Planet Earth abounds in rich diversity. Armies of insects, an abundance of birds, and flowers blooming around the world are only the more obvious manifestations of this grand variety.1 More significantly, plant and animal life seem to have become both more diverse and more complex over geological time. Today, our own species, already replete with many cultures and thousands of languages, is adding still more to the world's complexity with ever-expanding technologies. In fact, human success has been fostered by the natural diversity that surrounds us. In the final chapters, we will discuss how our own species has contributed to global complexity, but let's begin with natural diversity itself: how it is measured, how it is distributed, and how it got that way.

Trying to understand biodiversity and the processes that allow so many organisms to co-exist has been a challenge for the biological sciences over the last few hundred years. Sciences that call themselves ecology, biogeography, systematics, and phylogenetics are all attempts to comprehend the living world's rich variety. The subject of biodiversity—a convenient abbreviation for biological diversity—spans all these different disciplines. In addition, paleontology shows us how biodiversity has grown and expanded over geological time, while conservation biology examines ways of preserving our natural heritage. Surely human success would have been impossible without a foundation of biological abundance.

Considered in its widest sense, biodiversity includes genetic diversity and human cultural variety, as well as the interactions shared by all living things. The idea of diversity—the numbers of things—is a little different from our notions of complexity, in that complexity depends on the intricacy and number of parts comprising a particular system. Thus, beetles are more complex than jellyfish, while the rain forest is more complex than the tundra. And, as if all this weren't amazing enough, we humans are adding ever greater complexity to the planet ourselves. But starting with the nonhuman world, what are we talking about when we discuss biodiversity?

COUNTING SPECIES

Whether animal or vegetable, species are the common currency in discussing biodiversity. Species numbers are the first statistic we look at when considering a habitat's diversity. Each species has a limited range, constrained by its own specific physiological requirements. While some species may range across a continent, others are confined to very specific localities. All together, they make up the biota that characterizes a particular site or region. But how do we recognize and characterize the biological species? A useful working definition must be able to identify populations in the field and also permit comparisons around the world. In general, members of the same wild species share genetic information by interbreeding with others of their own kind, and not with other species! Because they don't exchange genes with other species, individual species can be distinguished from closely related species. We'll discuss this in more detail in chapter 3, but the species does seem to be the most fundamental and realistic element in our hierarchy of taxonomic categories. Folk taxonomies often agree with their scientific counterparts, supporting the reality of our species concepts.² Studying birds in the Arfak mountains of New Guinea, Ernst Mayr found that the local people recognized 136 kinds; after careful study, he came to recognize 137 species. Most species do seem to be “real” and not just a matter of opinion. More importantly, the world really does have a lot of different species.³

Species are the bottom rung in the hierarchy of living things. Higher categories are more inclusive, representing a larger sample of the living world. We place a group of species, which are more similar to each other than to other species, into a single genus (plural: genera), similar genera into families, similar families into orders, and similar orders into classes. Finally, classes are placed under a larger category: divisions (for plants) or phyla (for animals). Plants are members of the kingdom Plantae, and animals are members of the kingdom Animalia. Fungi, bacteria, and various microorganisms are arranged in additional kingdoms. This is our taxonomic hierarchy, a way of cataloging and making sense of the natural world—a methodology that has proven immensely useful over these last two hundred years. Wolves, for example, are considered a single species and placed in the genus Canis. That genus, which includes jackals and foxes, is placed in the family Canidae, the order Mammalia, and is part of the phylum Vertebrata (animals with backbones). Each of these higher categories is a larger and more inclusive grouping. Though initiated before the notion of evolutionary change had been developed, these ranks were easily incorporated into a historical framework covering hundreds of millions of years. In an evolutionary sense, higher, more inclusive ranks (such as orders and divisions) are older groupings, comprising descendants from a more ancient time. The largest divisions under kingdom Animalia are the phyla, and they differ greatly from each other. Fish (Vertebrates) are very different from jellyfish (Coelenterates), and neither is closely similar to clams (Mollusks). These major phyla represent categories that separated from each other more than five hundred million years ago. Species, in contrast, are seen to be the most recent twigs on a large evolutionary tree. And it is species that we will focus on most often in this book.

