A New History of Life (47 page)

On land, the dinosaurs had ruled for so long that with their passing, a whole set of new ecological relationships among the survivors had to be rather quickly worked out. And with the sudden absence of so many land animals, the evolutionary faucet of new species formation opened with one of the greatest gushers of diversity that the world has ever seen. The mammals, obviously, were the big winners on land, but birds made a comeback as well, and for some time competed with land mammals for various resources.

So it was into this ocean regime that the rock from space dropped. The great climate effects reverberated through the ecosystems for thousands of years, and there was great climatic instability added to the already slightly cooled world, both on land and in the sea. The biotic changes were no less devastating. For one thing, the disappearance of
the dinosaurs led to denser forests. Just as modern-day elephants go a long way toward maintaining open spaces in forests by their movement and destructive eating habits, so too the dinosaurs, of far greater size, must have really affected vegetation patterns. But with their sudden disappearance the forests thickened up; it was as if some exacting gardener suddenly walked off the job, letting long-tended and pruned trees run riot.

By the Late Paleocene, more than 7 million years after the catastrophic K-T extinction, global climate had stabilized. The planet had slowly warmed to produce globally warm temperatures. From oxygen isotope evidence we know that equatorial surface waters of the oceans were in excess of 20°C, reaching as much as 26°C in some places, and thus were quite similar to ocean temperatures in similar latitudes today. But the big difference to our world occurred at higher latitudes. In the Arctic and Antarctic the surface of the sea was between 10°C and 12°C, compared to the near-freezing temperatures of our time. Thus, the difference in heat from equator and pole was some 10°C to 15°C, which is about half of what it is today. Nevertheless, in spite of these temperature differences, the oceanic circulation patterns were fairly similar to those of today. Most important, oxygenated water masses that ultimately would end up on the bottom of the ocean formed at high-latitude sites, just as they do today.

After the K-T mass extinction of 65 million years ago, it took some millions of years for the surviving mammals to grow large enough to start affecting plant patterns. There have been many artistic images of tiny, rat-sized mammals crawling from bomb-shelter-like burrows in a world of stinking, rotting dinosaur corpses. For some months, those mammals that could eat carrion would have been in Nirvana. But soon enough there were but bones, and even these rotted away or were buried in fairly short order, forcing all of the mammals to strike out in a newly organizing series of food webs that were unprecedented. It was before the time of grass, so the herbivores of early Paleocene time were leaf or fruit eaters rather than grazers. Seemingly, there were few leaf eaters at all. Most of the teeth of the Paleocene mammals argue for a diet of insects, fruit, or soft shoots rather than tougher leaves;
others may have been root or tuber eaters. It was only in the latter half of the epoch that tooth morphology appropriate for eating leaves appeared in any number. But once opened, the evolutionary faucet fairly spewed out new kinds of mammals, ever-larger mammals among the new in a torrent of evolution. Then, only 9 million years after the great K-T mass extinction, once again the biotic world was affected by environmental crisis.

By the early Cenozoic era, the Earth had suffered through at least nine mass extinctions that we know of: the first was the great oxygenation event and the snowball it triggered, the second more than one billion years later was during the Cryogenian, then, in order, the late Ediacaran, late Cambrian, late Ordovician, late Devonian, late Permian, late Triassic, and late Cretaceous mass extinctions. The causes were amazingly varied: from sudden oxygen to too little of it; from the appearance of predators to the onset of anoxia coupled with hydrogen sulfide emissions to asteroid impact. But at the end of the Paleocene epoch, only 9 million years after the dinosaurs died out, there was to be a new assassin: methane, which precipitated one of the most rapid rises in global temperatures known. It is called the PETM: the Paleocene-Eocene thermal event.

This event was first discovered by oceanographers
⁸
who were not at all looking for any sort of temperature event of late Paleocene age. They were trying to get new data on the K-T mass extinction from deep-sea cores drilled by the US Ocean Drilling Program (ODP). But to drill down into the Cretaceous, the drills first had to pass through Eocene and then Paleocene sediment. Cores were taken from those depths while the drills went ever deeper toward their real quarry.

