?
But nothing lasts forever. By the time of Epihippus, the planet was once again on the brink of major change. The warmth was about to disappear. In its place would come a colder and much more challenging world.
For what may be the first time in Earth’s history, “seasonality” would begin. All life would have to adapt to this shift. The year-round empyrean feast of ripe fruits and tender buds ended. For many animals, finding food would become burdensome. Birds would have to learn to fly between the Northern and Southern Hemispheres twice a year. Whales would have to learn how to migrate through the world’s oceans on a seasonal basis. Bears would have to learn how to hibernate. Browsing mammals would have to learn how to stock up on fat during the warm season to carry them through the cold winters.
* * *
It’s hard for us to imagine in our twenty-first-century world, where we accept the cruel reality of sweltering summers and freezing winters, but for a good deal of Earth’s history, including most of the Eocene, the planet enjoyed fairly uniform temperatures. For example, during the Eocene, the world north of the Arctic Circle was so warm that crocodiles flourished there. There were no ice caps, of course, and so much freshwater flowed into the Arctic Ocean that a layer of freshwater sat like a lens over the salt water. The freshwater Azolla fern was plentiful. Forests of redwoods and walnut trees grew there. Paleontologists have found the remains of giant ants usually associated with the tropics.
Of course, the planet was still tilted and the poles still experienced long nights and long days during the solstice periods, but the warm environment was apparently so alluring that the animals made peace with the dark periods and lived there throughout the year. Fossils of primates and tapirs have turned up from this period in the Arctic rocks, along with snakes and alligators and turtles galore. Curiously, though, no early horse fossils have yet been found.
Then, toward the end of the Eocene, that frenzy of animal life was challenged. Their Elysian lifestyle came to a rather abrupt end. Consider that the fabulous and whimsical brontotheres, kissing cousins to the dawn horses, had ranged almost worldwide during the Eocene. Like the dawn horses, they had four toes on their front feet and three on their back feet. But unlike the dawn horses, they had taken great evolutionary risks. The first brontotheres were small, but then they exploded in size. Some grew to enormous heights, as much as eight feet at the withers. They were highly experimental. This risk-taking would prove to be their undoing.
This presents an interesting mystery: brontotheres and dawn horses appeared on Earth at about the same time, as far as we can tell, and are closely related, but while the horses remained evolutionarily conservative, brontotheres tried every evolutionary trick in the book. For example, they fooled around a lot with what are commonly called “horns” on their heads. These protuberances sit at the end of the skull, like rhino horns, but they are not horns at all. Horns are made of keratin, like our fingernails. The “horns” of the brontotheres are actually extensions of the skull. (Many animals evolve such skull ornaments, but, as Mike Voorhies had said, horses never did.)
In the short run, the brontotheres’ evolutionary strategy of experimentation paid off: they were among the most diverse groups of land mammals on the planet, and by the end of the Eocene a few species were as large as modern elephants, making them the largest of land mammals then extant. At one point there were so many different species of brontotheres roaming the planet that Matthew Mihlbachler once wrote that trying to keep their names straight was a “dizzying” task.
But despite this proliferation, by the end of the epoch they were gone. Perhaps their strategy of becoming highly specialized meant that eventually they became too specialized. When their favorite ecosystems disappeared, it was as though they’d had the rug pulled out from under them.
Also failing to make the shift from a warm to a cold world were North American primates. Fortunately for us, though, by the end of the Eocene they’d made their way into the heart of Africa, where they survived.
Horses, too, were severely challenged. Used to a life of leisure, where the grapes were there for the taking, horses had to develop coping strategies. Their agile ability to scamper from one hiding place to another underneath the copious canopy would no longer suffice as North America’s junglelike ecosystems evolved into more-open woodlands. In place of agility, the horses would need to opt for even greater speed as a defense. Even more important, their teeth would have to be able to grind rather than smoosh, as horses resigned themselves to eating much tougher forage.
Happily, just in the nick of time, just before the final, catastrophic end of the Eocene, a new and different horse appeared. Mesohippus was taller, faster, had only three toes on his front feet, and had teeth that were larger and flatter, with greater surface area. The improved ability to grind food was a great advantage.
And then, soon after Mesohippus came Miohippus, an extra-horsepower horse, even bigger and even tougher. Miohippus was a horse that most of us, at last, would recognize as a horse. He still had three toes, but his middle toe was clearly the weight-bearing toe. His face and skull were horselike and his backbone, while not like the backbone of the modern horse, had straightened out enough to allow him to somewhat gallop, rather than only “scamper.”
