Mammals: A Tiny Beginning

If the Permian‑Triassic extinction had been even a little more lethal, mammals might never have come into existence. Only a few lineages of synapsids straggled into the Triassic period, and most of them grew rarer still as the dinosaurs became more successful. But one of these lineages continued to evolve the equipment necessary for life as a mammal.

These doglike synapsids, known as cynodonts, evolved a new sort of skeleton. They evolved rib cages that could house diaphragms, allowing them to breathe more deeply and develop more stamina. They also probably evolved hair at this point, and began to nurse their young. They probably secreted fluid from glands in their skin that their young could swallow. At first this milk may have been nothing more than a liquid antibiotic that helped their young fight infections. But over time, evolution added protein, fat, and other substances that helped mammals grow quickly.

All of these innovations helped the ancestors of mammals to do a better job keeping their metabolisms high and maintaining a constant body temperature. As a result they could occupy new ecological niches that cold‑blooded vertebrates could not–for example, they could hunt at night. Their high metabolism made it possible for some lineages to evolve to smaller sizes as well. (Smaller animals have a harder time holding on to body heat, because the ratio of skin surface to body mass determines how quickly heat is lost.)

These small protomammals had sharper senses than their ancestors, and to handle this new sensory flow they evolved a new rind around their brains. Known as the neocortex, this layer was dedicated to sorting out the influx of sounds, sights, and smells, turning them into intricate memories and using them to learn about their surroundings. The warm‑blooded mammals could make good use of the neocortex, because their high metabolism required a constant supply of fuel. A snake can eat a rat and relax for weeks, but mammals cannot go long without food. A big brain with a neocortex let mammals build a map of places to find food and remember it.

To us humans, so proud of our brains, this achievement seems like a milestone that should have instantly altered the course of evolution. But for the great synapsid dynasty, it didn’t matter much. Synapsids only barely recovered from the Permian‑Triassic extinction before sliding down toward extinction again by the end of the Triassic.

“We think of mammalness as the superior way to be,” says Ward. “It wasn’t. Dinosaurs in head‑to‑head competition won out. They took over the world. We talk about an age of dinosaurs–well, they wrested it away from mammals.”

For 150 million years dinosaurs were the most diverse land vertebrates. Among their ranks evolved the biggest animals ever to walk on land. In 1999 researchers found pieces of the backbone of a long‑necked dinosaur in Oklahoma that they named Sauroposeidon. Judging from the size of the vertebrae, the paleontologists estimate that it stood six stories high. Sauroposeidon could have crushed the biggest mammals of the Mesozoic like pinecones. None of those early mammals managed to tip the scales at 5 pounds; paleontologists who hunt for them may sieve a ton of rock and find a single tooth the size of a pinhead.

“Mammals go way back–they’re as old as the dinosaurs,” says paleontologist Michael Novacek of the American Museum of Natural History. “But for about the first 160 million years, they’re not very dramatic animals. They really lived in the shadow of the dinosaurs, mostly small, possibly nocturnal creatures. They’re not very auspicious.”

Yet as inauspicious as they might have looked, mammals continued to evolve during the age of the dinosaurs. They branched into many different lineages, some still alive, some long extinct. The platypus belongs to the oldest lineage of mammals still alive today, known as monotremes. Monotremes still retain some of the characteristics of our own ancestors 160 million years ago. They have far less control over their body temperature than more recently evolved mammals. Female monotremes do not give birth to live young; instead, they lay soft‑shelled, pea‑sized eggs, which they carry in a slit on their belly. When the egg hatches, the infant nurses milk that oozes out of mammary glands. (Nipples had not yet evolved when monotremes branched off on their own.)

About 140 million years ago, mammal evolution produced two branches that would turn out to be the most successful of all. One was the marsupials, which include living animals such as the kangaroo, the opossum, and the koala. The male marsupial has a forked penis, which it uses to fertilize eggs in the female’s twin uteruses. A fertilized marsupial egg does not develop a shell; instead, the embryo develops for a few weeks until it is the size of a rice grain, and then it crawls out of the uterus. It makes its way into a pouch on its mother’s belly, where it clamps its jaws around a nipple.

