chapter07

Meteorites are rocks that have, quite literally, fallen from the sky. Using the isotopic ratios of a variety of elements within these meteorites, astronomers estimate that our solar system formed around 4,560 million years ago (mya), and that our planet solidified just a little later.1 Those must have been perilous times, as a solar system full of cosmic debris slowly organized itself into the stability we enjoy today. Craters covering both the Moon and Mercury are a reflection of heavy asteroidal bombardment in those early times. In fact, some planetologists claim that the giant gaseous planets, Jupiter and Saturn, with their significant gravitation, have vacuumed up a lot of solar system debris over these last four billion years. Larger comets and asteroids are still out there, some with erratic orbits that threaten us, but they are few and far between.

Astronomers believe that bombardment by comets and asteroids was especially intense around four billion years ago, bringing with them much of the water that makes our planet so distinctive. Just as significant, a large impact may explain our Moon's beginnings. According to the Big Whack hypothesis, a Mars-sized object collided with our planet to become one of the really lucky breaks in Earth's history. Thanks to this calamity, Earth ended up with greater mass and a more powerful magnetic field, a field acting both to deflect energetic particles fired at us by the Sun, and reducing the erosion of gases from our outer stratosphere. Best of all, by restraining our planet's axial wobble, our large satellite has given us more stable terrestrial climates.

IN EARLY TIMES

The early history of our planet has been divided into three main stages, called eons. The earliest is the Hadean Eon: 4,560 to 4,000 mya. This was indeed a hellish time, as our solar system was still convulsed by meteoritic mayhem. The second stage is called the Archean Eon: ending around 2,500 mya, when iron deposits began precipitating in oceans around the world. The third was the Proterozoic Eon (before animal life): 2,500 to 542 mya. The oldest rocks at the bottom of the Grand Canyon reach down into the Proterozoic; they bear no fossils of larger living things. Over the first 4,000 million years of Earth history, our planet gives no evidence of larger plants or animals. But there are larger objects, probably constructed by bacterial activity, in those ancient times.

The first larger fossil structures formed by living things are called stromatolites. Quite common in the early fossil record, they are rare today; found only in a few shallow tropical marine embayments. Stromatolites are often rounded cushion-like or columnar rocky structures, generally one to two feet (30–60 cm) in diameter and a few inches to six feet (10–200 cm) high. They can also form as flat layers in shallow water. What makes them distinctive is their thin, laminated structure. In living examples, microscopic layers of bacteria and algae live near the surface, trapping sand and debris in their gelatinous membranes. With time, new layers are formed and trap more debris, creating a layered cross-section. The earliest stromatolites are dated to about 3,500 mya.

Many geological processes can give rise to laminated structures. Cone-like stalagmites often have a layered structure due to chemical changes in the water from which they are formed. Wave action along shorelines can also produce thin-layered strata of differentiated sediments. However, the lamination within fossil stromatolites suggests they were built by thin layers of bacteria, together with trapped sediments. Rare today, they are found only in warm, highly saline water, where aquatic grazing animals are absent. Stromatolites, however, were common along shallow tropical sea shores around the world—until the advent of predatory animals.

Fossil evidence of larger organisms is first found around 560 mya, in Ediacaran times (635–542 mya).2 These organisms are mostly leaf-like devices, attached to the ground at their stalked-base. Others are tubular, and some are radial with smooth edges, resembling pancakes. Others had parallel transverse ridges, like little sleeping bags. Most are the size of your hand, but a few leaf-like forms grew to a meter (3.3 ft.) in height. None were more than an inch (2.5 cm) thick. None had external apertures or appendages. Clearly they couldn't eat anything and appear to have grown by adding segments at one end. More importantly, they were immobile, having left no trails. Also, there is no evidence of anything having taken a bite out of them. Numbering around fifty well-characterized species at this ancient time, I suspect that they were autotrophs, nourished by Cyanobacteria enclosed within transparent jellyfish-like tissues. Found only as impressions in ancient rock, we have no clue to their internal or cellular structure.³

Many Ediacaran sites were inhabited by numerous individuals of only a few species, just as is the case in some cold-water habitats today. A cold-water habitat may also explain why these fossils seem to show little bacterial degradation. Indeed, the world before Ediacaran times had suffered massive glaciation. Geologists believe that land surfaces had congealed into a supercontinent around a billion years ago, called Rondinia. Blocking ocean currents, this grand landmass caused our globe to suffer two major ice ages. Some even suggest that the most severe of these episodes resulted in a planet covered with ice: the Snowball Earth episode. For living things, this would have been a harrowing time, surviving only in the ocean's warmer depths. Worst of all, an ice-covered Earth, reflecting warm sunlight, might have remained frozen. However, with volcanic activity injecting CO₂ into the atmosphere, the planet warmed again, thanks to plate tectonics!

