IT’S HARD TO BELIEVE that crocodiles used to swim in warm waters off Greenland, or that our primate ancestors once chattered among broad palm fronds in a tropical forest within northern Wyoming. But scientists paint exactly this picture of the world circa 55 million years ago in the Eocene epoch, just 10 million years after the demise of the dinosaurs. During this time, the planet heated up in one of the most rapid and extreme global warming events recorded in geologic history. Sea surface temperatures on Earth rose almost 15 degrees Fahrenheit over a period of a few thousand years—a mere instant in the geologic timescale. And according to geologists Jim Zachos and Paul Koch of the University of California in Santa Cruz, this temperature spike triggered a wholesale reshuffling of life on the planet.

Zachos and Koch study chemical clues in the ocean and land, searching for evidence of how past climate changes altered Earth’s ecosystems. Zachos has connected the Eocene heat wave to drastic changes in ocean chemistry that caused a massive die-off of marine microorganisms worldwide. On land, Koch and his colleagues discovered that the global warming spike brought many newly evolved mammals to North America. Oddly enough, the mammals were strikingly smaller than both their ancestors and descendants. The first horse that whinnied in the subtropical forests of Wyoming, for instance, was the size of a modern Siamese cat.

What’s more, based on fossils recently unearthed in Asia, the researchers’ work has recently provided the first substantive evidence for a controversial theory of where modern mammals came from: Animals living in the hot Eocene world took advantage of warming northern latitudes to make their way from Asia to North America and Europe. These dwarfed creatures went on to evolve into the most common mammals on the planet today, Koch says.

In light of such discoveries, many earth scientists believe that the state of the planet at the beginning of the Eocene era could hold lessons for Earth’s future. With greenhouse gases locking in the sun’s warmth and global temperatures rising, the planet is heating up at least as fast as it did 55 million years ago, say Zachos and Koch. If global warming continues at its current rate, they speculate that future generations may well see a similar major impact on land and ocean ecosystems.

SIXTY-FIVE million years ago, at the end of the Cretaceous period, an asteroid impact brought an end to the dinosaurs and the Age of Reptiles. The fossil record indicates that mammals, which had lived in obscurity in the shadows of the dinosaurs, flourished soon after the giant reptiles died out. With no large competition for resources, little critters suddenly had a fighting chance to dominate the world. These first mammals were strange shrewlike creatures with sharp, jagged teeth. But by 55 million years ago, some of these diminutive beasts had grown to 6 feet tall or greater. Though bigger, these were still primitive mammals with short, thick limbs, clumsy feet and hands, and simple teeth capable only of easy maneuvers, like tearing. At the beginning of the Eocene, however, several new mammal groups arrived on the scene, bearing modern features like long, thin legs, feet and hands capable of grasping, and advanced teeth adapted for chewing. We recognize these new animals’ direct descendants today—in horses, camels, sheep, cows, and humans.

To zero in on how quickly this evolutionary transition occurred, paleontologists in the early 1900s turned to layers of sediment in northern Wyoming’s Bighorn Basin. The basin hosts a dazzling array of plant and animal remains from around 62 to 52 million years ago that were preserved as sediments slowly filled an ancient river valley. Over 150 years of fossil collection from the site reveals evidence of a warm Eocene world with tropical animal and plant life, like crocodiles and palm trees.

But something changed in the Wyoming habitat, dramatically altering its inhabitants. The mystery of what caused the transition to modern mammals was impenetrable—until pieces of the puzzle were pulled out of the earth in other parts of the world.

The modern oceans hold one of the crucial keys to understanding what had happened long ago. Until the 1960s, little was known about the record of Earth’s history stored in seafloor sediments. Then, scientists began probing ocean basins and studying the cores of sediment layers they pulled out. In 1990, a literally groundbreaking analysis of marine sediments showed that the Antarctic Ocean actually heated up, a lot and quickly, in the Eocene.

In an article published in the journal Nature, marine scientists James Kennett and Lowell Stott, both then at the University of California in Santa Barbara, reported that not only had the surface of the Antarctic ocean heated up about 20 degrees, but the entire depth of the ocean had warmed, changing its chemistry. The warming coincided with an extinction of almost 40 percent of microorganisms that lived in deep ocean waters, Kennett and Stott wrote.

Zachos, who was doing postdoctoral work at the University of Michigan at the time, reviewed the then-new Kennett and Stott paper. He remembers his reaction: “It was unlike anything I’d seen before.” Even after the asteroid crash that wiped out the dinosaurs, says Zachos, the ocean responded less drastically; only surface water chemistry changed. But whatever caused the Eocene warming altered the chemistry of the entire ocean, top to bottom. Zachos decided to investigate, and he’s still at it 13 years later, puzzling out the cause of the global warming.

