Biologists know of ~1.7 million different kinds of plants and animals, including microorganisms, now making a living on planet Earth. This number of species includes a broad range of current, non-extinct life all around us, from tiny insects and pesky weeds to giant whales and redwood trees. And since new life forms are constantly being discovered, especially among myriad microorganisms not yet found, the total number of living species could approach 10 million. Yet even with this enormous variety of extant life presently on Earth, as noted earlier >99% of all species that have ever existed are now extinct—a number still rising as biodiversity falls under the onslaught of 21st-century society.
The fossil record narrates the following history of relatively recent life on our planet: <1 billion years ago, multicellular organisms, having learned to utilize oxygen, quickly evolved into highly specialized creatures. These oxygen-breathing animals—the remotest ancestors of humans—swarmed in the sea, feeding on plants and on one another. Some could only float on the water, others anchored to undersea slopes, while still others had some mobility within the water and on the sandy bottom. Almost all the beings alive between 0.5-1 billion years ago had soft bodies. Hence fossil findings of the earliest respiratory organisms are understandably sketchy, for without bones or shells, few of them have remained intact to this day.
As the years wore on, life-styles multiplied and rapidly so with the passage of time. Each type of organism responded to changes in the oceanic, continental, and atmospheric environment. Each attempted to adapt for better survivability. By ~600 million years ago, tubular, soft-bodied fauna, such as the humble, long-extinct, Ediacaran worms (whose fossils now notably dot Australia and Newfoundland), ruled the world.
Suddenly and dramatically, yet for reasons only partly known, the fossil record of ~0.5-billion years ago erupts in numbers and diversity of species. Perhaps the reason was the establishment of a thick enough ozone layer or the efficient operation of the photosynthesis-respiration cycle, or maybe skeletonization began leaving behind hard evidence of soft-bodied animal predecessors. The cause is uncertain but the effect is clear: Whereas previously only rather simple life forms prevailed, in a relatively short period of a few tens of millions years—a mere geological moment—a bewildering array of new and more complex forms are seen among the fossils. Accordingly, paleontologists mark this “Cambrian explosion” as the start of a whole new age in the history of life on our planet—“biology’s big-bang,” best revealed perhaps by the plethora of novel fossils of the Burgess Shale in the Canadian Rockies and of the Chengjiang fauna in southern China's Yunnan Province.
For ease in reference, all Earth times older than ~0.5-billion years are called simply the Precambrian, while all times since are subdivided into more recent ages: The Paleozoic (Greek for “ancient life”) began ~540 million years ago, the Mesozoic (“middle life”) ~250 million years ago, and the Cenozoic (“recent life”) ~65 million years ago. These relatively recent time intervals delineate three great waves of animal advancement in the evolutionary history of life on Earth.
TABLE 6.1 summarizes these major ages in the geological history of planet Earth. As noted, each age is sometimes divided into periods, and for the most recent timespans these periods are further subdivided into geological epochs.
Paleozoic The Paleozoic fossil record shows that the oldest known fishes originated >500 million years ago. At first, small and jawless yet with bodies completely covered in bony plates of armor, they were among the forerunners of true vertebrates. They probably dined on seafloor invertebrates while using their external skeletons to defend against predators. Soon thereafter, minute spores of plants appeared on the land nearly 500 million years ago; megafossils of land plants themselves enter the record ~50 million years later as algae adapted to drier environments. These were followed by the first insects and amphibians ~400 million years ago, and the first forests and reptiles ~50 million years later. Over the course of 200 million years or so, not only had life greatly multiplied and diversified in the sea—tens of thousands of marine species are documented—but it had also spread from its oceanic nursery. As depicted in Figure 6.10, life had begun to come ashore and to colonize the land.
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FIGURE 6.10 – The Paleozoic Age saw many forms of life flourishing on planet Earth. In this artist’s conception, some life ekes out a sparse living—mostly in the sea, such as the trilobites and sponges on the ocean floor and the jellyfish-like creatures nearer the ocean surface. Yet, as also suggested by this painting, simple life forms were beginning to make their way onto the land. (Smithsonian) |
Naturally, as with all frontiers in science, some uncertainty (and often controversy) prevails—the sign of a robust, active subject, frankly. In this case, the most elementary, microscopic forms of life may have actually landed much earlier than 0.5-billion years ago. Unusual carbon-rich clays, but no fossils, have been found in ancient South African rock layers that are surely 1 and possibly even >2 billion years old. Just as carbon deposits at a few Greenland sites dating back nearly 4 billion years imply a possible origin for life earlier than the oldest fossils of ~3.5-billion-year age, bacterial landlubbers may have been reluctantly stranded on ancient lake shores or tidal flats earlier than at the start of the Cambrian, several hundred million years ago.