The enigmatic Ediacarans were followed by a period of small, fragmented shelly fossils. This period, around 545 mya, is when fossil trails and burrows first became common. Such was the fossil evidence for larger life forms during these early times. Then, quite suddenly, life propelled itself forward.

ANIMALS ARRIVE: THE “CAMBRIAN EXPLOSION”

There is nothing like the Cambrian until the Cambrian.

—Andrew Knoll4

Between about 542 and 530 mya, the ocean world exploded with larger forms of life. As just noted, rocks around 545 mya exhibit an increase in burrowing activities and trace fossils, together with a mix of small, shelly fossils. Then, about 535 mya came the Cambrian Explosion itself. The earliest stage includes the mollusks, brachiopods (lamp shells), and echinoderms (starfish and sea urchins). By 530 mya, the fossils include trilobites and the ancestors of animals, which would later give rise to scorpions, horseshoe crabs, and fish. Within a period of less than ten million years, the rocks suddenly include the remains of many animals bearing heads, tails, legs, spines, eyes, and other useful features. Most of these new creatures left an indelible mark in the record of the rocks for a very simple reason: they had a hard, resistant covering. Surely a crowd of hungry neighbors was the reason for growing a tough fossil-friendly epidermis.

Most surprising, the Cambrian explosion of animal fossils features a wide range of animals with fundamentally different body plans. There are no “proto-animals” that gradually diversify into more varied forms. Rather, the Cambrian explosion is marked by an array of very different creatures from its very beginning. Both coiled and clam-like mollusks, a variety of worm-like animals, segmented trilobites, primitive crustaceans, chelicerates (ancestors of spiders, mites, and horseshoe crabs), starfish-like echinoderms, and weird forms with no modern counterparts are all here and all together. As the Cambrian curtain rose, the stage was already populated by a variety of differing lineages.

Paleontologists have been puzzling over the mystery of this sudden and dramatic event for over two hundred years. The Cambrian period had been preceded by extensive ice ages, perhaps even a Snowball Earth, and the enigmatic Ediacarans.⁵ However, this was well before the explosive appearance of so many different forms of animals appearing in grand synchrony at the beginning of the Cambrian. The discovery of minute animal fossils, less than a tenth of an inch (2.5 mm) in size, in rocks around 560 million years old, indicates that many ancestral lineages were already there—but they were miniscule! Why might these different lineages have expanded in size during so small an interval of time?

The powder—as represented by the little fossils—was there; but what lit the fuse? What caused the sudden increase of size in separate animal lineages at this one moment in time? It makes sense that if the Earth had gone through a severe cold period, only the smallest forms could survive, protected in small areas of warmth within the ocean. But why suddenly diversify and expand between 542 and 530 mya? Only one scenario makes sense: a major change in the world's environment allowed different animal lineages to become larger at the same period in time. Whether these creatures built their hard parts of carbonates or silicates or phosphates, all appeared during this same short time interval.

The simplest and most plausible theory for the Cambrian explosion claims that life itself had changed the world. Beginning much earlier, blue-green bacteria had been busy splitting water in photosynthesis. Perhaps a billion years later, eukaryotic algae added their efforts to this life-affirming activity. Using the hydrogen atoms of water to fix carbon and build energy-rich compounds, photosynthetic Cyanobacteria and algae released free oxygen into the atmosphere. Ongoing oxidation reactions with various other elements would purge the atmosphere and oceans of free oxygen over many millions of years. However, sometime around 2.5 billion years ago, oxygen levels began to rise.6 A striking aspect of our planet's crust is that nearly all the world's banded-iron formations were deposited between 2,500 and 1,800 mya. Thanks to an atmosphere rich in carbon dioxide, huge amounts of ferrous iron were held in solution in the oceans. As free oxygen became more abundant, ferrous iron reacted with oxygen to precipitate as ferric iron deposits in a grand “rusting” of the planet! Today these ancient banded-iron formations represent about 90 percent of our industrial iron reserves.

