Origin Story, page 14
The Cambrian explosion ended in a series of extinction events starting about 485 million years ago. Many species of trilobites walked the plank. So did many of the stranger Cambrian species, whose fossils have been found in the Burgess Shale in Canada and in the Chengjiang region in China.9 The Ordovician period also ended in a mass-extinction event 450 million years ago, when 60 percent of all genera may have vanished.
The greatest of all mass extinctions came at the end of the Permian period, 248 million years ago. This time, more than 80 percent of all genera vanished, including the last of the trilobites. The precise causes of this mass extinction remain uncertain. It might have been due to rising magmas that broke through the crust in massive volcanic eruptions that sent enough ash into the air to block photosynthesis. We find modern evidence of this in a large volcanic region of Siberia known as the Siberian Traps. The eruptions pumped huge amounts of carbon dioxide into the atmosphere, so when the dust settled, carbon dioxide levels spiked, oxygen levels fell, and the oceans warmed. When Earth burped, the biosphere shuddered. By some estimates, oceans may have been as warm as thirty-eight degrees Celsius, hot enough to kill most marine organisms and stop nearly all photosynthesis in the seas. Warmer oceans could hold less oxygen and support less life, and deep beneath their surface, thawing balls of frozen methane known as clathrates may have released huge bubbles of methane. This was a greenhouse mass extinction; it killed by heating rather than freezing.10 In an extreme greenhouse world, large organisms survived only in the cooler polar environments in the far north and south of the vast supercontinent of Pangaea.
Greening the Land and Oxygenating the Atmosphere
Beneath the violent changes of the early Phanerozoic, a new biosphere was building. The spread of plants, fungi, and animals onto land transformed the Earth’s surface. Particularly important was the spread of photosynthesizing plants onto land, because they consumed huge amounts of carbon dioxide and released huge amounts of oxygen. That reset the biosphere’s thermostats, creating a new climatic regime with higher oxygen levels and lower carbon dioxide levels than ever before. In its essential features, that regime has lasted until today.
Colonizing the land was extremely difficult, a bit like colonizing a new planet. Life had evolved and flourished in water for three billion years. Every cell had evolved in a bath of salty water. Organisms floated in water, extracted from it the gases and chemicals they needed, and fished in it for their food. Away from water, they needed support systems as elaborate as any space suit. They needed tough skins that could hold in water and prevent their bodies from drying out. But those skins also had to be permeable enough to let in carbon dioxide or oxygen. There was a tricky balance here. Leaves handle these opposite demands by way of tiny pores called stomata that allow carbon dioxide in and let water leak out. The size and number of stomata change depending on the surrounding temperature, humidity, and carbon dioxide levels.
How could organisms reproduce out of water? How could they protect eggs or infants from the terrible fate of desiccation? Water also provided buoyancy, and there wasn’t much buoyancy on land. For tiny insects like fleas, this didn’t matter. They were too light to worry about gravity, which is why a flea can happily jump off a cliff. But for large organisms, gravity was a problem. They needed bracing from girders of bone or wood if they were to stand up. Once standing, they needed elaborate plumbing through which liquids could be circulated against gravity to every cell in their bodies. Plants circulated liquids through roots and internal channels, exploiting water’s ability to clamber upward through narrow passages using capillary action. Animals developed special pumps (aka hearts) to circulate liquids and nutrients and remove toxins.
Serious colonization of the land by metazoans began only after the late Ordovician extinction, 450 million years ago. That’s when, for the first time, a few intrepid groups of plants and animals tiptoed out of the oceans and onto the land, encouraged, perhaps, by the energy boost from increasing levels of atmospheric oxygen.
The first vascular plants, with tissues that could circulate liquids and nutrients, showed up on land about 430 million years ago. Fungi and animals soon followed them. Simple, scorpionlike arthropods may have flourished on land as early as the first vascular plants. Early amphibians certainly walked the land by 400 million years ago, the date of amphibianlike fossil footprints found in Ireland and Poland. Amphibians evolved from fish that could breathe out of water and walk in the shallows of drying lakes and rivers, like modern lungfish. But all amphibians have to stay near water, where they lay their eggs. The first amphibians were the first large, land-based vertebrates. Some were as large as you and I.