Unfortunately, there's a problem in determining biodiversity by counting species: which animals or plants should be given priority in our enumerations? We have neither the time, the money, nor the ability to find and count them all. At the present time, around 1.8 million species have actually been described and published, far too many to handle in any reasonable survey.4 Efficiency requires that we follow the principle of least effort. Actually, we have really good numbers for only a few major lineages. Forget about insects; they're too numerous, too small, too poorly sampled, and too difficult to identify. Except for butterflies, trying to compare the numbers of insect species around the world is not currently feasible. We have no choice but to rely on the numbers of better-known creatures; for terrestrial surveys these are birds, mammals, reptiles, amphibians, and the higher (vascular) plants.

Birds are especially popular as indicators of diverse locales; they are an obvious choice for two reasons. First, birds have been intensely studied; of all animals, these are the ones we know the best. Being colorful, often noisy, and active mostly by day, they've gotten a lot of attention. Because many mammals come out only in the dark of night, they've avoided the scrutiny to which birds have been subjected. Plants, despite being conspicuous and stuck in the ground, present other challenges. Identifying higher plants successfully usually requires finding flowers or fruits and seeds—easier said than done. For some tropical trees, many years may pass before they flower and fruit. Desert floras can be similarly opaque: plants flower only after sufficient rainfall, being both leafless and flowerless over much of the year. With less than generous rainfall many desert species will not flower at all. Come back during the rainy season next year and try again. For these reasons, plant identification and enumeration require many years of continuing effort. Similarly, getting good lists of birds, mammals, reptiles, or amphibians can take years of extensive mist-netting, trapping, and observation. All told, finalizing species lists for local plants and animals requires years of effort. These efforts are continuing, with new discoveries being made on a regular basis. Recent work with Australian geckos and Madagascar's frogs have made clear that there are still many “unrecognized and undescribed” species to be added to our inventories.5

THE POWER BEHIND BIOLOGICAL DIVERSITY

Generally speaking, plant diversity is fundamental to biodiversity—and for two reasons. By transforming the physical energy of sunlight into chemical energy, plants support the local food chain. Living things cannot survive and reproduce without the timely acquisition of energy. Photosynthesis by plants, algae, and Cyanobacteria (once called blue-green algae) are the source of this energy. Within the sea, microscopic phytoplankton capture most of the Sun's energy, while on land, green plants are the foundation of the food chain.

In addition to capturing energy, larger plants provide terrestrial environments much of their structural architecture. Bird diversity, in particular, is strongly constrained by the variety of nesting sites, and such sites are dependent on the numbers of trees and shrubs. Also, with more varied plant species there will be a greater variety of fruits, seeds, and fresh green shoots, exactly what many insects, birds, and mammals are looking for. In most cases, the numbers of land animals and higher plant species will be closely congruent in any given habitat.6

Habitats are where biodiversity is found, and we will be referring to these as different biomes throughout the book. A biome is essentially the local or regional vegetation, together with all the other living things present within this community. Delimiting different biomes might seem arbitrary, but we do require means of describing different and contrasting communities around the world. Clearly, the vegetation of the Arctic tundra, the Sonoran desert, and the tropical rain forest are all very different. The vegetation that defines these biomes is marked by its stature, density, productivity, and its annual cycle of life activities. Many of the world's biomes also differ greatly in the species numbers they support, each constrained largely by the environmental stresses they experience. Discussions regarding biomes have also been framed in terms of life zones or ecosystems: areas of similar climate and vegetation structure in different parts of the world. Hiking up the side of a tall tropical mountain allows you to experience a variety of “life zones,” beginning from a hot and humid rain forest, through cooler montane cloud forests, and ending in treeless alpine meadows. Biomes, or life zones, are a major aspect of the living world, and we will discuss them in chapters 4 and 5. Setting the subject of biodiversity aside for a moment, let's reflect on diversity beyond our own illustrious planet.