AN EVOLVING ATMOSPHERE

Earlier, the Earth's geology was mostly gray, but now, with oxygen doing its work, reddish rocks give evidence for “The Great Oxidation Event” around 2.2 billion years ago. Life itself had begun to change the atmosphere. Nevertheless, oxygen remained at very low levels, resulting in what paleontologists call the “boring billion” years. It wasn't until between about 635 and 548 mya that isotopic ratios in sulfur compounds give evidence that oxygen concentrations were rising to modern levels.7 Oxygen pressure, in all probability, was the key factor in having so many different kinds of larger animals all show up in the record of marine fossils at the same time. Powered by oxygen-consuming respiration, larger animals require more oxygen. It was not until oxygen became a larger portion of the atmosphere that larger active animals were able to evolve. Here was the most sudden escalation of complexity in the history of life. An earlier world, devoid of larger animals, was now alive with nearly all the major animal phyla we know today.

The Cambrian explosion may have been the opening salvo in the proliferation of larger animals, but it could not have been their true beginning. Studies of DNA relationships among the major animal phyla, extrapolating back in time, suggest that these lineages had differentiated much earlier.⁸ How else might Cambrian rocks contain such a wealth of different lineages of animals? They had originated while still tiny and were already evolving along independent trajectories. As oxygen levels rose, and the firepower for energizing larger animals became available, different lineages all responded in a similar way: producing a diverse array of much larger forms. Suddenly, the environment became more diverse and much more dangerous. The “boring billions” were long gone, a world with larger, fiercely competitive, animals was a completely new world.

The Cambrian period began suddenly, with animal lineages continuing to expand and diversify over time. The Cambrian was followed by the Ordovician (488 to 443 mya), a period marked by a grand increase in species richness but terminated with a sudden loss of many species. The Silurian (443 to 416 mya) and the Devonian (416 to 360 mya) periods followed. Each of these periods was characterized by suites of dominant marine animals, and each transition was marked by faunal collapse or change. Overall, the number of animal genera increased from around seven hundred in the late Cambrian (500 mya) to fifteen hundred in the Ordovician (450 mya), then fell, only to return to higher levels over the next two hundred million years.9 During these early times, the sea was alive with fascinating creatures that have left a rich fossil heritage. The first bony fish are found around 500 mya, but they lacked jaws; lampreys and hagfish are their living descendants. Fish with jaws began their own grand radiation about 416 mya. Around that same time, another profound advance in life's history was underway.

PLANTS PIONEER THE LAND

Land plants changed the world of processes as surely as they changed the world of places.

—Oliver Morton¹⁰

The grand procession of life we've just reviewed was entirely marine; moving onto the land was a more formidable challenge. We find the first evidence of microscopic spore-tetrads in the fossil record around 470 mya. Nothing living in the sea produces such spores; they represent something new and different. These spore-tetrads appear to be the reproductive disseminules of the most simple, and most ancient, land plants. For the ensuing fifty million years, these spores are the only evidence we have regarding the possible presence of land plants. No stems, no leaves, no roots—just spores. Sad to say, small plants decay quickly, leaving little for the fossil record. Spore-tetrads are four spores stuck together in a single unit. Recall meiosis, where two cell divisions produce four haploid sex cells. The spore-tetrad is the direct product of meiosis, with the four haploid daughter cells remaining stuck together. These spores have tough moisture-retaining walls, allowing them to survive long dry periods and travel widely. Built with a tough covering of sporopollenin, spores readily became fossilized. (Fungi and bacteria also produce spores, but these are quite different in form and chemistry.) Spores-in-tetrads are the first evidence that plants were adapting to a terrestrial environment.

Just as higher oxygen levels were essential to the metabolism of larger animals in early Cambrian seas, increasing oxygen concentrations was important in the advance of life onto land. Higher concentrations of oxygen gave rise to a thin layer of ozone (O³) high above the atmosphere. This high-elevation ozone layer acts as a shield surrounding the globe, absorbing much of the ultraviolet (UV) radiation the Sun beams our way. Because high-energy UV light can damage living tissues, the ozone layer plays an important role in our biosphere. Think of what strong sunshine can do to unprotected light-skinned people. For creatures adapted to living in water—where UV light is absorbed—the ozone shield helped make the land environment less hostile.

