Chapter Content
Okay, so, like, you know, if you really think about life from a human perspective, which, let's be honest, it's kinda hard not to, it's just... life's a weird thing, right? It's all eager to get going, super excited to start, but then, like, once it's going, it doesn't seem to be in any real rush to get anywhere.
I mean, think about lichens, you know? They're probably the toughest visible creatures on the planet, but they're also some of the least ambitious, seriously. They're happy enough, I guess, growing in sunny, old graveyards. But they especially love, like, thriving in places where nothing else wants to be, you know? On windswept mountaintops, in the Arctic wilderness where it's basically just rocks, wind, cold, and basically no competition, or Antarctica, where, I mean, nothing else really grows! You'll see huge patches of lichens, like 400 different kinds, clinging faithfully to, like, every weathered rock.
For a really long time, people just didn't understand how they even did it. I mean, they grow on bare rocks. There's no obvious, like, nutrients, and they don't produce seeds. So, people, even, like, educated people, thought they were rocks in the process of *becoming* plants! Seriously, one observer, a Dr. Hornschuch, apparently said, all excited, way back when, that "lifeless stones automatically transform into living plants!"
But if you look closer, you see that lichens aren't magic. They're actually, like, a partnership, a super interesting one, between a fungus and algae. The fungus, it secretes acids that dissolve the rock surface, releasing minerals. And then the algae transforms those minerals into enough food to keep them both alive. It's, like, not exactly a thrilling arrangement, but hey, it works! There's over 20,000 kinds of lichens in the world.
And, you know, like most things that thrive in harsh conditions, lichens, they grow really slowly, super slowly. A lichen might take, like, half a century to grow to the size of a shirt button. Like, David Attenborough wrote that those lichens that grow to, like, plate size are "almost certainly hundreds, if not thousands, of years old." It's hard to imagine a smaller kind of achievement, right? "They simply exist," Attenborough said, "demonstrating a touching fact: that even life at its simplest level, exists apparently just for its own sake."
And that's it, you know? That's all life wants, and it's easy to miss that. As humans, we tend to think life has to have a purpose. We've got plans, we've got ambitions, we've got desires. We want to constantly exploit our whole, like, intoxicating existence. But, like, what is life for a lichen? Its survival instinct, its desire to live, is as strong as ours, maybe even stronger. I mean, if you told me that I had to be a lichen on a rock in the woods for decades, I think I'd lose the will to live. But a lichen? Nope, it won't. Basically, like all living things, they suffer, they endure insults, just to live a little longer. So, yeah, life wants to exist. But, and this is the cool part, in most cases, it doesn't want to do much.
Which is maybe a bit odd, 'cause life has had a long, long time to, like, show some ambition, you know? Imagine shrinking the Earth's 4.5 billion-year history into a single day. So, life starts pretty early, with the first simple, single-celled organisms appearing around, like, 4:00 AM. But then, for the next 16 hours, not much happens, like seriously. It's not until almost 8:30 PM, when five-sixths of the day is already gone, that Earth manages to produce anything of note, and even then, it's just a layer of, like, restless microbes. Then, finally, the first marine plants pop up. Twenty minutes later, there are the first jellyfish and the mysterious Ediacaran fauna, which, you know, Reginald Sprigg found first in Australia. At 9:04 PM, trilobites show up, followed almost immediately by those gorgeous, you know, Burgess Shale animals. It's almost 10:00 PM before plants start to appear on land. And then, not long after, with less than two hours left in the day, the first land animals follow. Thanks to a good ten minutes or so of awesome weather, by 10:24 PM, the Earth's covered in Carboniferous forests, the remains of which have become our coal. The first winged insects make their debut. It's just after 11:00 PM when dinosaurs slowly lumber onto the stage, dominating the world for, like, three-quarters of an hour. And then, at 20 minutes before midnight, they vanish, and the age of mammals begins. Humans? We appear at 1 minute and 17 seconds before midnight. On this scale, our entire recorded history is just a few seconds long, and a single human life is, like, a blink of an eye.