When these younger cores were eventually examined and measured for the carbon and oxygen isotope found in the shells of tiny, single-celled protists known as benthic Foraminifera, the registered temperatures, as well as the ration of carbon 12 to 13, looked like they had to be in error: they showed that when a series of cores were
compared, those with strata pulled up from ancient, deeper-water parts of the ocean showed warmer paleotemperatures than those from shallower paleolocalities. Even in the frigid Antarctic today, water cools with depth, and back in the surely much warmer Paleocene, deeper water should be obviously colder than shallower. But the numbers here said just the opposite. Warm, deep waters and cool, shallow waters. Over a relatively short period of time the deep ocean had anomalously warmed.

Near the Paleocene-Eocene boundary there is a striking increase in global volcanic ash.
⁹
Like dust, this fine material makes its way to the seafloor from the atmosphere, but is put up there by volcanic eruption, not atmospheric storms. This increase could only be due to a sudden increase in global volcanic activity, about 58 to 56 million years ago. Further work in many places around the globe confirmed these findings as being global phenomena, not anomalous events limited to one ocean basin.

The late Paleocene tropics remained about the same (hot) temperature, but the Arctic and Antarctic regions warmed markedly. In the Paleocene the difference in seawater temperature between equator and pole was a hefty 17°C (it is an even heftier 22°C now). By early Eocene times, however, the difference had shrunk to only 6°C. And as the high latitudes warmed, the heat exchange between the two regions slowed, reducing both the number and ferocity of storms. The world went calm and got very hot, just as it did so many times before. This was yet another greenhouse mass extinction.

The carbon isotope record across the Paleocene-Eocene boundary in two cores also yielded a surprise. They showed a short-lived negative excursion—the kind of record that occurs when the amount of plant life is reduced—a hallmark of mass extinction. Other paleontologists began looking at the survival record of bottom dwellers from the region, looking specifically at the common benthic, or bottom-dwelling, Foraminifera—and found evidence of a catastrophic mass extinction on the bottom. Was it simply sudden warming of the deep that wiped out the cold-adapted species in short order? These results were published in the early 1990s, and soon after a Japanese
paleontologist named K. Kaiho published studies inferring that the fate of the benthic forms was decided not by rising temperature in the great depths of the sea, but by falling oxygen levels on the bottom. This made a lot of intuitive sense, for warm water can often become eutrophic and oxygen poor.

A deep bottom warming and a lowering of bottom oxygen, even a warming of the surface waters. What was the ultimate cause? The K-T asteroid impact event caused sufficient havoc in shallow waters to kill off almost all of the surface and upper water column plankton, but left the deep relatively unscathed but for the loss of nutrients from above. Warming the deepest part of the ocean could conceivably happen if the large parts of the sea bottom quickly warmed, but this would require an entirely new kind of deep ocean volcanism. The sea bottom does have areas of high heat flow, but these are confined to the relatively narrow mid-ocean mountain chains where seafloor spreading—the ocean bottom growth phase of plate tectonics—takes place. Even much faster plate movement by increased rate of volcanism along these mid-ocean rift systems would not do the trick. It was correctly surmised that the entire warm bottom had come from the warm, tropical surface waters where evaporation would make the surface waters saltier and denser. This warm and saline water was then transported along the sea bottom, even as far as the cold, high-latitude sites of Paleocene age.

Some aspect of ocean currents and the normal export of cold, oxygenated surface water down onto the deep-sea bottoms was not working in the Paleocene ocean. The deep, thermohaline circulation system—the main way that the ocean stays mixed—was just the opposite of how such currents work in our current ocean. The first victims were the tiny organisms requiring oxygen, the benthic forams of the deep sea. Many of these species died out, and did so relatively quickly in an event that lasted about four hundred thousand years. Still, to count as a mass extinction at all, it would have to be shown that it was not just the ocean that was affected, but land fauna as well. So the search was on for events on land.