THE TRICK TO tracking the chemistry of ocean water lies in studying two forms of carbon, called isotopes, which have slightly different weights. Most of the carbon in the carbon dioxide we breathe is dubbed 12C, or “light” carbon. It is the most abundant carbon isotope in water, air, and plants. A tiny portion of all carbon in nature is the slightly heavier isotope, 13C. Carbon dioxide dissolved in magma and fossil fuels like methane has a distinctively low amount of the heavy isotope.

Scientists examine ancient sediments and fossils for the relative amounts of heavy and light carbon they hold, in order to figure out where gases in the air and the ocean came from at different times. Marine critters keep great carbon isotope records because they build their shells from the carbon in the water they live in. When these organisms die, their shells settle on the ocean floor, accumulating hundreds of feet of sand grain-–sized skeletal remains —a silent testimony to the environment the animals once lived in.

Kennett and Stott found a sharp decrease in the amount of heavy carbon in 55-million-year-old marine fossils, a decline that caused the relative ratio of 13C to 12C to plunge. Most scientists agree that in order to drop the ratio so sharply, a gas with very low amounts of 13C must have literally flooded the atmosphere. Some researchers theorize that numerous volcanoes spewed carbon dioxide directly into the atmosphere.

But in 1995, Gerry Dickens, then a graduate student at the University of Michigan, instead argued that only methane gas had enough light carbon to produce the early Eocene plunge. He proposed that a belch of methane escaped from ice in seafloor sediments as the Earth warmed.

Zachos’ studies over the past dozen years support the methane-belch theory. Based on his own and colleagues’ recent work, Zachos calculated that up to 2 trillion tons of methane bubbled out of the oceans. Zachos and Dickens say that methane combined with oxygen in the air and water, forming carbon dioxide and essentially suffocating marine life. But whether volcanic activity or a methane belch was the culprit, the greenhouse gas locked in the sun’s warmth, sending global temperatures soaring.

Zachos and other experts of past climate change have studied, in excruciating detail, evidence of this heat spike, which they call the Initial Eocene Thermal Maximum. After the Kennett and Stott paper was published, recalls Zachos, “that basically set off a flurry of activity, with people running to all these existing outcrops and cores.” They found that the sharp drop in carbon isotopes is recorded in every ocean sediment core that scientists have collected dating to 55 million years ago. Ocean cores from seafloor locations as farflung as Blake’s Nose in the north Atlantic off Florida and the Kerguelen Plateau in the southern Indian Ocean all recorded the event. “It was almost within a year or two that we pretty much knew that this was a global signal,” Zachos says. “This was not something unique to one ocean.”

With the isotope record in hand, paleoclimatologists could link the extinction of seafloor-dwelling critters to increased temperatures in the ocean. “It was exciting: There was a connection that we could attribute to this global warming event,” says Zachos. Later, in 1999, researchers from Bremen University, in Germany, and the Woods Hole Oceanographic Institution, in Massachusetts, showed that the sharp drop in the carbon ratio took place in less than 10,000 years. On a geologic timescale, this shift is virtually instantaneous.

WHILE MARINE SCIENTISTS were busy looking for the carbon isotope signal in ocean cores, earth scientists searched for similar evidence in the land fossil record. Back in 1991, Zachos met Paul Koch, then a new graduate student in Michigan, and the two joined forces. “Paul and I came up with this idea to correlate the marine and terrestrial records using carbon isotopes,” recalls Zachos.

Shifts in ocean chemistry directly impact the land because the atmosphere acts as a big conduit, shuttling carbon between ocean and land habitats. On land, plants take up carbon from the air and soil. Animals incorporate that carbon into their teeth and bones through the plants or other animals that they eat. Decay of animals and plants returns the carbon to the soil.

To investigate the land record, Zachos and Koch went to the Bighorn Basin of Wyoming, 100 miles east of Yellowstone National Park. Fossilized plant parts tell us that 55 million years ago, Bighorn Basin was a hot, humid subtropical forest with a river snaking through it. Elm-like trees related to species living today in Panama, Texas, and Oklahoma shed pits on the ancient riverbed. Dawn redwoods, large sequoia related to those now found mainly in central California, sprouted from the fertile floodplains.

Today, the Bighorn Basin is a maze of bare, red-striped gray mudstone hills as far as the eye can see. Each stripe represents an interval of time, composed of sediment that cemented millions of years ago. “You can just see time laid out in front of you,” says Gabe Bowen, a graduate student working with Koch in Santa Cruz.