During this super-compressed day, continents are, like, crashing together at, like, crazy speeds. Mountains rise and fall. Oceans appear and disappear. Ice sheets advance and retreat. And, like, throughout this entire time, about three times every minute, there's a flash somewhere on the planet, showing that a Manson-sized or bigger meteorite has slammed into the Earth. It's really amazing that *anything* survived at all, in this unstable environment, with the meteorite impacts. Actually, you know what? Not that many things *did* survive for very long.
To really understand how insignificant our arrival is in this 4.5-billion-year movie, maybe, like, try this. Stretch your arms out as far as you can, and imagine that that width represents the whole history of the Earth. On that scale, according to John McPhee, the distance from the fingertips of one hand to the wrist of the other hand represents all of pre-Cambrian time. All complex life is contained in just one hand, and “you can take a medium-grained nail file and remove all of human history with a stroke."
Which, thankfully, hasn't happened yet, but probably will at some point. And, you know, I don't wanna sound all depressing here, but there's another really, really important thing about life on Earth, it goes extinct, and often, like really often. I mean, even though species struggle to preserve themselves, they often collapse and die. And the more complex they get, it seems, the faster they go extinct. And maybe that's why so many life forms aren't very ambitious, huh?
So, whenever life does anything bold, that's a big deal. And we're gonna talk about life moving forward to another stage, leaving the oceans. That's one of those big deals.
I mean, the land, it's a scary place, super hot, dry, bathed in intense ultraviolet radiation, and it doesn't have that, like, relative ease of buoyancy of moving in water. To live on land, animals had to totally rework their structure. If you hold a fish by both ends, the middle sags down because its spine isn't strong enough to support itself. To survive out of water, sea creatures needed a new, load-bearing, internal structure, and that's not something that adjusts overnight, not at all. And, especially important, any land animal, obviously, has to learn to take oxygen directly from the air, not filter it from the water. So, none of these were trivial difficulties, all of them had to be overcome. On the other hand, animals had a big incentive to leave the water because the environment underwater was getting more dangerous. Continents were merging into a single landmass, Pangaea. Which meant fewer coastlines than before, so fewer coastal habitats. So, yeah, competition was fierce, and, also, a new, omnivorous, and unsettling predator appeared. An animal whose body was, like, perfectly suited for attack. And that animal hasn't really changed much since it showed up, and it's sharks. So, you know, the best moment for, like, a replacement for water had finally arrived.
Around 450 million years ago, plants started to colonize the land, and with them came the essential, small mites and other animals, which plants needed to decompose their dead organic matter, you know, to recycle it. Larger animals took longer to show up, but about 400 million years ago, they, like, tentatively crawled out of the water. A lot of illustrations show the first brave land-goers as some kind of super ambitious fish, maybe looking like a modern mudskipper, jumping from one pond to another in the dry season, or, you know, even like a fully formed amphibian. But, like, actually, the first visible, mobile residents on land were probably more like modern woodlice, or pill bugs. Those little guys, they're crustaceans actually, and when you lift up a rock or a piece of wood, they just scatter in, like, a frantic panic.
Things were good for animals that had figured out how to breathe oxygen from the air. During the Devonian and Carboniferous periods, when land life greatly increased, the concentration of oxygen in the air was really high, up to 35% (now it's close to 20%). So, animals could grow to be incredibly big, incredibly fast.
And, you know, you might be wondering, how do scientists even *know* what the oxygen concentration was, like, hundreds of millions of years ago? The answer is isotope geochemistry, which is, like, a pretty obscure but super amazing field. The ancient oceans of the Devonian and Carboniferous were home to tons of tiny plankton, hiding in little protective shells. Then, as now, the plankton absorbed oxygen from the atmosphere, and combined it with other elements, especially carbon, to form, you know, durable compounds, like calcium carbonate, to build their shells. This is all part of the long-term carbon cycle, which might not sound exciting, but is essential to making Earth habitable, which, you know, we already talked about elsewhere.