The other lineage gave rise to our own sort of mammals, known as the placentals. Unlike marsupials, placental mammals keep their babies in the uterus until they are much larger. They can do so because the embryo is surrounded by a placenta, a special kind of tissue that can draw food from the mother. Placental mammals are born much more developed than marsupials. In some cases, such as rabbits, they are still blind and have to stay hidden in a warren. But in other cases, such as dolphins or horses, they are ready to move on their own almost immediately.

There are precious few fossils of placental mammals older than 65 million years that belong to living orders, but the few there are suggest that they began to diverge into the living orders about 100 million years ago. The first lineage to branch off would much later produce anteaters, sloths, and armadillos. These animals lack many of the traits that other placental mammals share; they don’t have a cervix, for example, and their metabolism–while higher than a platypus’s–is still slower than other placentals. The fact that these mammals branched off first doesn’t make them missing links in our own evolution (just as monkeys are not missing links in human evolution). It doesn’t mean that we descended from armadillos, sloths, or anteaters; it doesn’t even mean our ancestors had armor like armadillos, claws to hang upside down from trees like sloths, or long tongues like anteaters. Once these lineages split off, the mammals went on evolving new adaptations of their own.

Paleontologists suspect that other groups of living mammals emerged around 80 million years ago. The Insectivora would eventually give rise to moles, shrews, and hedgehogs. The Carnivora produced dogs, cats, bears, and seals. The Glires would produce rabbits and rodents. The Ungulata would give rise to horses, camels, whales, rhinos, and elephants. And the Archonta would eventually include in its ranks bats, tree shrews, and our own branch, the primates. But all of this diversification was still tens of millions of years away. The ancestors of living placental mammals were practically indistinguishable from one another. It would take another mass extinction to reveal what mammals could become.

Death From Above

In northern Italy you can find a beautiful rosy limestone, called Scaglia rossa, which Italian builders like to use to construct their villas. Just north of the town of Gubbio, the 1,200‑foot Bottaccione Gorge is walled with the stuff. Geologists have determined that the rock at the bottom of the gorge was laid down 100 million years ago, when placental mammals were just beginning to diverge into their living groups. The rock built up continuously for the next 50 million years. It was still forming 65 million years ago at the end of the Cretaceous period, when the dinosaurs vanished, along with 70 percent of all species. It continued forming for another 15 million years, as mammals evolved into the dominant vertebrates on land. Wedged between the rock from the Cretaceous period and the rock from the Paleocene period that followed is a strip of clay only a half‑inch thick, like a smear of jelly in a sandwich. Below the strip, the rocks contain calcium carbonate skeletons of plankton; their bodies make up most of the rock. In the layer of pure clay, no plankton can be found; above it, the limestone starts again, but it lacks many of the old species of plankton. In that half‑inch strip our destiny may have been determined: it marks a global catastrophe our ancestors survived but the giant dinosaurs did not.

An American geologist named Walter Alvarez hammered out chunks of this clay in the mid‑1970s and brought them back home with him. Alvarez hoped to find in the Scaglia rossa the precise boundary between the Cretaceous and the Tertiary and assign a date to it. If all went well, he hoped to find a way to identify the same boundary in rocks in other parts of the world. Every few million years Earth’s magnetic field flips, so a compass points south instead of north. The field lines up magnetic crystals in rocks, and geologists can measure its direction millions of years later. Alvarez wanted to find layers of rock below and above the boundary that had formed when Earth’s magnetic field had flipped. It might be possible then to go to other rocks and find the same sequence of flipping and use it to put a bracket around the boundary.

Alvarez showed his rock to his father, Luis, when he came home. Luis Alvarez was not a geologist himself, but he was a hungry‑minded scientist nevertheless. He had won the Nobel Prize for physics in 1968; he had helped to invent the bubble chamber that would lead to the discovery of subatomic particles, and he had x‑rayed the pyramids to look for hidden tombs. The rock that his son brought home fascinated him. What had happened in the oceans at the close of the Cretaceous period to stop the limestone factory and then start it again?

His son’s plan to find a paleomagnetic bracket for the Cretaceous‑Tertiary boundary didn’t work out. North and south traded places too slowly at the end of the Cretaceous to be much use for dating. But Luis had another idea–they could use the steady rain of interstellar dust as a clock. Meteorites and other material drifting through space have chemical compositions very different from that of rocks on Earth. They have, for instance, much more of a rare element called iridium in them. (Most of the iridium that helped form the molten Earth 4.5 billion years ago sank into its core along with other metals.) Tons of microscopic debris fall into Earth’s atmosphere every year, sprinkling steadily on land and sea below. Walter and Luis set out to determine how to measure the rate at which iridium landed on Earth by measuring its levels in the Gubbio rocks.