Surely, blue-green bacteria, various algae, and a variety of fungi were already living on moist land surfaces many millions of years before the arrival of green plants. Today, they form the dark green borders you see at the muddy edge of ponds, and the yellowish green glop you find in moist depressions. These are algae and bacteria still doing what they've been doing for a very long time. But such communities flourish only with continuing moisture, and they quickly become inactive when desiccated. Overall, they don't amount to much more than slop and glop. In contrast, the first terrestrial plants probably resembled thin prostrate little liverworts. Still with us today, liverworts are restricted to moist forests and shaded muddy soils. (Together with mosses, liverworts are members of the Bryophyta.) Liverworts come in two general categories: flat, strap-shaped green forms, half an inch (1–2 cm) wide, growing close to moist substrates. Today, Marchantia is found around the world. In contrast, the leafy liverworts have very small leaves (a quarter inch / 2–5 mm, or less), borne on slender stems that rarely exceed three inches (8 cm) in length. Leafy liverworts are often found hanging from branches in the moist understory of wet forests. Lacking erect stems, and with thin tissues, these kinds of plants make poor candidates for fossilization. In all probability, liverwort-like plants produced the earliest fossil plant spores, leaving no fossils of themselves.11

Around 425 mya, the fossil record changes: spore-tetrads decline in abundance, and solitary spores become common. Bits of plant tissues begin to show up in the fossil record as well. Land plants are beginning to make changes. Lignin to strengthen tissues made of cellulose was an important innovation. In addition to reducing water loss through their outer surfaces, a waxy layer protected against ultraviolet radiation, microbial attack, and corrosive chemicals. Unfortunately, having a waxy covering presents a problem: how can a plant absorb carbon dioxide from the air if it's covered with wax? Moist interior cell surfaces must be exposed to air in order to absorb carbon dioxide for photosynthesis. And that raises yet another problem; exposing moist cells to air will result in water loss by evaporation. Because carbon dioxide concentrations are low in air, a great deal of water will be lost in the process of absorbing carbon dioxide. Stomates were the solution to this problem—little openings on the surfaces of the plant. Called the stomatal apparatus, or stomate for short, the little openings have two peripheral guard cells that can open and close in response to moisture stress. Opening for gas exchange and closing to conserve water loss made stomates an essential land plant innovation.

Unfortunately, gas exchange results in water loss, and larger plants must replace water with their roots, and this means a larger plant needs plumbing! Tubular cells within the stem die and lose their contents. Connected at their open ends, and with their insides empty, these dead cells become conduits, allowing water to move upward unobstructed. We call such tissues the plant's vascular system. With a vascular system, water absorbed from the soil can be carried up to the photosynthesizing tissues, replenishing the water lost through evaporation and used in photosynthesis. Combined with cellulose, lignin helped create stronger vascular tissues and stiff stems. In strong contrast, simpler liverworts lack lignin, stomates, roots, and plumbing, which is why they are both small in size and restricted to wet environments; their tissues must absorb moisture directly from their surfaces.

By around 420 mya, we find the first fossil plants with erect stems reaching 4 to 8 inches (10–20 cm) high. Their simple green stems carried spore-producing structures at their tips. Though lacking leaves, they did have a vascular system, able to transport water up from below and keep their green tissues moist. Because their roots were small, these plants remained short and slender. Nevertheless, and for the first time, the terrestrial world was becoming a lot more interesting.

PLANTS GROW LARGER

With the development of stomates, lignin, plumbing, and roots, land plants moved into high gear. Plants grow with meristems: embryo-like tissues that can continue growing to form new leaves and stems. Meristems are generally found at the tips of stems and roots, where they continue building new tissue. (Plants grow by the continuing growth of their meristems, unlike animals, which lose their embryonic tissues during early development.) Around 400 mya, plants invented something new in the way of meristems. Cambium is a thin tubular meristem surrounding the center of the stem and paralleling the entire long axis of the stem. A single cell layer thick, the cambium produces new cells on both its inner and outer sides. In this way, the cambium forms new vascular tissue, causing the stem to expand in girth. Becoming just a little thicker and a little stronger each growing season, the cambium builds new plumbing as well as additional structural support. By creating strong wood fibers toward the interior, cambium produced woody tissue along the entire length of the stem. With cellulose reinforced by lignin and a long tubular cambium, plants began building trees.

The whole point of a tree is to colonize the air, where sunlight is plentiful.

—Diane Ackerman.12

The earliest trees were leafless. Rather, they looked somewhat like palms, with a solitary trunk and shorter dissected greenish branches at the top; branches that would fall to the ground as the tree grew taller.13 Not until around 370 mya did trees begin building leaves. Thin leaves, with broad flat surfaces to intercept sunlight, are a more efficient way of harvesting light, but they require more plumbing and elaborate branching for support. As time progressed, different lineages developed a variety of tree forms. By around 365 mya, moist tropical estuaries and river basins were the home of forests! Soon, trees were reaching as high as 80 feet (24 m), while fern-like plants graced the understory. All this greenery was a major advance for the world's biodiversity. In fact, around 300 mya, giant millipedes and dragonflies with two-foot wingspans, enlivened these ancient forests (probably because of higher oxygen levels at that particular time). Overall, the Carboniferous period (363–290 mya) was marked by widespread forests in river basins and estuaries. These were the times during which massive coal deposits were formed (the primary energy source for our electrified society).