DIVERSITY IN THE UNIVERSE AT LARGE

Variety, we've discovered, is almost everywhere we look. Stars vary greatly in size, luminosity, and even color, while galaxies come in a variety of shapes and sizes. A profound revelation during the last half of the twentieth century was finding unexpected diversity within our own solar system, thanks to interplanetary probes. No one had imagined that the larger moons of Jupiter and Saturn might be so different from each other. Europa, a satellite of Jupiter, seems to have a smooth covering of solid ice and might even possess a watery ocean deep within.7 Another Jovian satellite, Io, has a dramatically colorful surface, marked by frequent and violent volcanic outbursts. Likewise, the moons of Saturn come in great variety. Venus, where science fiction had imagined steaming jungles, proved to have a torrid surface baking under a suffocating atmosphere. Despite imaginative conjectures, subtly changing colors on Mars were not seasonal vegetation changes but dust storms occasionally sweeping across its frigid deserts. Even rocks falling from the sky—meteorites—vary in structure and composition. These many astronomical discoveries were concordant with something everyone already knew: history matters!⁸

Each of the major planets and their many satellites have had their own special history, and each is different on that account. Likewise, planet Earth has had its own unique historical trajectory. The Big Whack hypothesis claims that a Mars-sized object sideswiped Earth early in our planet's history. This impact added mass to Earth's interior, even as it threw debris high into orbit—debris that would condense to become our lovely Moon.⁹ Swinging around our planet every twenty-eight days, the Moon constrains Earth's axial gyrations. Our spin axis is tilted at an angle of about twenty-three degrees to the plane in which we orbit round the Sun. This tilt gives us our seasons, recurring each and every year. But without our Moon, Earth's axis might wobble back-and-forth by as much as forty degrees over millions of years. Thanks to a large Moon, our axis ranges over only two degrees. By constraining our wobble, the Moon gives Earth a more stable platform for terrestrial life. On the other hand, and as the Big Whack hypothesis suggests, the solar system was quite chaotic during its early history.

Craters covering the Moon and Mercury document an early period of asteroid and cometary collisions. Astronomers propose a “late heavy bombardment” (4.1 to 3.8 billion years ago), which may have been due to resonance perturbations, as the giant outer planets reordered themselves.10 This scattering of comets and asteroids throughout the solar system may have been another lucky break in Earth's history, peppering our planet with debris from the farther reaches of the solar system. Because the outer regions of our solar system contain more moisture than the inner, this early influx brought us lots more water! Without that precious liquid—in grand abundance—oceans would not encircle our globe, rain would not fall across broad continents, and we, quite simply, would not be here.

Even more intriguing is the conclusion that solar systems experience complex dynamics in their early history and that it's only a matter of good luck that we have the arrangement we do, with rocky planets close to the Sun and large gaseous planets more distant.¹¹ The discovery of “Hot Jupiters” closely circling other stars made it clear that orbital interactions can result in a wide variety of planetary configurations. Jupiter, in fact, may be another factor in Earth's vibrant biodiversity. By far the largest planet in our solar family, Jupiter circles the Sun in an almost perfectly circular orbit. A more eccentric orbit would have perturbed other planetary orbits and resulted in a very different solar system. Thanks to our largest planet being five times as far from the Sun as we are and having a near-circular orbit, our solar family has been stable for a very long time.¹² In addition, Jupiter and Saturn have served as vacuum cleaners, sweeping up errant asteroids and comets with their gravitation, reducing the likelihood of Earth being bombarded in more recent times.

A VERY FORTUNATE PLANET

It remains to be seen whether truly earth-like planets are commonplace or whether a unique series of events during the solar system's formation gave rise to our very special world.

—John Chambers and Jacqueline Mitton13

We are here because of many lucky breaks: fortuitous events that have contributed to making our planet so comfortable for life. To begin, Earth circles the Sun in what's been called the “Goldilocks Orbit” (or the Habitable Zone). In the children's fable that bears her name, Goldilocks, a very fussy little lady, insisted that her porridge be not too cold, not too hot, but just right! Our distance from the Sun keeps Earth's surface not too hot and not too cold—just as Goldilocks demanded. Thanks to a thin blanket of greenhouse gases trapping the Sun's warmth, water remains wet over most of our planet's surface. Just as important, Earth's orbit is near-circular, resulting in seasons that are not as severe as they would be with a more elliptic (eccentric) orbit. Temperate areas do have warm summers and cold winters, but these are caused by Earth's axial tilt—not the orbit's eccentricity.