Land-dwelling plants had a particularly large impact on the atmosphere, as they inhaled carbon dioxide and exhaled oxygen. Levels of atmospheric oxygen rose fast after the Ordovician period, increasing from about 5 to 10 percent of the atmosphere to levels much higher than they are today, perhaps to 35 percent, before stabilizing. Since about 370 million years ago, oxygen levels have mostly remained between 17 percent and 30 percent of the atmosphere.11 We know this because over this entire period researchers see evidence of spontaneous fires, and fires cannot ignite if oxygen levels fall much below 17 percent. Oxygen levels probably peaked during the Permian period (from 300 to 250 million years ago).
One indicator of rising oxygen levels was the appearance of coral reefs, which need huge amounts of oxygen. The first large coral reefs appeared in the Ordovician period. Corals are really vast symbiotic colonies of tiny, genetically identical invertebrate animals. At a stretch, we might regard them as vast, sprawling animals with a hard but somewhat shapeless skeleton. Each coral hosts colonies of single-celled photosynthesizing organisms that supply it with energy. Coral reefs offered cozy lodgings for many large organisms, including trilobites, sponges, and mollusks.
Rising oxygen levels fueled a second wave of metazoan colonizers of the land during the Devonian period, which started about 370 million years ago. The first plants with woody skeletons that allowed them to stand up against gravity appeared about 375 million years ago, and the first forests appeared soon after. They fixed huge amounts of carbon through photosynthesis, so as the Earth turned green, carbon dioxide levels fell to perhaps a tenth of earlier levels.12 The impact of the first forests was particularly significant because as yet, there were no organisms that could break down the lignin in wood. That’s why forests from the Carboniferous period (from 360 to 300 million years ago) were mostly buried beneath the soil, along with the carbon they had drawn down from the atmosphere. Over time, they fossilized to form the coal seams that later powered the industrial revolution. About 90 percent of today’s coal deposits were buried during the period of high oxygen levels, from around 330 to 260 million years ago. With plenty of oxygen, forest fires were easily ignited by lightning strikes. So the Carboniferous and early Permian world, though chilly, probably had the acrid smell of forest fires, a smell no one will detect on other planets in our solar system because they lack the high oxygen levels and the woody fuel sources needed for the propagation of fire.
Carboniferous forests may have doubled rates of photosynthesis, and that effectively doubled the biosphere’s total energy budget, allowing the production of many more organisms.13 Plants tweaked Earth’s geological thermostat, because they sped up the weathering of rocks by grinding and dissolving them into soils that could carry buried carbon more easily into the oceans; from there, some carbon was subducted deep into the mantle. Buried carbon could no longer react with oxygen to form carbon dioxide, so oxygen levels rose. This is why the amount of free oxygen depends roughly on the amount of carbon subducted into the mantle, so levels of atmospheric oxygen and carbon dioxide tend to move in opposite directions. Rising oxygen levels also allowed new chemical reactions in the crust, creating many of the four thousand different types of minerals found on Earth today.14
Between 450 and 300 million years ago, from the end of the Ordovician period to the beginning of the Permian, forests and land-based metazoans transformed Earth’s surface, turning the continents green and resetting the biosphere’s thermostats to create the Late Phanerozoic atmospheric regime of high oxygen levels and low carbon dioxide levels.
Long Trends: Larger Bodies and Bigger Brains
Like the history of complexity in general, the history of big life was shaped by chance and necessity. Mass extinctions illustrate the dramatic role played by chance. Without them, today’s biosphere would look very different. But evolution was never a matter of chance alone. Some changes were more likely than others. So, though serendipity shaped the history of big life, there were also large trends that persisted despite the turmoil caused by asteroid impacts, volcanic eruptions, and mass extinctions. The long trends are as important to us as the sudden catastrophes.