As in so many other situations, positive advances on one front produce negative effects in another area. The spread of forests during the Carboniferous had a global effect. Vast quantities of CO₂ were removed from the atmosphere and ended up in dead vegetation, eventually becoming coal. Because CO₂ acts as a thermal blanket for our planet, this period of vegetative exuberance was followed by the most extensive glaciation of the last 580 million years. Deposited at roughly the same time period around the world, coal deposits mark a significant episode in our planet's long history. But why wasn't all this organic matter degraded more rapidly? Nowadays, termites, some beetles, and many fungi are decomposers of wood, preventing our forests from becoming piled high with dead wood. A recent study, combining fungal phylogenies and fossil dating, claims that lignin-digesting “white rot fungi” first appeared around three hundred million years ago.¹⁴ Using enzymes that can tear apart lignin, these wood-decaying fungi may have put an end to the Carboniferous!

Land plants are another dramatic example of how the living world has changed over geological time. Life on land—both plant and animal—initiated the grandest advance in the history of biodiversity and ecological complexity on our planet. But what might have induced any plant or animal lineage to abandon a moist aquatic home to live in a far more stressful environment on land?

THE CHALLENGE OF LIVING ON LAND

Why might plants and animals have adapted to a hostile land environment, when remaining in a comfortable liquid medium seems much the simpler choice? With natural selection operating only in the here-and-now, adaptations for some possible future environment are unlikely. The most reasonable answer to this question is that adaptation to life on land actually began by trying to stay alive within an aqueous environment. Both plants and fish living in shallow inland waters were often confronted by periods of desiccation, as their creeks, marshes, and mud puddles dried up. Many tropical regions have long and severe dry seasons during which no rain falls. Rain forests are the exception; they make up less than 30 percent of the tropics’ potential natural vegetation. Most tropical regions are seasonally dry, ranging from deciduous woodland and grass savannas to hostile desert.

Tropical regions with limited rainy seasons have many rivers, swamps, and marshes providing rich aquatic habitats. A majority of these same wetlands lose their standing water over the dry season. Such conditions create life-threatening challenges for local aquatic plants and animals. Today, East African lung fishes burrow deep into the mud as their watery home dries up. There, within a mucous lining and living on stored fat, they can breathe air and survive until the rainy season returns. Long ago, lobed-fin fishes used their forward fins to move through shallow streams blocked by tree-falls and aquatic vegetation. With time, these vertebrates have come to dominate the terrestrial world.

A variety of different animals, in fact, crawled out of their watery world and became adapted to life on land. The aquatic ancestors of spiders, insects, millipedes, a variety of different snail lineages, worms, and the land vertebrates (tetrapods) all became terrestrial independently. Each of these lineages came from a different aquatic phylum, not closely related to the others, having made the same journey onto land. In strong contrast, all land plants appear to have originated from a single lineage of complex freshwater green algae: the Charales.15 Freshwater habitats, bordered by moist soils, provided the environment from which plants could pioneer the land. Lacking the movement and versatility of animals, plants had a far more difficult task in adapting to the land environment and, it appears, only one such attempt was successful. Also, and unlike most of the land animals, land plants acquired an entirely new body plan as they made the transition. Beginning with a larger diploid plant body—something quite rare among the algae—this lineage also developed an embryo and new growth modalities. With these innovations in place, plants began their invasion of moist tropical lowlands.

To summarize, early land plants probably lived along streams and ponds that dried up during part of the year. Their first adaptation was the microscopic spore or spore-tetrad, which could survive severe drying and be scattered across wide areas, germinating—if lucky—in a distant pool of quiet water or moist mud. Spore-tetrads are the earliest evidence of land plants. Waxy cuticles, stomates, and a vascular system were later innovations. And, when the tubular cambium came on line, woody stems fashioned a more diverse and complex vegetation.