Eventually, all those tiny creatures died and sank to the bottom of the ocean, and were gradually compressed into limestone. And in the tiny atomic structure that the plankton took to their graves, there are two super stable isotopes, O-16 and O-18. (Don't worry if you forgot what isotopes are, just remember that atoms with extra neutrons are isotopes.) Now, geochemists use this, because those isotopes accumulate at different rates, depending on how much oxygen or carbon dioxide was in the atmosphere when they formed. By comparing the rate at which these two isotopes were stored in ancient stuff, geochemists can figure out stuff about the ancient world, you know? Like, the concentration of oxygen, the temperature of the air and oceans, the extent and timing of ice ages, and, like, a ton of other stuff. Combine that with other fossil remains, like pollen concentrations, and scientists can pretty confidently reconstruct entire scenes that humans never saw.
Now, the reason oxygen was able to accumulate to such high concentrations during early land life was mainly because there were vast numbers of tall tree ferns and swamps all over the world. Those things naturally disrupt the normal carbon recycling process. Leaves and other dead plant material didn't completely rot, but accumulated in rich, moist sediments, and were eventually squeezed into vast coal seams. And, you know, even now, those coal seams support a huge amount of economic activity.
High concentrations of oxygen, they clearly encouraged organisms to grow tall. The oldest evidence found of land animals, is a track left on a rock in Scotland by an arthropod-like creature 350 million years ago. It was over a meter long! And by the end of that period, some arthropods would grow to be more than twice that length.
Because of these quietly foraging animals, insects of that period, they gradually developed a countermeasure to avoid those quick-reaching tongues, and they learned to fly. This probably isn't surprising. Some insects gradually became used to this new way of moving, and became really good at it. And their technique hasn't really changed since then. Dragonflies, they flew then, and they fly now at speeds of over 50 kilometers per hour, and they can stop fast, hover, and fly backwards. On a proportional scale, a dragonfly can reach altitudes that are much higher than any human aircraft can reach. "The US Air Force," as one commenter wrote, "put them in wind tunnels to see how they did it, and came away humbled." They also devoured the thick air. In the Carboniferous forests, dragonflies grew as large as crows. Trees and other plants also grew to be particularly tall, with calamites and ferns growing to 15 meters high, and lepidodendron to 40 meters high.
The first terrestrial vertebrates, the first land animals from whom we evolved, are, to some extent, still a mystery. Partly because of the lack of relevant fossils, and partly because of the quirks of a grumpy Swedish dude named Erik Jarvik, whose, like, eccentric interpretations and secretive demeanor delayed progress in this area by almost half a century. Jarvik was a member of a Swedish research team that came to Greenland way back to look for fish fossils. And they were particularly looking for a lobe-finned fish, which was supposed to be the ancestor of the so-called tetrapods, meaning us and all other walking animals.
Most animals are tetrapods, and living tetrapods share a common trait: four limbs, and each limb has a maximum of five fingers or toes. Dinosaurs, whales, birds, humans, even fish, are tetrapods. This clearly indicates that they came from a common ancestor. It is believed that the clues to this ancestor could be found in the Devonian Period around 400 million years ago. Before then, no animals walked on land. After then, many animals walked on land. Luckily, that team found just such an animal, a one-meter-long animal called Ichthyostega. Analyzing that fossil fell to Jarvik. He started in 1948 and the study lasted 48 years. Unfortunately, Jarvik didn't let anyone interfere with his research. Paleontologists around the world had to be content with two brief preliminary papers. In his papers, Jarvik noted that the animal had four limbs, each with five fingers, confirming its ancestry.
Jarvik died and when he did, other paleontologists scrambled to take a closer look at the specimen. And they discovered that Jarvik was way off on the number of fingers or toes, there were actually eight on each limb, and he hadn't noticed that the fish probably couldn't walk. Judging from the structure of the fins, they probably couldn't support its own weight. So, needless to say, this didn't contribute much to understanding the first land animals. Today, three early tetrapods are known, but none are related to the number five. So, yeah, we're still not entirely sure where we came from.