Sea plants, such as unicellular algae and then mosses, were probably the first life forms to migrate from the water and along previously barren, rock-strewn coasts—an act of heady exploration of an alien landscape not too unlike humans later exploring the Moon. Animals that depended on this vegetation then apparently followed their source of food, among the first of the air-breathers being the insect scorpions. Some early fish may have crept ashore intentionally, whereas others likely washed up onto beaches during storms or were left high and dry during droughts. Those species of fish able to use their fins to successfully negotiate the land became four-legged, air-breathing amphibians; those that tried and failed became extinct. Others became reptiles, which prospered among a bewildering variety of flying bugs that had a field day exploring new spaces. The result was an explosive invasion of the land well prior to the breakup of the ancestral supercontinent of Pangaea. Details are sketchy because geologists have no clear record of where shorelines were so long ago, what the climate was then like, and just how fast the environment was changing at the time.
The fossil record does provide incontrovertible evidence for an evolutionary trend: Descendants of those first shore plants became the world’s first forests, and certain descendants of the amphibians eventually became the animals that lived in those forests. Some of that fossil record is richly detailed beyond our imagination. Who would have predicted that nearly half of all fossils would be trilobites, lobster-like creatures of which the horseshoe crab is the closest living relative? Shown in Figure 6.11, some trilobite species had heads, some apparently not; others had a dozen eyes, still others none at all. Most were quite small, measuring a few cm in length, yet some stretched 0.5 m from head to tail. Though all trilobites have been extinct for the past 250 million years, paleobiologists are reasonably sure that some version of them gave rise to most of today’s animals.
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FIGURE 6.11 — A fossil of a trilobite, an enormously widespread invertebrate organism ~500 million years ago. They appear in the fossil record suddenly and with great diversity at the time of the Cambrian “explosion,” yet show no trace of earlier, ancestral forms. Most fossils found today are trilobites, much like this one that measures ~0.3 m (or a foot) in length. All trilobites are now extinct, though horseshoe crabs (and probably lobsters) are among their descendants. (Smithsonian) |
Fossils also record when, and sometimes how, many of the features regarded as important evolutionary advances arose, including true bones, paired appendages, and cartilaginous jaws. Bone itself was a new material, needed later to support body weight when animals left the water. Made of cartilage mineralized largely with calcium, bone is nearly as solid, yet more flexible, than cast iron. And not just spinal cords had come forth, but gills too, allowing rudimentary lungs to filter water and remove dissolved oxygen to power these aquatic newcomers. Not least, jaws served as the impetus for many of the sophisticated vertebrate qualities that followed. Whereas invertebrate fauna, such as worms and snails, ingest organic particles in mud for food, small fishlike creatures that evolved jaws had a distinct advantage beginning not quite 500 million years ago: Their jaws allowed a greater intake of energy by biting off larger pieces of other organisms. And their offspring eventually led to most vertebrates on Earth today, including human beings.
Once again, much as for many advances along the arrow of time from the early Universe to humankind itself, a general trend is evident whereby increased energy use (per unit mass) parallels the rise of complexity. Evolutionary biologists don’t normally think in terms of energy expenditures, but when attempting to unify the natural sciences—a central feature of this Web site—energy is a powerful concept, as noted toward the end of the CHEMICAL EPOCH.
By the end of the Paleozoic Age (Figure 6.12), life was firmly implanted at sea, on land, and in the air. Some 250 million years ago, with Pangaea still in tact, there existed a broad opportunity for living. The land in particular, with its green expanse and virgin forests, enabled animal life to proliferate with astonishing diversity. Species multiplied rapidly, so much so that the fossil record documents, just to give one homey example, nearly 1000 different kinds of roaches coexisting at the time. The household version—the common cockroach—is a direct, and very durable, survivor of the late Paleozoic.
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FIGURE 6.12– This painting captures a scene toward the late-Paleozoic Age. Life had diversified and become more robust—as depicted here both by the variety of (now extinct) fish and also by an increased presence of plants on the land. (Smithsonian) |
All life on Earth had become dominated by the cold-blooded reptiles, a whole new life form that had, over millions of years, evolved from vertebrate amphibians. The conquest of the land was complete, as reptiles spread out to fill every conceivable niche on the planet. As ancestors of nearly every animal now on Earth, the reptiles of 200 million years ago had developed supple backbones, mobile legs, and keener brains than any other creature inhabiting Earth until that time.
Mesozoic The Mesozoic fossil record shows that many forms of life not only thrived but also evolved toward greater complexity. As depicted in Figure 6.13, plant life flourished, diversifying wildly while taking additional steps toward its currently existing 0.5-million green species. Simple angiosperms appeared in dazzling colors and rich scents (though true flowers much later), all for the purpose of attracting pollinating insects. And the first birds took flight, most as small as today’s sparrows.