Other scientists had tried to use the same method and failed, but fortunately the Alvarezes didn’t know that. They measured the iridium in the rocks from the end of the Cretaceous and found that it was enormous, 30 times the levels that were in the limestone Walter had taken from above and below the layer of clay. The steady rain from space couldn’t have left that much iridium. But their measurement was not a fluke: Danish scientists had looked at rocks near Copenhagen that dated from the end of the Cretaceous and found an even bigger spike of iridium.

The Alvarezes began to wonder if Earth had gotten a giant delivery of iridium from space at the end of the Cretaceous period. That might support a wild‑eyed idea from a paleontologist named Dale Russell. The dinosaurs (at least the big ones) had disappeared at the end of the Cretaceous, in a mass extinction that claimed an estimated 70 percent of all species, including giant marine reptiles and the pterosaurs that filled the skies. Russell threw out the possibility that an exploding star in the sun’s neighborhood had done them in. A supernova might have released a flood of charged particles that would have raced through space and fallen into Earth’s atmosphere, causing mutations and death.

The Alvarezes knew that iridium is one of the elements that are forged during a supernova. Perhaps along with the lethal charged particles, the supernova could have sent a surge of iridium to Earth. But when the Alvarezes investigated Russell’s idea, they discovered that a supernova couldn’t have been the cause. In addition to iridium, exploding stars also produce plutonium 244, which ought to have left a mark in the Gubbio clay. The Alvarezes found none.

Their thoughts turned instead to the possibility of a giant comet or asteroid striking Earth. Luis remembered reading about Krakatau–the eruption had launched 18 cubic kilometers of dust into the atmosphere, and 4 cubic kilometers lofted to the upper reaches of the stratosphere. Fast‑blowing winds carried it for two years around the planet, masking the sun and creating blazing sunsets. Luis suggested that the impact of a giant asteroid was like a magnified Krakatau. He speculated that when the asteroid slammed into the ground, its debris flew back up into the air, along with the terrestrial rock gouged out by its crater. Together they formed a thick dark shroud around the planet. Without sun, plants withered and photosynthetic plankton in the oceans died. With nothing to eat, herbivores starved; the carnivores disappeared soon after.

The meteorite, the Alvarezes calculated, had to have been about 10 kilometers across. It would have been as if Mount Everest had been fired into the planet like a bullet. Impacts of this scale were common during Earth’s early years, but they tapered off 3.9 billion years ago. Since then, giant asteroids and comets have probably struck the planet only once every 100 million years. An impact at the end of the Cretaceous would have been a rare event, but not an unexpected one.

The Alvarezes published their impact hypothesis in 1980, and in the decade that followed, other geologists looked for more clues as to what happened at the end of the Cretaceous period (known as the K–T boundary). They found more and more evidence that something huge hit the planet 65 million years ago. At more than 100 sites around the world geologists had found the layer of clay that marks the end of the Cretaceous, and iridium consistently appears in it. Researchers have also found bits of shocked quartz in the clay that could only have been created under intense pressures such as those created in an impact.

For more than a decade, however, the Alvarezes were dogged by the absence of the crater that such an impact would have left. It was possible that the impact had occurred in the ocean and was now covered over by seafloor sediments, or that plate tectonics had sucked it into Earth’s mantle, or that a volcano had covered it over. But the Alvarezes kept looking for a smoking gun. One reason that they kept searching for it was that their critics were cooking up alternative explanations. Some researchers argued that volcanoes, which are such a strong candidate for the Permian‑Triassic extinction 250 million years ago, could have been the culprit at the K‑T boundary. The same sort of volcanic activity that carpeted Siberia at the end of the Permian period also poured lava across India at the end of the Cretaceous. These eruptions could have brought up iridium from deep within the Earth, and could have created the intense pressures necessary for shocking quartz as well.