But, hey, we made it anyway, even though getting to our current awesome state was probably not always smooth sailing. Since life began on land, it has consisted of four so-called great dynasties. The first dynasty included slow-moving and sometimes rather clumsy, primitive amphibians and reptiles. The most famous animal from this age is Dimetrodon, a creature with wings on its back, often confused with dinosaurs. It was actually a synapsid, like we used to be. Synapsids were one of the four main groups of early reptiles, the other three being anapsids, diapsids, and parapsids. Those names just refer to the number and position of the holes found on the sides of their skulls. Synapsids have one hole in the lower temporal region, diapsids have two holes, and anapsids have only one hole in the upper temporal region.
Eventually, each major group was divided into subgroups. Some thrived, some declined. Anapsids produced turtles, which at one time seemed about to be dominant, to become the most advanced, and most deadly species on this planet, though that's kind of ridiculous. But because their evolution was relatively slow, they maintained long-term survival rather than dominance. Synapsids split into four branches, and only one made it through the Permian Period. Luckily, we happen to be on that branch. It evolved into a family of primitive mammals called therapsids. These reptiles made up the second great dynasty.
The therapsids were unlucky because their cousins, the diapsids, were also highly fertile in their evolution, and some evolved into dinosaurs. Therapsids gradually proved to be no match for dinosaurs. They were unable to compete with these fierce new animals, and generally disappeared from the record. However, a small number evolved into furry, burrowing, small animals, and existed for a long time as small mammals, waiting for the right time to come. The biggest of them wasn't even the size of a house cat. Most were no bigger than mice. In the end, this would prove to be a way to survive. But they would have to wait almost 150 million years, waiting for the third great dynasty, the age of the dinosaurs, to come to an abrupt end, giving way to the fourth great dynasty, and our own, the age of mammals.
Every major transformation, and many smaller transformations in between and afterwards, depend on the important and, you know, contradictory driving force, extinction. On Earth, species death is, like, honestly, a way of life. I mean, it's such an interesting fact. No one really knows how many organisms have existed since life started. The number that gets used most often is 30 billion, but some people estimate that that number could be as high as 4 trillion. No matter the total number, 99.9% of all the species that have ever existed are no longer with us. "The basic estimate is," as David Raup of the University of Chicago liked to say, "all species are extinct." For complex animals, the average lifespan of a species is only about 4 million years, about the same as humans have existed so far.
Of course, extinction is always bad news for the victims, but it seems to be a good thing for a vibrant planet. "The opposite of extinction is stagnation," says Ian Tattersall of the American Museum of Natural History. "Stagnation is rarely a good thing in any field." (I should probably point out that we are talking about extinction here as a long and natural process. Extinction caused by human carelessness is something else entirely.)
Crises in Earth's history are always related to subsequent leaps forward. The decline of the Ediacaran fauna was followed by the Cambrian creative explosion. The Ordovician extinction 440 million years ago cleared the seas of large numbers of motionless, filter-feeding animals, creating favorable conditions for fast-swimming fish and large aquatic reptiles. Those animals, in turn, were ideally placed, when yet another disaster struck life a blow at the end of the Devonian Period, to send colonists onto land. Throughout history, things like this have happened from time to time. If these events hadn't happened exactly the way they did, and exactly when they did, we almost certainly wouldn't be here right now.
The Earth has witnessed five major extinction events, in the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods, and a lot of smaller extinction events. The Ordovician (440 million years ago) and Devonian (365 million years ago) wiped out about 80%-85% of species each. The Triassic (210 million years ago) and Cretaceous (65 million years ago) each wiped out 70%-75% of species. But the real heavy hitter was the Permian extinction about 245 million years ago, which marked the beginning of the long age of the dinosaurs. During the Permian, at least 95% of the animals known from the fossil record left the stage, never to return. Even about a third of insect species disappeared, which was their worst loss ever. That was as close as we've ever come to total annihilation.