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FIGURE 6.13 — The Mesozoic Age saw a continued increase in the diversity of life forms especially among the land plants, and not least the first appearance of the mammals—all of which, however, were completely dominated by the dinosaurs such as the T. Rex rendered here by an artist. (Smithsonian) |
A—perhaps the—highlight of the Mesozoic Age was the first appearance of the mammals, warm-blooded animals able to derive body heat from digested food and thus stay comfortable in cold environments. Fossil evidence reveals that three types of mammals originated midway through this nearly 200-million-year period, partly because of widespread environmental change caused by the breakup of the giant Pangaean landmass that predated the modern continents as we know them. The earliest mammals, having the size and weight of a paperclip, were probably ancestors of the present-day anteater and aardvark—primitive creatures that had fur and nursed their young with milk, but, like reptiles, laid eggs instead of bearing live young. Another, more advanced group of mammals probably bore their young live like their descendants, the modern kangaroo and the koala bear. These young were so small and immature, however, that they had to be incubated in a fur-lined pouch under their mother’s belly. Toward the end of the Mesozoic, true mammals appeared, laying no eggs at birth and needing no pouch for their young. Though the last 65 million years is often popularly called “the age of mammals,” their Lilliputian ancestors were clearly occupying many ecological niches well into the Mesozoic, possibly as long ago as 200 million years.
This outline aside, details of the mammals’ line of ascent during the Mesozoic Age are somewhat obscure, as they were completely overshadowed by the mightiest reptiles of all time—the dinosaurs. Nothing at all like the snakes, lizards, or crocodiles of present times, in their prime ~100 million years ago the dinosaurs roamed the Earth with skill and power, overrunning the air, land, and sea until they completely and devastatingly dominated our planet. Taking their name from the Greek words deinos (terrible) and sauros (lizard), these monstrous beasts were mostly house-sized land creatures often weighing as much as 25 tons. Their relatives included awesome seagoing reptiles capable of swallowing today’s great white shark in one gulp and fearsome airborne brutes having wingspans comparable to those of modern fighter aircraft. Their fossils have been uncovered on all the world’s continents, except at the poles.
The popular, stereotyped dinosaur was downright dumb—cold-blooded and small-brained. In chilly climates, or even at night, the metabolisms of such huge reptiles would have become sluggish, making it hard for them to move around, secure food, and thus survive. However, a new and controversial view has recently been embraced by some leading paleontologists. Studies of dinosaur fossils suggest that many of these monsters might have had large, 4-chambered hearts, like those of mammals and birds. Such a heart could have pumped blood through organs, enabling the dinosaurs to sustain a high level of physical activity. If these revised interpretations are correct, some dinosaurs were probably warm-blooded and thus relatively fast-moving creatures, even after dark. What's more, though the dinosaurs clearly had small brains compared to those of today’s mammals, they were still smart for their time. Indeed, no species able to rule Earth for >100 million years could have been too dumb. By comparison, humans have thus far dominated for hardly a few million years.
All the flying, swimming, or landlocked prehistoric predators disappeared with bewildering abruptness near the end of the Mesozoic Age. The presence of the dinosaurs simply vanishes from the fossil record. No one is certain why, nor does anyone know how the mousy mammals were able to survive throughout the ~150-million-year reign of terror when these beasts were overwhelming and the mammals were scared. Whatever the cause of their demise, it affected not only the dinosaurs but also many other forms of life. Fossil records demonstrate that, ~65 million years ago, nearly half of all plants ceased to exist, including >80% of plant species in present-day North America. Most mammals, reptiles, and birds also then perished; fossils of all animals larger than ~20 kilograms (~40 pounds) are absent from geological layers for tens of millions of years thereafter.
Dinosaur Failure Explanations abound for the dinosaurs’ complete and total extinction. Devastating microbial plagues, magnetic-field reversals, sea-level changes, as well as deep volcanic eruptions and severe climatic shifts, any and all of these perhaps triggered by asteroid impacts or supernova explosions, have been proposed. Each of these ideas has some merit, though none is entirely convincing. Out of seeming desperation, some researchers even joke that the dinosaurs died of constipation, since a large variety of oily plants on which they likely feasted also became extinct at about this time.