Getting whacked by the proto-Moon is likely to have been another lucky break, giving us our twenty-three-degree tilt. This tilt has important effects. Even along the equator, the Sun's path across the sky moves southward from July to December, then northward between January and June. Aligned with the Sun, the intertropical convergence zone sweeps seasonal rains back and forth across the tropics each year. This is why tropical monsoons appear with such regularity at the same time each year. Without our axial tilt, tropical rains would fall continuously along only a narrow equatorial band. What Earth's wobble does through the year is to spread rain more widely across the tropics. This same wobble provides long days during the Arctic's short summer, warming tundra vegetation, sustaining forests in Siberia, and allowing wheat production in Alberta.¹⁴ Annual seasonality is a significant factor supporting terrestrial biodiversity, all thanks to a tilted axis.

Earth's size is also just right! A larger planet would have much stronger gravitational attraction, making agile terrestrial animals—such as monkeys—impossible. A larger planet would also trap a denser atmosphere, perhaps suffocating the surface under a Venus-like blanket. On the other hand, a smaller planet with weaker gravitation simply can't hold onto a sufficiently dense atmosphere, essential in sustaining active terrestrial life. And there's more. Earth's daily spin means that days do not become too hot, and nights do not become too cold. A faster spin would result in continuous gale-force winds, making the evolution of terrestrial vegetation unlikely. A slower spin would result in long, frigid nights, followed by long, torrid days. Under such conditions, marine life might be able to survive, but life on land would be impossible.

Earth is unique within our solar system in yet another fundamental way: we've got plate tectonics! Thanks to a planet rich in water, hydrated minerals—heated at depth—give our planet a dynamic crust. Beginning perhaps three billion years ago, the Earth's crust broke into plate-like sections. With new magma rising along fissures in the ocean floor and spreading laterally, continents are pushed around Earth's surface on giant plates. Slamming into each other, one plate must override the other, subducting the ocean floor while forcing mountains high into the sky. Mountains, as we shall see, are a huge boon to terrestrial biodiversity. Just as important, plate dynamics maintain stable continental platforms well above sea level. Parts of Canada, Australia, and Greenland have geology more than three billion years old. Continuing erosion should have worn down these surfaces a long time ago. Such eroded surfaces would have ended up around eight thousand feet below the sea—all around the globe. Surely, without plate tectonics we'd all be fish!

Plate tectonics has played another critical role as well. Carbon dioxide is absorbed by rain water, producing weak carbonic acid. In this form it reacts with rocks to form carbonates and can be used by marine animals to build their many shells. By removing carbon dioxide from the air, these processes sequester carbon at the bottom of the sea. That's bad news: carbon dioxide will become ever more rarified in the atmosphere and, without this greenhouse gas, the planet's surface might cool into a frozen ball. Enter plate tectonics! In regions where the ocean floor is subducted under advancing plate margins, sea floor sediments are heated at depth and volcanoes form, belching carbon dioxide back into the sky!15 By returning sequestered carbon back into the atmosphere, plate dynamics have been essential in keeping our planet warm and life-sustaining over the last three billion years. Plate tectonics helps maintain continents high and dry, builds lofty mountain chains, and reinjects carbon dioxide into the atmosphere. And there's more: by forming hot cauldrons beneath colliding plate margins, elements and minerals can become concentrated into rich ores. No other member of our planetary family has a similar process and none have concentrated ores in near-surface deposits. (“Mining” the Moon or Mars is fantasy.) Without plate tectonics we'd have little in the way of metallurgy.

The more we learn about our planet and our solar system, the more special they appear to be. Interstellar space is a near vacuum, with temperatures only a few degrees above absolute zero. Take a spacewalk without your spacesuit and you'll be freeze-dried on the spot. In contrast to the frigidity of space, star interiors burn at millions of degrees, even as their outer shells glow at thousands of degrees. In this grand universe, Earth-like habitats with moderate temperatures are very scarce indeed.