When Koch himself first set to picking apart the sediment layers ten years ago with Zachos and University of Michigan paleontologist Phil Gingerich, they sampled the stripes at 5-meter intervals. They collected preserved teeth and pieces of soil carbonate, in which Koch found the same drop in carbon isotope ratios recorded in marine fossils. “Organisms on land and in the oceans were responding to this climate change, like, boom, dramatically,” remarks Koch.

The land record revealed a few new crucial pieces of information absent in the marine record. First, according to Koch’s data, after heating up, the Earth remained warm for about 80,000 to 200,000 years. More importantly, digging within a 40-meter sediment stripe that marked this interval of global warming, paleontologists found dramatic changes in the animals living in Bighorn Basin at the dawn of the Eocene. Whole new orders of mammals—groups of closely related families of animals—appeared, including many families never before seen in North America.

Koch and Gingerich were astonished to find that the chemical change recorded in land sediments coincided with one of the most bizarre events in the fossil record: the dwarfing of early mammals. Based on fossil tooth size, paleontologists discovered that within the 40-meter layer representing the hottest temperatures of the early Eocene, animals were half the size of both the mammals that came before them and those that followed. “Before it, there are animals characteristic of the Paleocene. In the 40-meter [layer] there are strange, small animals. Above it, you find normal-sized animals again,” says Koch. “There’s this genealogical evolution that’s dropping forms on the landscape.” Animals’ weights, estimated from fossil tooth size, were 60 percent lower.

While paleontologists already knew that animals of the Eocene substantially differed from those of the preceding epoch, no one had been able to pinpoint exactly when the transition occurred, or why. The discovery of the carbon isotope shift in Bighorn Basin sediments is the first evidence to unequivocally correlate any stage of mammal evolution to climate change, according to Gingerich.

Why animal size shrank during the heat wave is anybody’s guess. Koch and Gingerich, among others, speculate that body size is related to temperature or food supply. Koch says that animals in warm climates tend to have smaller bodies. “Look at white-tailed deer from one end of their geographic range to the other,” says Koch. “There’s little tiny ones in Guatemala, there’s big honkin’ ones in Michigan. And this happens in a lot of different species.” Still, he says no one knows what the evolutionary mechanism behind the dwarfing is. Some have suggested lack of nutrition made the animals smaller. Plants may have been to blame.

According to Scott Wing, curator of paleobotany at the Smithsonian’s Natural History Museum in Washington, D.C., the fossil record shows that plants stayed put during the initial temperature spike. He thinks they forsook the opportunity to spread to new habitats in favor of soaking up the abundant carbon dioxide where they already lived. “It’s very odd that so little seems to have happened to plants at that time,” says Wing.

Plant resilience during the climate shift may have been bad news for animals, according to recent investigations by Wing and Gingerich. When plants take more carbon into their tissues, they produce less protein in their leaves, so their nutritional value for animals drops. Plants also use the extra carbon to produce more compounds that herbivores find hard to digest. As a result, the researchers speculate, animals grew more slowly.

Not only were early Eocene mammals remarkably small, they were also extremely successful. “These animals are basically the evolutionary roots of a huge radiation in the tree of life. They started whole new branches,” says Koch.“What’s interesting is that not only are there lots of first appearances [of mammals], but they’re all first appearances that are going to go on to do lots of business,” he says.

The Bighorn Basin research documents the introduction of three entirely new orders of mammals to North America—a major development, considering that there are only 21 orders in total today. The trio includes artiodactyls, or hoofed mammals with an even number of toes, whose direct descendants are cows, pigs, sheep, deer, and camels; perissodactyls, or hoofed animals with an odd number of toes, which gave rise to modern horses, rhinos, and tapirs; and perhaps most meaningful of all, because we are their direct descendants, the primates. Chris Beard, curator of paleontology at the Carnegie Museum of Natural History, agrees with Koch. “What happens right at the boundary, at least in North America, you get modern types of mammals. They’re still primitive, but at least they’re things we can place on a family tree: ancestral primates, horses and such.”

The standing theory is that these mammals were immigrants. Going into the early Eocene, the planet’s continents were arranged differently than they currently are; although they sat at roughly the same latitudes as today, continents were bunched closer together. The polar regions weren’t covered with ice, but they were still too cold for comfort for mammals. But as the globe warmed during the heat pulse 55 million years ago, researchers speculate, land animals that had been living in mid-latitudes migrated northward in search of cooler haunts. The poles warmed up too, enough to make them more inviting to wandering animals, who moved into northern latitudes. Europe was covered by a shallow sea, leaving only one link—the Bering land bridge—to fresh territory in North America.