Currently, the most popular idea is that the dinosaurs rapidly expired because a huge extraterrestrial object collided with Earth ~65 million years ago—another obvious and dramatic interaction of astronomy with biology. Studies of impact cratering on the Moon do imply that ~60 meteorites >3 km across have probably hit the Earth since the start of the Cambrian ~550 million years ago. Most of the direct evidence has eroded away on Earth, but it’s there for all to see on the weatherless Moon, and if the Moon were belted then the more massive Earth must have been bombarded at least as much. Even the smallest of those collisions would have released a blast of kinetic energy equal to that of >106 nuclear bombs. So the notion that asteroids have belted our planet—and could have caused biological upheaval—is not as far fetched as widely thought a generation ago. In an about-face in scientific circles, we now realize that Earth is under a regular barrage by foreign objects in the cosmos. It’s the natural scheme of things for a Solar System littered with debris from its formative stages.
In the specific case of the dinosaurs, a large, ~10-kilometer-wide asteroid or comet almost surely struck the Earth, causing great quantities of dust (mostly its pulverized self) to become airborne. The dust, in turn, reached the altitude of the jet stream, encircled the planet for several years, darkened much of the atmosphere, shut down photosynthesis, and disrupted the base of the food chain by killing off many plants. A thin horizontal band of reddish clay enriched with the rare element iridium found sandwiched between layers of 65-million-year-old ancient limestone (one from the Cretaceous and the other from the Tertiary geologic periods) is the main piece of evidence supporting the asteroid-impact idea. Although rare on Earth’s surface (since most of it long ago sunk to the planet’s interior), the iridium in the clay is hundreds of times more abundant than in native crustal rock, yet matches levels found in meteorites. This idea is not without problems, however: If the iridium then “rained down” out of the atmosphere, why is its abundance so highly varied from place to place on Earth’s surface?—unless the highest iridium content is pointing toward a probable impact zone, seemingly somewhere in the Americas. Where is the crater left by the impact?—though a good candidate is the so-called Chicxulub crater buried under ~1 km of partly submerged sediment on the north coast of what is now Mexico’s Yucatan Peninsula. And if instead the asteroid landed in the ocean—the most likely scenario given that ~¾ of Earth’s surface is water—then how did it uplift dust and debris high into the air? Perhaps, argue some opponents, the iridium-enriched clay was laid down by upwelling volcanoes and had nothing to do with an extraterrestrial impact. Or, perhaps the blast of an impact also prompted intense volcanism, in which case they both wreaked havoc.
For whatever reason the dinosaurs perished, environmental change of some sort, and probably dramatically so, was responsible. It would be useful to continue to seek the cause of their extinction, for there’s no telling if sudden, global change might strike again. As the dominant species on Earth, we are the ones now poised to perhaps lose the most.
Despite our knowledge of dinosaurs, no human ever saw one alive—unless today’s birds are an evolutionary offspring of the dinosaurs, as some paleontologists now think. Those movies showing hulking dinosaurs terrorizing cave dwellers (let alone modern cities) are simply wrong. Dinosaur remains lay hidden for ~65 million years before Homo sapiens discovered their prior existence less than two centuries ago. Our great-great-grandparents and all those who preceded them knew nothing of the dinosaurs. Ours is the first generation to realize that we probably wouldn’t be here had these great hulks not gone extinct. Only when the dinosaurs disappeared did the spectacular rise of the mammals—including human beings—begin.
Cenozoic The onset of the Cenozoic Age, ~65 million years ago, saw the appearance of an almost entirely new cast of characters. The huge landmass of Pangaea had fully broken up into continents nearly familiar to us now. The dinosaurs were completely gone, along with nearly ¾ of all life on Earth to that time. The earlier reptilian dominance over the mammals had been totally reversed. Flowering plants bloomed widely. Fruits first grew upon the landscape. And the planet had returned to its pre-dinosaur, Paleozoic tranquility. Clearly, the mammals had taken over the world, although they had apparently done so by default. In a certain sense, the meek had indeed inherited the Earth.
Fossils as recent as ~50 million years old show that most mammals had small brains, large jaws, clumsy and inefficient feet and teeth. None was larger than today's squirrels, and most had evolved eye sockets, adapted to night vision. Life wasn’t too tough, though, as those fossils show that they freely multiplied, swelling in numbers and diversity. As always, change was rampant. Ice ages came and went; continents split and drifted. In generation after generation, life forms constantly fine-tuned their daily routines for better survivability. Accordingly, many of these early mammals passed into extinction, to be replaced by better-adapted stocks.
In a relatively short time, the mammals had evolved into an amazing assortment of creatures. About 40 million years ago, the ancestors of such modern mammals as the horse, the camel, the elephant, the whale, and the rhinoceros, among others, gradually ventured forth, though often in shapes and sizes nearly unrecognizable compared to their contemporary descendants. Most of these life forms, in turn, improved their overall performance between 40 and 20 million years ago, converging by means of seemingly endless changes toward the multitudinous, yet threatened, biodiversity of flora and fauna seen around us today in the 21st century.