"It was truly a massive extinction, a slaughter. Something the Earth had never seen before," Richard Fortey says. The Permian event was particularly devastating for marine animals. Trilobites disappeared completely. Clams and sea urchins almost went extinct. In fact, all marine animals were in tatters. It is believed that all in all, on land and in the water, the Earth lost 52% of the "families," that level is higher than "genus" and lower than "order" on the life hierarchy (that's the subject of the next chapter), and about 96% of all species. It would take a long time, some estimate as long as 80 million years, for the total number of species to recover.
We need to remember two things. First, these are just speculations based on the data. It is estimated that the number of living animal species at the end of the Permian ranged from 45,000 to 240,000. If you don't know how many living species there are, you're not really in a position to calculate the exact proportion of extinct species. Second, we are talking about the death of species, not individual animals. As for individual animals, the number of deaths is likely to be much higher, in many cases, virtually all of them. The species that survived into the next stage of life almost certainly owe their existence to just a few injured and disabled survivors.
Between the several massacres, there were many smaller and lesser-known extinction events (the Hemphillian, the Frasnian, the Famennian, the Rancholabrean, and over a dozen others) that didn't greatly damage the total number of species, but were often a serious blow to certain populations. In the Hemphillian event that occurred about 5 million years ago, herbivores, including horses, were almost wiped out. Only one species of horse was left, which occasionally appears in the fossil record, showing that it was once on the verge of extinction. Just imagine a human history without horses, without herbivores.
For almost every case, whether large-scale or medium-scale, we are confused and unsure about what actually caused it. Even after removing the less practical views, there are still more theories explaining extinction events than there are actual events. At least 20 possible culprits have been suggested as causes or major accomplices, including global warming, global cooling, sea-level changes, a sharp reduction in ocean oxygen (called hypoxia), infectious diseases, massive methane leaks from the seafloor, meteorite and comet impacts, violent hurricanes of a so-called "superstorm" variety, intense volcanic eruptions, and catastrophic solar flares.
Solar flares are a particularly interesting possibility. No one knows how big solar flares can get, because we've only been observing them since the space age. But the sun is a big engine, and the storms that arise from it are extremely huge. An ordinary solar flare, that we might not even notice on Earth, releases the energy equivalent to 1 billion hydrogen bombs, hurling 100 billion tons of dangerous high-energy particles into space. The magnetosphere and atmosphere usually work together to throw these particles back into space, or guide them safely to the poles, where they create the Earth's beautiful auroras. It is thought that an extremely large eruption, say 100 times larger than an ordinary flare, could damage our thin defensive layer. That glow would be magnificent, but would almost certainly kill most of the creatures exposed to it. And, chillingly, according to Bruce Tsurutani of NASA's Jet Propulsion Laboratory, "It wouldn't leave a trace in the history."
All of this leaves us, as one researcher said, "with lots of speculation and little evidence." Cooling appears to be linked to at least three major extinction events (the Ordovician, Devonian, and Permian events), but beyond that, there's almost no consensus, including whether an event happened quickly or slowly. For example, scientists disagree on whether the Devonian extinction (after which vertebrates migrated to land) occurred over millions of years, thousands of years, or in a spectacular day.
One of the reasons why it is so difficult to give a convincing explanation of extinction is that it is very difficult to wipe out life on a large scale. We've already seen that from the Manson impact. You can take a hard hit, but still recover fully, even if you feel a bit overwhelmed. As such, Earth has endured thousands of impacts. Why was the KT event 65 million years ago so destructive as to doom the dinosaurs? Well, first, it was really awesome. Its impact force reached 100 trillion tons. Such an explosion is hard to imagine, but as James Lawrence points out, if you detonated a Hiroshima-type atomic bomb on every living person on Earth today, you would still be about 1 billion bombs short of the power of the KT impact. Yet even that may not have been enough to wipe out 70% of life on Earth, including the dinosaurs.