After many millions of years of evolutionary cautiousness, horses were on the move. “Just in the nick of time” is not an exaggeration. World temperatures had been slowly dropping, but then the planet suddenly plunged into a deep freeze. The precipitous drop was every bit as abrupt as the early Eocene’s temperature increase. This sudden temperature change, which occurred about 34 million years ago, brought a cascade of consequences.
In Europe the cold caused an extinction event known as La Grande Coupure. “The Great Cut.” As I mentioned, Europe during the Eocene was an archipelago of islands. When the temperature dropped, newly formed glaciers froze colossal amounts of the planet’s water. Sea levels dropped and these islands became connected. The animals, long accustomed to the privacy and comfort of peaceful island lif
e—disappeared.
In their place a new group of animals arrived. This was one of the strange, inexplicable anomalies in the European rock record that caused Darwin’s emotional crises. There was a line of demarcation in the rock record. Below this line lie all the many different European Eocene mammals; above it are found very different animals—animals that had been common in Asia. This line is so clear and so obvious that early European paleontologists could see it, although they couldn’t explain it. Having no idea that the world had abruptly plunged into a deep cold spell, Charles Darwin along with other scientists agonized over the Coupure, which seemed to contradict his slow-and-steady thinking about evolutionary change.
Today, though, we have a pretty good idea of at least some of the details. The Coupure was a perfect-storm incident involving deep-sea phenomena and plate tectonics. Researchers postulate a one-two punch of events, one event that occurred gradually and another that occurred comparatively suddenly. First there seems to have been a general decrease in the atmospheric gases that kept the planet warm. Some paleogeologists suggest that the tectonic plate of India, long parted from Africa and slowly drifting north, collided with Asia and began forcing the rise of the Himalayas, which in turn slowly cooled the planet by depleting carbon and oxygen from the atmosphere. Areas thick with foliage, like Messel, were replaced by more-open areas.
Coupled with this was the final coup de grâce: the tectonic independence of Antarctica. Just as India had broken free from Africa and headed toward Asia, and just as the North American plate was separating from the European plate and opening up the Atlantic Ocean, Antarctica had been slowly separating from South America and from Australia. Finally, about 34 million years ago, Antarctica severed all physical contact with the other continents. It sat, by itself, right over the South Pole. Scientists suggest that this event had several profound effects.
First, Antarctica grew an ice cap and became a global Ice Queen who ruled the world with a frigid fist. So much water was tied up in the ice that Europe was no longer a series of isolated islands, but an expanse of above-sea-level land connected to Asia. As such, Europe was open for conquest. The Asian animals, better suited to the new conditions, rushed in.
North America, too, experienced changes. Sea levels dropped, exposing former seafloor to colonizing plants. Because so much water was tied up in glaciers and ice caps, expanses of grass appeared for the first time. Animals like Miohippus, capable of living on those season-dominated, drier, open plains, also appeared.
The planet’s ocean currents shifted. After Antarctica came to power and was comfortably ensconced in her South Pole castle, she was surrounded by a circumpolar current that acted like a moat. This current fed cold water into the world’s other ocean currents, changing their flow and bringing about a colder planet.
Chris Norris had said that horses had a unique story to tell, and it turned out that their story was one of adaptability—but an adaptability that depended on context. Horses were born into a silver-spoon existence, into a world where food was easily available and temperatures were comfortable. Think Porgy and Bess: “Summertime, and the livin’ is easy.”
Then the world changed, and the horses were tested and hardened. Fortunately, they did manage to evolve enough to hang on and pass into the chilly, challenging, brave new world of the Oligocene. Their unusual digestive systems and their longer legs helped them, but it was their teeth, according to Matthew Mihlbachler, that did the heavy lifting.
* * *
Mihlbachler once pulled more than seven thousand North American horse teeth out of a multitude of drawers and cabinets at the American Museum of Natural History, hoping to correlate horse evolution with climate change. He unlocked and relocked cabinet after cabinet.
“This is by far the world’s largest collection of fossil horses,” he said. “If you go from one end to the other of this floor, you’ll witness the entire evolution of horses. We opened up every drawer on the floor and looked at the upper teeth of every fossil horse in that collection.”
He is a patient man.
“We looked at the cusps to see if they were worn or flat. We got a huge spreadsheet going. We also went to Yale. There’s a few gaps in our collection that Yale fills nicely. We took those results and we converted the cusp sharpness into a scale. We mapped all that tooth wear data through time and compared it with paleoclimate data.”