Recently, Koch and his student Bowen, working with collaborators at the American Museum of Natural History in New York City and Louisiana State University, unearthed evidence supporting the controversial idea that creatures from Asia made their way across that bridge to North America, giving rise to the modern mammals. That theory was first proposed by 19th century paleontologists Roy Chapman Andrews and Henry Fairfield Osbourne, because they believed evolutionary innovation happened at high latitudes and then spread southward. The Eurasian continent, the biggest landmass positioned at such a high latitude during the Eocene, fit the bill.

Andrews and Chapman failed to find evidence for their theory. Beard, the Carnegie paleontologist, remarks, “Some people have parodied it as a Sherwin Williams model,” referring to the paint company’s logo that shows paint dripping down over a globe. Although scientists agree that the same mammals appeared in Asia and North America around the same time, there was no way to know where the creatures had evolved first.

The new findings by Koch’s group and his collaborators shed light on that question. They’ve detected the same initial Eocene carbon isotope signature in fossil soils from China and Mongolia. And from their excavations, they discovered that at least one dwarfed animal type—a creodont, a now-extinct carnivorous, hoofed animal with an odd number of toes—that first showed up 55 million years ago in Bighorn Basin also appeared earlier in two different parts of Mongolia. This work suggests that hoofed, odd-number toed creatures existed in Asia at or before the beginning of the Eocene, says Koch.

Beard says the findings confirm that Asia was the birthplace of modern mammals. “The climate warmed, and that allowed all these animals that had evolved in Asia … to leave Asia and basically take over the world,” he says. But other researchers urge caution. “I think the truth is we don’t exactly know where most of these animals are coming from,” says Gingerich. “The only thing we know for sure about them is that they are coming from somewhere else.”

Koch is now busy in Asia trying to find more predecessors of the North American Eocene mammals. He is also trying to link climate change to evolution during other time periods, using fossil teeth and soil minerals to figure out past rainfall and air temperature, and animal diet and migration patterns. Meanwhile, Zachos just returned from a two-month ocean expedition, sampling ancient sediments off Venezuela from the JOIDES Resolution, an ocean drilling vessel. Zachos was looking not only for more evidence of the global impact of warming 55 millions years ago, but also for its cause. He and his shipboard colleagues believe they found more evidence of a giant methane burp at the start of the Eocene.

WHY LIFE ON EARTH would respond so dramatically to climate change remains unclear, but the planet’s unique qualities provide some clues. Earth, as far as we know, is the only place in the universe supporting complex life forms. Water may have once flowed on other planets, and some planets may even host simple organisms, but only Earth has been able to maintain the delicate balance of air, mineral, and water chemistry vital to all living beings. When this balance is disturbed, the consequences can be major.

Zachos and many of his colleagues theorize that the Earth’s temperature can change slowly, incrementally, with no visible impact—to a limit. They call that limit a climate threshold, and once it is crossed there is no going back. “Several of us suspect that the … rapid release of methane was initiated by gradual warming that pushed the climate system across a physical threshold,” Zachos says. According to Zachos, crossing the threshold could mean facing a climate system gone haywire. With air and water temperatures and pressures and ocean salinity all playing roles on the climate stage, it is hard to pinpoint when and where that line is crossed. But Zachos thinks the climate system begins to crumble when polar regions warm up.

The ocean is much like air when it comes to circulation. As blowing winds stir up air, swirling water currents stir up the seas. Water on the surface of the North Atlantic warms, shimmies north and south to the poles, cools and sinks. But if polar waters warm, too, the relocated water would fail to cool, and would remain on the surface. The pockets of water that once warmed and cooled, rose and sank, would stop flowing. Ocean circulation would slow down.

Imagine stagnant, humid, unmoving air: It’s hard to breathe. That may be what oceans were like to their resident organisms once the warming took hold. Zachos thinks that an extra-warm push at the poles was enough to cross the climate threshold, which in turn triggered a bizarre response in land and sea creatures.

And it could well happen again, he warns. Up to 2 trillion tons of greenhouse gas were released into the atmosphere 55 million years ago. Today we pump 7 billion tons into the air each year from the burning of fossil fuels alone. As a result, Zachos points out, the carbon isotopic signature of ocean surfaces today has already begun to shift. Since the same kind of shift was the precursor to upheavals in the planet’s ecosystems in the past, the current trend might foreshadow similar changes in the future. But according to Gingerich, climate lessons from the past are not all doom and gloom. “The good news coming from Wyoming is that the Earth’s biota worked its way out of it,” he says. “The bad news is that it took about 80,000 to 200,000 years.”