Most importantly, our Sun is not an ordinary star. First, the Sun is a solitary star, without a costar to bollix-up the regular orbiting of its planetary family. The Sun is also a larger star; more than 80 percent of stars are smaller and give off less energy. Keeping water wet requires that a planet must circle closer to a smaller star, and that's a problem! A closer planet is very likely to become bound to its star in tidal lock. With one side always facing its star, finding terrestrial life forms on planets circling smaller stars seems utterly unlikely. (Earth holds the Moon in tidal lock, which is why we always see the same face of our splendid satellite. Mercury and Venus are both in tight resonance with the Sun, revolving only very slowly.¹⁶)

Another pleasant astronomical detail is that because the Sun is a larger star it is more stable than many smaller stars. Our Sun came into being about 4,560 million years ago. A single solar hiccup could have put an end to life on Earth over these last few thousand million years. Lesser stars are not as uneventful; they flare up from time to time. In addition, our star orbits the crowded and explosive center of the Milky Way Galaxy at a safe distance. Who knows what havoc a nearby supernova explosion might wreak upon a biosphere? More than four thousand million years of calm stability have resulted in an ongoing scenario of ever-more complex life on planet Earth. Viewing ourselves from a broader astronomical perspective, we are the lucky inhabitants of a very rare and special place.

Many have claimed that, with billions of likely Earth-like planets, civilizations like ours must be common in our galaxy. However, the more we learn, the more unlikely that appears. SETI—the Search for Extraterrestrial Intelligence program—has been scanning the firmament for radio signals over more than forty years, and they have failed to intercept a single coherent message.17 More fundamentally, complex biological beings did not evolve to traverse the vastness of interstellar space; if interstellar travelers exist they'll be robots capable of “sleeping” over many thousands of years. Remember that stars are separated in distances measured by “light years” and, with light speed at 186,000 miles (300,000 km) in a single second, interstellar travel by living things remains a fantasy. Setting astrobiology (“the science without a subject”) aside we'll confine ourselves to the planet on which we live.¹⁸

OUR EVOLUTIONARY EPIC

Over the last five centuries, science has fashioned a grand history for our species, our planet, and the universe itself. Bringing all this information together has given us a coherent narrative across time. We call it the Evolutionary Epic. This scientific effort resembles earlier mythical epics, fashioned by cultures around the world, as people tried to make sense of the world around them. Though deficient in purpose and devoid of meaning, our scientific epic is extravagantly rich in detail, while ranging over enormous scales in both time and space. From within the nucleus of an atom to galaxies being flung ever farther apart, science has revealed a cosmos larger and more complex than anything imagined earlier.19

Though scientists may delight in so grand a historical narrative, many people find it unsatisfying. Lacking an explanation for the universe's origin, with little linkage to revered sacred texts, and seemingly without purpose, a great majority of people continue to find comfort in traditional religious narratives. But science is different; it is nothing more than a pragmatic way of trying to understand the world around us. While many scientists examine the real world through carefully controlled experiments, the origin and elaboration of biodiversity are historical questions. In these instances, we formulate historical scenarios and then seek evidence from nature to support or reject a given scenario. It's very much like detectives trying to solve a crime. In the late 1800s, Thomas Huxley hypothesized that humans originated in Africa for the simple reason that our closest relatives—chimpanzees and gorillas—live there. After more than a hundred years of effort, we now have evidence supporting Huxley's original conjecture. Fossils in the ground, as well as DNA within our bodies, indicate we humans originated in Africa.

Religious fundamentalists have responded to the evolutionary epic by claiming that the complexity of many biological systems can only be explained as having been “designed,” implying the direct intervention of God. While that's fine for most people, for science there's a problem. How can this general hypothesis be tested or evaluated? Furthermore, if God has used magical or miraculous means to fashion the universe, there is simply no way that we humans could comprehend the universe; we're no good at magic. On the other hand, if the “Creator” used the natural laws we see around us in creating our world, then science can hope to understand the world. If God sort of kick-started the universe and then gave it a set of rules and regulations (the strength of gravity, the speed of light, the strong nuclear force, natural selection, etc.) then we should be able to figure things out.20 Following this strategy, scientists of all faiths have found common ground in studying the world in which we live.