Mihlbachler found evidence that following the 34-million-year-old temperature drop, horses changed their diet by shifting from eating fruits to eating other forage—forage that gradually became more and more abrasive and that grew close to the ground, so that horses would ingest a lot of grit along with their grass. Some horse species had teeth that were able to cope with this shift. Those who did not, disappeared.
Those who survived had tougher teeth. If you’ve ever been to a beach and accidentally gotten a mouthful of sand, you’ll understand the problem that horses encountered when they began to graze rather than browse. Learning to eat grass—a formidable food that defended itself from predators like horses by incorporating more and more razor-sharp silica into its blades—was probably not a pleasant task. Silica is the same material from which glass is made. If you’ve ever walked through a field, picked a blade of grass, and cut yourself by running your finger along the fine edge of the blade, then you’ll understand the problem the horses faced. Your skin has been sliced by what’s essentially a sharply honed knife.
Silica can be dangerous stuff. We humans can chew on a blade or two of grass over the course of a day, but we would never be able to survive on the stuff. For horses, gaining the ability to eat grass was a supreme achievement. In his study of the seven thousand horse teeth, Mihlbachler saw that colder temperatures, plant shifts, dietary changes, changes in silica content, and changes in horse teeth were all correlated. It was as if horses and grasses were waging a fairly constant war.
Interestingly, other animals were not able to respond this way. Mihlbachler looked at the teeth of camels, which were also plentiful in North American rock layers. But camels, it seems, did not have the same talent. Their teeth did not change as much.
“Horses are really tracking global temperatures, according to their teeth,” he said. “The horses just kind of change with the times. I don’t know what it is about horses—their teeth, their digestive system—they are pretty successful through it all.”
In other words, once horses came through the evolutionary bottleneck represented by the scarce remains of Epihippus, they may have changed in some basic way.
“Horses are unique,” he said. “Something happened to them that allowed them to adapt to whatever was thrown at them. Whatever it was that happened to them, it allowed them to track their changing environments so closely that we can use their fossilized teeth to track the history of climate change over the long term.”
I had seen that modern wild horses were able to make small evolutionary changes that helped them adapt to where they lived, but I had no idea that this talent could be traced all the way back to a specific period in time, to a specific climate shift and to a specific tectonic event.
Forged in extreme warmth, then tempered by cold, horses managed to cross from the Eocene into the Oligocene as animals who had survived fire and ice and could withstand an almost unbelievable variety of conditions.
* * *
In another row of cabinets, Mihlbachler pulled out a drawer filled with ancient horse brains.
“These are Radinsky’s brains,” he said, with a certain reverence.
A leader in the field of paleontology, Leonard Radinsky died prematurely in 1985, saddening his colleagues so much that when one researcher found an early jaw of an animal in China that could have been a dawn horse ancestor, he named the fossil in the deceased scientist’s honor: Radinskya.
What Mihlbachler found in the drawer were not really brains, of course, but horse brain endocasts—casts of the cranial cavity, the part of the skull that houses the brain, of extinct animals. Endoc
asts come in three basic varieties—those that are themselves fossils, having formed naturally of sand and other minerals inside an ancient animal’s skull; those physically created by a research scientist who fills a fossil skull with composite material; and those that are virtual scans, made with modern computer technology. Some of the endocasts Mihlbachler found in the museum drawer were fossils Radinsky had worked with. Others were casts Radinsky had made.
Field researchers had known from early days about the existence of naturally formed endocasts. Workers had dutifully collected and cataloged them for future research along with fossilized bones, but not a lot happened with them research-wise for quite a while. The natural endocasts, often poorly formed, just didn’t seem to yield much information. Most scientists believed there was little to be gained by studying them.
Then Tilly Edinger, a German Jew associated with the Senckenberg Museum before World War II, was fortunate enough to emigrate after the horror of Kristallnacht in 1938. In Europe, Edinger had already begun to show that endocasts could provide usable information because they revealed some details about the exterior shape of the brain. When she came to the United States, she, like Thomas Henry Huxley before her, found what was lacking in European museums: a consistent series of horse fossils. Realizing that she could follow the evolution of the brains of horses over 56 million years, she spent much of the following decade writing a major scientific paper. In her seminal 1948 “Evolution of the Horse Brain,” Edinger asserted that even the evolution of the senses in horses could be traced, in a limited way, by studying the changes in endocasts. The result was the codification of an entirely new research field: paleoneurology.
The Horse Page 10