Geologists continued to search for a crater, and in 1985 the first clues of one emerged. Geologists found some peculiar deposits in Texas dating from the K‑T boundary that contained coarse sands and pebbles. They could only have been carried there by a giant tsunami that had originated from somewhere south of the site. Perhaps, the researchers reasoned, an impact had generated the giant waves. Meanwhile in Haiti other geologists found K‑T rocks containing globules of glass, which were predicted to have been formed in the impact, as molten rock hurled into the sky quickly cooled. Unlike the dust and vapor that the impact would have kicked up, these globules were too heavy to travel very far. The crater, researchers realized, must be within a few hundred miles of Haiti. Together, the tsunami and the glass pointed to an impact site somewhere around the Gulf of Mexico.

In the 1950s, Mexican geologists had discovered the remnants of a giant circular structure dating from about the end of the Cretaceous, buried off the coast of the Yucatán Peninsula. It had been pretty much forgotten, but with the discoveries in Haiti and Texas, it took on a new importance. When geologists revisited the site (called Chicxulub after a nearby town), they brought with them equipment that could detect buried rock formations by the subtle changes in the gravitational field they create. The geologists mapped out two concentric circles that looked as if they had been traced by a heavenly compass. All signs pointed to a 100‑mile‑wide crater buried under the sediment. Other researchers drilled into the rings and brought out rocks that they then dated. The age of the rocks − 65 million years old–matched both the Alvarezes iridium marker and the glassy globules in Haiti.

In 1998 a geologist named Frank Kyte from the University of California at Los Angeles found what may actually be a piece of the thing that hit the Yucatán. He was looking over a cylinder of rock drilled from the floor of the Pacific Ocean. The dark brown clay was loaded with iridium and shocked quartz, marking the K‑T boundary. Kyte sliced the clay at the boundary, and there he discovered a lone pebble, measuring 2 millimeters across. Its chemical composition was unlike anything on Earth, but exactly like that of many meteorites. Here, Kyte suggests, was a chip from a giant asteroid. It had broken free during the impact at Chicxulub and flown high over the Yucatán, arcing through the stratosphere before plopping into the Pacific.

As geologists have gotten a better sense of the asteroid that hit the planet 65 million years ago, other researchers have been looking for clues to its effect on life. At the end of the Cretaceous, they’ve found, the Yucatán was covered by a shallow sea, less than 100 meters deep, its bottom made of rocks rich in sulfur and carbon. Giant seagoing lizards may have swum under the shadow the asteroid cast just before it struck. The asteroid entered our atmosphere at a speed somewhere between 20 and 70 kilometers a second, creating a giant shock wave that ignited a jet of flame in its path. The fiery tail leveled trees for thousands of kilometers.

Computer models suggest that when the asteroid hit the water, it may have sent a tsunami out across the oceans, rising as high as 300 meters. The waves roared onshore, and the riptide dragged back entire forests, drowning them 500 meters underwater. An instant after the asteroid struck the water, it hit the bottom of the sea and vaporized 100 cubic kilometers of rock. The impact sprayed rock and asteroid 100 kilometers into the sky, above the stratosphere. An earthquake 1,000 times more powerful than anything in recorded history made the entire planet shiver. Geologists drilling in the Atlantic have found evidence that it triggered undersea landslides along the eastern seaboard of North America as far north as Nova Scotia, flowing 1,200 kilometers out from shore. Meanwhile, a fireball emerged from the crater and spread out hundreds of kilometers. The blackened sky was probably filled with thousands of shooting stars, molten hunks of rock that soared over the planet, igniting more fires wherever they landed.

The world burned; smoke hid the sun. Plants and phytoplankton died in the prolonged darkness, and the ecosystems that were built on them collapsed. When the smoke cleared a few months later, the world may still have been dark and cold. The impact may have vaporized the sulfate deposits in the Yucatán rocks, which combined with oxygen to form droplets of sulfur dioxide. The hazy clouds they formed may have reflected sunlight away from Earth and could have lingered for a decade. But as the haze faded, the impact ravaged the planet in yet another way, by warming it. The carbon in the limestone that was heaved into the atmosphere turned to the greenhouse gas carbon dioxide; the asteroid also sprayed the air with water vapor, an even more powerful greenhouse gas.

The heat, the cold, the fires, and the other disasters caused by the impact may have destroyed more than two‑thirds of all species on Earth. The Alvarezes found a single culprit for the K‑T extinction, but as in the case of the Permian‑Triassic extinction, it used many weapons.