The KT meteorite also had an advantage, that is, an advantage if you are a mammal, and it landed in a shallow sea only 10 meters deep, at a very suitable angle, and the oxygen concentration at the time was 10% higher than it is today, so it was easier for the world to catch fire. In particular, the seabed in the landing area was made of sulfur-rich rock. As a result, the impact turned a seabed the size of Belgium into a sulfuric acid mist. Over the next few months, the Earth was hit by acid rain, acid concentrations were high enough to burn skin.
In a sense, there is also a bigger question than, “What destroyed 70% of the species that existed at that time?” That is, "How did the remaining 30% survive?" Why was the event the end of every dinosaur that existed, while other reptiles such as snakes and crocodiles got through the disaster safely? As far as we know, not one species of toads, water salamanders, newts, or other amphibians in North America went extinct. "Why did these fragile animals escape this unprecedented disaster unscathed?" Tim Flannery asks in his wonderful history of prehistoric America, The Eternal Frontier.
The situation in the ocean was very similar. Ammonites disappeared altogether, but their cousins, the nautilus, survived, even though they had similar lifestyles. Among plankton, some species were virtually wiped out altogether, for example, foraminifera lost 92%, while other organisms like diatoms, despite their similar shape and living alongside foraminifera, suffered less damage.
These are hard to explain contradictions. As Richard Fortey said, "It never seems quite satisfactory to just call them 'lucky.'" If everything was covered in black, choking smoke for months after the event (and it seems that was the case), it is hard to explain how many insects managed to survive. "Some insects, like beetles," Fortey points out, "can live on wood or other things around them. But what about animals like bees that fly in the sunlight and need pollen? It is not easy to say clearly why they survived."
Especially the coral. Corals need algae to survive, and algae need sunlight. Both need stable minimum temperatures. In recent years, there have been numerous reports of coral dying due to a change in seawater temperature of just 1 degree Celsius or so. If they are affected by even small changes, how did they survive the long winter caused by the impact?
There are also many inexplicable regional differences. Extinction seems to have been far less severe in the southern hemisphere than in the northern hemisphere. To a large extent, New Zealand in particular seems to have survived intact, and it has few burrowing animals and even its plants have largely survived, while the fire intensity elsewhere suggests that the disaster was global. All in all, there are still many questions we don't understand.
Some animals were flourishing again, including turtles, which is kind of surprising. Flannery pointed out that the period after the extinction of the dinosaurs could easily be called the Age of Turtles. Sixteen species survived in North America, and three more appeared soon after.
Obviously, living in water is a very good thing. The KT impact wiped out almost 90% of land-based species, while only 10% of species living in fresh water were affected. Water clearly provided heat and fire protection. It may also have provided food in the years that followed. All of the land-based animals that survived had a habit of retreating to a safe environment in times of danger, into the water or underground, both of which could provide considerable protection from the outside disaster. Animals that survived by scavenging food also had an advantage. Lizards are generally not harmed by bacteria in rotting corpses, and they never have been. In fact, they often have a liking for it. Obviously, there were large numbers of rotting corpses around them for a very long time.
It is often wrongly suggested that only small animals survived the KT impact. Actually, crocodiles were among the survivors. Not only were they large, but they were also three times larger than modern crocodiles. However, in general, it is true that most survivors were sneaky, small animals. When the world was pitch black and full of danger, nocturnal, non-picky, cautious, small, homeothermic animals were right where they needed to be. These are exactly the skills our mammalian ancestors had. If we had evolved to be more advanced, we might have already gone extinct. However, like any living organism, mammals felt very adapted to that environment.
However, it doesn't seem that mammals rushed in to take over every place. "Evolution may hate vacuums," paleontologist Stephen M. Stanley wrote, "but vacuums often take a long time to fill." For perhaps as long as 10 million years, mammals were cautious, keeping a small body size. In the Tertiary Period, if you were the size of a red cat, you could be king.