Science, as Nicholas Wade points out, “consists largely of facts, laws and theories. The facts are the facts, the laws summarize the regularities in the facts, and the theories explain the laws.”²¹ Evolution, whether of the planet or life forms, is paleontological fact. In contrast, Evolution by Natural Selection is a theory that helps explain the “design” we find in nature. Together, evolutionary fact and evolutionary theory have given us a single coherent narrative. And why not? There is only one real world out there! Meteoritic studies (using elemental isotope decay rates) indicate that the solar system is about 4,560 million years old. This long history has provided a wealth of time for life to develop and proliferate. Paleontology gives us a picture of Earth history that coincides with times determined by isotope decay ratios within cooled and solid lava. In addition, fossil-bearing sedimentary rocks record how life has changed over time, expanded grandly, then fallen back several times, only to recover and form even more diverse biotas. Indeed, the geological time scale is linear and consistent around the planet. Mastodons and humankind coexisted during the recent ice ages; neither was present when dinosaurs ruled the world. Similarly, the great coal measures of the Carboniferous period had roaches, dragonflies, and a few amphibians, but neither dinosaurs nor mammals occur in these more ancient deposits. Biblical fundamentalists claim that an enormous flood carved out Arizona's Grand Canyon, but they cannot explain thousands of feet of canyon walls, successively layered with geological strata. Nor can they explain why the lowermost strata at the Grand Canyon are completely devoid of animal fossils, while the uppermost contain dinosaur fragments. Today there is no question: our planet's long history has provided the time, the lucky breaks, and the many opportunities—along with a few calamities—to produce the exuberance of living things we see today.

BIODIVERSITY: ON LAND AND IN THE SEA

A surprising fact regarding today's biodiversity is that the numbers of species of plants and animals living on land far outnumber species living in the oceans.22 Estimating the world's habitable volume, from the deep ocean to the highest mountains, we find that over 90 percent of that volume lies within the sea. Why might such a large volume have fewer species? The oceans do have a great diversity of animal phyla, from jellyfish and a variety of worms to crustaceans, mollusks, fish, and plant-like bryozoans. Life began within the sea and, obviously, that is where the greatest number of ancient plant and animal lineages are to be found. Green algae, brown algae, red algae, diatoms, and many microscopic forms are all abundant in the ocean. These lineages represent divisions of great age. This is diversity at the highest ranks: animal phyla and plant divisions. However, diversity at the lowest rank—species numbers—presents a completely different picture. Insects alone number around 800,000 described species, far outnumbering the 250,000 described plant and animal species in all the oceans! Marine biologists contend that there are many unrecognized species in the sea, and that may be correct. However, we needn't worry ourselves about such marine life, mostly microscopic and still waiting to be described. Instead, this book will concentrate on the numbers of larger terrestrial creatures.

Though land areas amount to only about 29 percent of the Earth's surface, scientists estimate that a bit more than half of the world's photosynthesis takes place on land.²³ Photosynthesis by green plants, various algae, and blue-green bacteria captures the energy of sunlight to build energy-rich carbohydrates and other foods. Whether on the land or underwater, photosynthesis keeps 99 percent of the living world from running out of gas.²⁴ Energy is the fundamental currency of life. It makes no difference if you're a bacterium or a ballerina, when you run out of energy your life has ended. Despite the huge volume and large surface areas of ocean water, the ability of photosynthesis to capture sunlight and absorb carbon dioxide is limited in marine environments. There is insufficient light for photosynthesis in deep water, and many areas of the ocean are poor in nutrients. Worst of all, there are hordes of animals, large and small, ready to devour those who photosynthesize within the sea. Also, a vast majority of marine photosynthesizers are microscopic and short-lived; sea grasses, larger kelp, and other algae are uncommon. Neither the Cyanobacteria nor microscopic algae can provide the physical and nutritional support afforded by trees, shrubs, and grasses. All told, estimates of biomass (the weight of living matter) in the oceans range from five to ten petagrams (5 to 10 billion metric tons). This contrasts with around 560 petagrams for biomass on land. That's perhaps an eight-fold difference, and it is a major reason for greater biological species richness on land.25 What's more, Vaclav Smil estimates that standing phytomass (plant biomass) on land may be as much as two hundred times greater than phytomass within the sea.²⁶