One long trend was toward bigness. That’s the trend that gave us metazoans in the first place. It also encouraged the evolution of larger and larger metazoans, because being a giant often made good evolutionary sense. After all, larger organisms have fewer predators. Try getting your teeth into a blue whale! Large organisms also need less food for each unit of body weight, and it’s usually easier for them to avoid the catastrophe of desiccation.15 Besides, the high-oxygen atmospheric regime that emerged early in the Phanerozoic eon provided the extra energy needed to power megametazoans. Indeed, it seems likely that very large organisms flourished best when oxygen levels were highest, which usually meant during periods of low carbon dioxide levels and cooler climates. This was true in the oceans as well as on land, because cold water can hold more oxygen than warm water.
As oxygen levels rose, many different evolutionary lines experimented with larger bodies. During the Carboniferous and Permian periods, we begin to see mega-insects and mega-vertebrates. This was when you might have seen dragonflies with fifty-centimeter wingspans, or ninety-centimeter-long scorpionlike creatures weighing twenty kilograms. The first reptiles appeared in the Carboniferous period, which started about 320 million years ago. They were part of a new group of animals, the amniotes, which include reptiles, birds, and mammals. Unlike amphibians, amniotes could reproduce away from the water because their young developed in protected eggs, pouches, or wombs. Reptiles would eventually include some of the largest animals that have ever strolled, waddled, lolled, or galloped across the land.
The mass extinction at the end of the Permian period was followed by a new adaptive radiation during the Triassic period (from 250 to 200 million years ago). This is when we see the first large dinosaurs. (Not all dinosaurs are large!) In the later Triassic period, though, oxygen levels began to fall once more, the world began to warm, and life got tougher for massive metazoans. The Triassic world ended abruptly two hundred million years ago in another greenhouse mass-extinction event. Those dinosaur families that survived evolved highly efficient mechanisms for breathing in an oxygen-deprived world. These mechanisms may have encouraged bipedalism (think T. rex and modern birds), because in bipedal reptiles, the chest is more open and breathing is not checked by motion the way it is in the waddling walk of four-legged reptiles. During the Jurassic period (from around 200 to 150 million years ago), oxygen levels rose again, until they approached the levels of today’s world. And dinosaurs got bigger once more. The largest tramped over the Earth in the Late Jurassic and Cretaceous periods, between 160 and 65 million years ago. Equipped with more efficient lungs than their Triassic ancestors, they used the large amounts of energy available in an oxygen-rich atmosphere to power their huge bodies.
The first true birds evolved during the Late Jurassic period. They, too, depended on high levels of atmospheric oxygen, because as every pilot knows, flight demands a lot of energy. Archaeopteryx, one of the earliest of all birdlike creatures, left fossils that were discovered in Germany in 1861, just two years after the publication of Darwin’s The Origin of Species. It lived around 150 million years ago and was about the size of a crow. For Darwin, its discovery offered powerful evidence for his theory of evolution by natural selection because it showed the existence of transitional species, halfway between reptiles and birds. Archaeopteryx had many birdlike features, but it also retained reptilian features such as claws, a bony tail, and teeth. Recent finds have shown that many species of birds with teeth evolved during the Cretaceous and coexisted with flying dinosaurs.
Mammals, like the other amniotes (reptiles and birds) also appeared after the Permian mass extinction. Mammals would eventually produce some giants, too, but not for almost two hundred million years. Before that, they mostly lived in modest obscurity in the shadows of a world ruled by dinosaurs. Throughout the Triassic, Jurassic, and Cretaceous (from 250 million years ago to 65 million years ago), most mammals were small, burrowing creatures, a bit like modern-day rodents.