But once they started, mammals greatly increased their size, sometimes to ridiculous proportions. For a time, there were rhinoceros-sized guinea pigs and two-story rhinoceroses. Wherever there was a vacancy in the predator chain, mammals immediately stepped forward to fill it. Early raccoon family members migrated to South America, discovered a vacancy, and evolved into bear-sized and ferocious animals. Birds also grew to be disproportionately large. For millions of years, a flightless, predatory bird called "Giant Bird" may have been the fiercest animal in North America. It must have been the most majestic bird that ever existed. It was 3 meters tall, weighed over 350 kilograms, and its beak could tear off the heads of almost any animal it hated. Its family reigned supreme for 50 million years. Yet until a skeleton was discovered in Florida in 1963, we had no idea it existed.
This leads to another reason why we lack confidence in the causes of extinction: the poor fossil record. We have already briefly talked about the impossibility of any skeleton becoming a fossil, but the inadequacy of such records is even more serious than you think. Take dinosaurs, for example. Actually, most of the museum exhibits are man-made. The huge Diplodocus, prominently placed at the entrance of the Natural History Museum in London, has brought joy and knowledge to generations of visitors, and is made entirely of plastic. The model was built in Pittsburgh in 1903 and gifted to the museum by Andrew Carnegie. There's an even more grandiose scene in the lobby of New York's American Museum of Natural History: a huge Barosaurus skeleton, protecting its young from an allosaurus that is baring its teeth and lunging forward. This is an impressive exhibit; the Barosaurus reaches perhaps 9 meters high towards the high ceiling, but it's also completely fake. Every one of the hundreds of bones on display is a model. If you visit almost any natural history museum in the world, in Paris, Vienna, Frankfurt, Buenos Aires, or Mexico City, you're seeing old models, not old bones.
The reality is that we actually don't know much about dinosaurs. Throughout the age of the dinosaurs, fewer than 1,000 species have been identified (about half of which are known from a single specimen), about a quarter of the number of mammal species alive today. Don't forget that dinosaurs ruled the Earth for almost three times as long as mammals have. So either there were very few types of dinosaurs, or we only know the tip of the iceberg (I can't help but use this suitable cliché).
The age of dinosaurs was millions of years long. But so far, not one dinosaur fossil has been found. Even at the end of the Cretaceous Period, the prehistoric period we study the most because we have a lasting interest in dinosaurs and their extinction, about three-quarters of the species that existed at that time may not have been discovered. Thousands of animals bigger than Barosaurus or more majestic than Tyrannosaurus rex may have roamed the Earth, and we may never know. Until recently, all of our knowledge of dinosaurs from this period came from only about 300 specimens, representing only 16 species. Due to the lack of fossil records, many people believed that dinosaurs were already declining at the time of the KT impact.
In the late 1980s, Peter Sheehan, a paleontologist at the Milwaukee Public Museum, decided to conduct an experiment. He marked out an area in Montana's famous Hell Creek Formation, and chose 200 volunteers for a careful survey. The volunteers carefully screened and picked up every remaining tooth, every vertebra, and every bone, everything that previous diggers had left behind. The work took three years. When it was over, they found that they had more than doubled the dinosaur fossil record from the end of the Cretaceous Period, for this planet. The survey confirmed that dinosaurs were still quite numerous by the time the KT impact event occurred. "There is no reason to think that dinosaurs were gradually disappearing during the last 3 million years of the Cretaceous Period," Sheehan said in the report.
We are used to thinking that it was inevitable that we ourselves became the dominant species of life, so we cannot understand that we are here only because the impact from outer space happened at the right time, and because of other accidental chances. We share only one thing in common with other creatures, and that is that for almost 4 billion years, at every necessary moment, our ancestors successfully crawled through a series of doors that were about to close. Stephen Jay Gould famously put it succinctly, "Humans exist today because our particular family has never been interrupted, not even once, at a billion critical moments that could have wiped us out of history."
We made three points at the beginning of this chapter: life wants to exist, life doesn't always want to do great things, and life sometimes goes extinct. Maybe we can add one more point: Life is moving forward. And we'll see that life often moves forward in extremely surprising ways.