Wholesale changes among oceanic organisms because of this greenhouse event occurred on the land as well.
¹⁰
The newly discovered
extinction in the deep sea stimulated paleontologists to look anew (and collect anew as well) at the fossil record of Paleocene land animals, to see if there was an extinction on land at the end of the Paleocene epoch. It did not take long to see that a great turnover
had
occurred among the mammals. Accurate dating soon showed that the extinctions on land and sea took place simultaneously.

In terms of the fossil record on land, the event itself seemed to mark nothing less than the start of our modern-day mammalian fauna. While there were numerous kinds of mammals by the latter part of the Paleocene (thirty distinct families are recognized from the collected fossils), many of these were small, and some belonged to groups no longer present, including survivors of small and rodent-like forms, many kinds of marsupials, some raccoon-like ungulates (a strange paradox, having the new entirely herbivorous ungulates taking on a meat-eating role in the Paleocene). There were also true insectivores and the first primates (like the insectivores, still at small size). But by late Paleocene time there were larger forms as well, and some of these were truly bizarre.

Dog- to bison-sized forms called pantodonts were leaf eaters that branched out into living a semiaquatic lifestyle like hippos, or living in trees, as well as having larger forms moving about on all fours on the forest floors. In general they were stout of body, with short legs, and one cannot help but surmise that, at least compared to modern herbivores, they were very clumsy and inelegant walkers. Yet large as they were, by the end of the Paleocene they were joined by even larger herbivores, the giant Dinocerata, which looked like huge rhinos even to the strange sets of knobs and horns on their skulls.

In the piles of strata marking the transition from Paleocene to Eocene, a reduction of species occurs, and over time—not instantly—new kinds of bones appear. Many come from kinds more familiar to us. The first even- and odd-toed ungulates appeared; more modern carnivores related to current groups soon evolved to eat the new herbivores, and all had to take into account an event that changed the very climate of the world. The lesson from past mass extinctions is that new occurrences would not have evolved as they did unless substantial
extinction had opened the door to the possibility of new morphologies. This too happened at the end of the Paleocene.

Our colleague Francesca McInerney has given us a wonderful summary based on her work in the North American West that can help us describe the PETM. First, she noted that this event is highly relevant to us humans, as the amount of carbon released into the atmosphere, about 12,000 to 15,000 gigatons, is roughly equivalent to what we humans are releasing over time by our industries and energy use. The temperature change caused elevated greenhouse gases during the PETM made the world 5 to 9 centigrade warmer than it is now. The actual event lasted on the order of 10,000 years. Plants before and after were different from those during the event, when all the gymnosperms, the pines and their kind, disappeared. The plants that were present in her field area, as discovered by paleobotanist Scott Wing of the Smithsonian, were mainly plants that until the PETM lived in lower latitudes and thus at higher temperatures. After the event the old plants came back, as did the insects that were present prior to the 10,000 years of literal hell on Earth. But not so the mammals. This event caused a wholesale change in the North American mammalian fauna.

A final note. Had there been large ice sheets such as we have today they would have rapidly melted. That causes sea level to rise. In our view this is the single most dangerous aspect of human-caused warming: we are melting Antarctic and Greenland ice that will over the coming centuries inundate huge areas of current human farmland. The highest known rate of sea level rise is currently on the south China coast, one of the most heavily populated areas on Earth with sea-level rice farms.

From the Eocene to the start of the 23.5–5.3-million-year-ago Miocene epoch, the world slowly began to cool. At first, during the Eocene, this was almost imperceptible, and in fact there was a global tropical forest
with crocodiles living inside the present-day Arctic Circle. But in the Oligocene this cooling accelerated, creating a different kind of major climate, and changing what had been a near uniform global climate to one with extreme seasonality. At the same time, giant continental ice sheets began to form on Antarctica, and probably Greenland as well. These swelling ice sheets caused a rapid and dramatic fall in sea level. At higher latitudes, forests gradually gave way in many places to grassland meadows and savannas. But other changes were taking place as well, changes in the atmosphere that would prove to have enormous consequences to the history of life.