Mammals are a class of warm-blooded animals related to the other amniotes, the reptiles and birds. But mammals differ from reptiles and birds in crucial ways. The mammal brain has a neocortex, which makes mammals superb calculators. They have fur (yes, even humans have fur, though less than most mammals), and for the most part, mammals take more care of their offspring. It was Carl Linnaeus, the founder of modern taxonomy, who first called animals in our class mammals, after another distinctive feature: all mammals nourish their young with milk from mammary glands. For paleontologists, the most visible distinguishing feature of mammal fossils is their teeth. Even the earliest mammal teeth have cusps so that the upper and lower teeth can mesh together, allowing them to chomp down on new types of food and grind it more efficiently than most reptiles do.
Mammals illustrate another powerful evolutionary trend, the tendency toward more elaborate information processing. This is apparent throughout the Phanerozoic but particularly among animals and most strikingly among mammals.
We have seen that all living organisms are informavores. They collect information, process it, and act on it. In the simplest organisms, including prokaryotes, the second (processing) stage is rudimentary, often amounting to little more than a sort of on/off switch, as in: “It’s too hot here, so wag your flagella clockwise and move away fast.” Simple pain and pleasure reflexes guide a lot of effective information processing even in simple metazoans.
But as organisms became larger and more complex, they needed more information about their environments. Natural selection equipped large organisms with a desire for more information, because good information was vital to their success. That’s why, when a human solves a puzzle, the brain gets the same buzz it gets from food and sex.16 Natural selection also gave large organisms more sensors and more types of sensors: for sound, pressure, acidity, light. And natural selection evolved a growing repertoire of possible responses. As the number and range of inputs and outputs increased, the processing stage became more elaborate, so more nerve cells were devoted to that task. In animals, nerves gathered in nodes, ganglions, and brains, forming networks of transistor-like switches that linked hundreds, millions, or billions of neurons that could compute in parallel. That allowed them to model important features of the external world and even to model possible futures. No brainy creature (not even you or I) is in direct contact with its environment. Instead, we all live in a rich virtual reality constructed by our brains. Our brains generate and constantly update maps of the most salient features of our bodies and our surroundings, just as climate scientists model changing environments today.17 Those maps enable us to maintain homeostasis. They help us respond appropriately, most of the time, to the never-ending swirl of changes all around us.
Decision-making works at several different levels in brainy creatures. Some decisions need to be made quickly if there’s not enough time for careful deliberation. Other decision-making mechanisms are slower and more ponderous but offer more options. The simple on/off switches of pain sensors control a lot of behavior in even the most complex metazoans. Put your hand into a flame and you will remove it before you can think about it. The emotions, dominated by the limbic system, also allow rapid decision-making by creating predispositions and preferences that drive many important decisions and get them right most of the time. Charles Darwin understood that the emotions are decision-makers that have evolved through natural selection to help organisms survive. The antelope that wants to hug lions is unlikely to pass on its genes to any offspring. The most basic emotions, those least amenable to conscious control, seem to bubble up inside us. They include fear and anger, surprise and disgust, and also, perhaps, a sense of joy. They predispose us to react in certain ways and send the chemical signals that prepare our bodies to run or focus, to attack or hug.18 Emotions drive decision-making in all animals with large brains, and some emotions, like fear, are probably present in all vertebrates and maybe in some invertebrates, particularly the most intelligent ones such as the octopi. The preferences emotions create for particular outcomes and behaviors lie behind the human sense of meaning and ethics.
The faculty we often describe as reason is just one of many biological decision-makers. It adjudicates on important decisions if the brain is big enough, if there is plenty of time available, and if other systems are deadlocked and can generate no clear answers. Do I really need to waste this much energy running if that is not really a lion? Is my rival making phony threats or do I need to respond?
Sensations, emotions, and thought together create the inner, subjective world that all humans, and probably many other large-brained species, experience. The state that we describe as consciousness seems to be a mode of sharply focused attention summoned by the brain, as if to a court of law, when new, difficult, and important decisions have to be made. That suggests that consciousness is present to some degree in many organisms whose brains are large enough to provide the necessary working space for really complex decision-making.19 But it is not needed for routine decisions.