Biodiversity on land, however, is not simply a product of greater photosynthetic potential. Terrestrial habitats vary from frigid polar regions and parched deserts to evergreen tropical rain forests. They range from mangroves along tropical sea coasts, sub-desert thorn bushes, a grand variety of forests, to mountain-top grasslands. Terrestrial ecosystems are both hugely diverse and they differ profoundly in how stressful they are for their living occupants. You won't ever dry out if you spend your life living in the sea, but desiccation is a serious problem in most terrestrial environments. Also, temperatures vary over only a modest range within the world's oceans. Marine environments offer nothing comparable to the stress of living on a Midwestern prairie through the year, where wintertime temperatures can plunge to –30° C (–20° F) and summertime temperatures can reach 40° C (102° F). Clearly, the challenges of living on land are far more varied than those within the sea. Terrestrial substrates are diverse as well, ranging from rich loams and deep mud to clay, loose sand, and bare rock. In fact, the soil itself may house thousands of bacterial species in a single gram.²⁷

Another factor promoting terrestrial biodiversity is that many land surfaces are separated by broad oceans. Such isolation has resulted in distinctive land biotas on different islands and distant continents. Fish can be very diverse around a coral reef, but there is the potential of sending their little ones off to join another coral community a thousand miles away. In contrast, few rain forest inhabitants can journey across a hostile ocean. The Congo rain forest may look like an Amazonian rain forest or one in Asia, but a closer look will reveal the many different species and genera comprising these widely distant biomes. A terrestrial world, divided by oceans large and small, and rich in environmental variety, has produced an astounding number of plant and animal species.28

Not only is there isolation between land masses separated by the sea, there can be isolation within the larger land areas themselves. If you are adapted to living on a mountain top, your offspring might not be able to reach another, distant, mountain top. Likewise, if you are a freshwater fish living in a river on the west side of a mountain range, it is unlikely your offspring will cross that range to swim in rivers on the eastern side. Of the approximately 31,000 species of fish gracing our planet, a full third live within fresh water, even though fresh water makes up only 1 percent of the water on our planet. The Amazon basin is estimated to have as many species of fish as live in the entire Atlantic Ocean! With so much diversity on land surfaces, and since we are creatures of the land ourselves, we will focus on terrestrial biodiversity and our own history.

This book celebrates a simple fact: our planet has become increasingly complex over time, whether with geological dynamics, continually evolving life forms, or new human technologies. We will explore the many factors that have propelled this extraordinary epic forward over the last four thousand million years.

To better appreciate our planet's biological wealth we'll address a number of questions. To begin, we'll examine the insects and try to understand what has made them so successful. In chapter 2, we'll ask how bacteria fit into discussions of biodiversity, and why larger, nucleated cells were so important an advance in the history of life. For many readers this may be a bit of a slog, but living things are complex and we need to understand basic processes in our quest to understand life's forward momentum. In chapter 3, we will discuss the factors actually generating new species.²⁹ Chapters 4 and 5 will survey species numbers around the globe, look for general patterns, and try and understand why some regions harbor unusually high numbers of distinctive biotas. In chapter 6 we'll tackle the question of how so many species manage to live together in the same biome at the same time.

Shifting from contemporary ecology, we'll turn to paleontology in chapter 7, examining the history of biodiversity and increasing complexity over the last 600 million years; in chapter 8 we will examine the factors driving these ongoing trends. In chapter 9 we'll discuss the origin of what we like to think of as nature's most complex organ: the human “mind.” With this device, and after forming effective symbioses with a few plants and animals to better feed ourselves, the stage was set for explosive cultural advance. Chapter 10 will review how settled human communities increased their technological prowess, and chapter 11 will review the four billion years that have led to human dominion. Our last chapter will examine how expanding technologies and our increasing numbers have become so transformative for the biosphere—and so problematic. Beginning our journey, let's start with a really successful crowd: beetles, the six-legged kind.