Land plants proved to be globally transforming, not only as a major advancement of complex life, but also for increasing the numbers of living species. Plant cover intercepted the impact of rainfall, reducing erosion. The dead remains of plants added organic matter to the soil and more nutrient run-off into the sea. A leafy plant cover provided shade, protection from desiccating winds, and a rich source of food for herbivores. By absorbing sunshine and evaporating moisture into the air, plants reduced temperatures under a hot tropical Sun. Within a three-dimensional land flora, animals had new opportunities for relentless diversification.

SEEDS: ANOTHER INNOVATION FOR LAND PLANTS

Living descendants of the early vascular plants include the club mosses (Lycophyta), as well as the horsetails, ferns, and their allies (Pterophyta). Today, the lycophytes and horsetails rarely grow more than a few feet tall, but during earlier times some members of these lineages grew to become tall trees. Living ferns range from a few inches in height to over fifty feet tall. Slender tree ferns with broad and graceful leafy fronds are found only in wet tropical forests today, resembling plants that lived 300 mya. Together with giant lycopods, horsetails, and smaller ferns, tree ferns helped create ancient swamp forests. Among the giants in these ancient forests, Sigillaria grew to over 90 ft. (30 m) and Lepidodendron reached 130 ft. (40 m). Entombed in rivers and deltas, these forests formed the great coal deposits of the Carboniferous period. As these early forests were flourishing, a new plant innovation came upon the scene.

The development of the seed marked a major advance. In a way, the seed represents for plants what internal fertilization and a tough desiccation-resistant amniote egg represented for land vertebrates: reproduction without water! Even today, liverworts, mosses, ferns, and most amphibians require external moisture for fertilization. Frogs and toads call loudly from ponds and streams in early spring; they can only make love in a watery environment. Their sperm cells must swim through water to reach eggs deposited by the females they have embraced. Thanks to internal fertilization, reptiles can engage in sex without having to find a river or pool in which to couple, and a leathery egg meant that their young did not have to develop underwater. Reptiles soon spread far and wide across the landscape. Similarly, the invention of pollen gave seed plants an alternative to swimming sperm cells. Pollen, resembling tough little spores, can travel many miles without becoming desiccated. Carried by the wind—or by animals—pollen can reach the ovule, where the egg cell resides, and effect fertilization. In contrast, ferns, mosses, and liverworts require thin films of water for their sperm to reach an egg cell. Just as in the case of animals, these sperm have long wiggly tails that propel them toward the chemical signals of the egg cell.

Carried by the wind through the air or transported by animal agents, pollen grains emancipated seed plants from the necessity for water as the medium for fertilization. Germinating near the ovule in response to chemical signals, pollen grains split open to produce a microscopic tube that grows toward the egg cell. Carrying the male nucleus to where it can unite with a female nucleus achieves fertilization—all without having liquid water at hand. The fertilized ovule then develops into a seed, which can be dispersed to form another plant. Seed plants were now able to propagate in a variety of terrestrial ecosystems where water was scarce. Seed plants (Spermatophyta) included the extinct seed ferns, the living cycads, conifers, ginkgo trees, the strange gnetophytes (Ephedra, Gnetum, and Welwitschia), and the flowering plants (Angiosperms or Magnoliopsida).

The triumph of the seed was two-fold. First, pollen grains did away with the necessity for liquid water in fertilization. Secondly, both pollen and ovule simplified the two-stage life cycle of more primitive plants. Technically speaking, the original haploid male plantlet has transformed itself into the pollen grain! Similarly, the female haploid plantlet became a small part of the ovule produced by the diploid mother plant. With the proper chemical messaging, a pollen grain germinates at the ovule, grows toward the egg cell, and effects fertilization, and the resulting diploid embryo matures within the seed, ready to create an adult (diploid) plant.

FLOWERING PLANTS: ANOTHER ADVANCE FOR BIODIVERSITY

The emergence of seed plants was followed by yet another major innovation in land plant evolution. In rocks dated around 140 million years old, we find the first good evidence for flowering plants or Angiosperms: their distinctive pollen and a few fossils. (Because plants decay rapidly, many plant lineages have left only a meager fossil record.) Expanding rapidly during the latter half of the age of dinosaurs, the flowering plants have been instrumental in creating more complex ecosystems. Today, flowering plants dominate most of the world's land surfaces. Of a conservatively numbered 300,000 species of land plant, flowering plants have over 260,000 described species.16 Best of all, Angiosperms come in a huge variety of shapes and sizes. Giant baobab trees, oaks, alders and acacias, potatoes and barley, water lilies and roses, cacti, grass, and orchids are all flowering plants. Enormous diversity in size and structure mark the Angiosperms. But what is it that makes the flowering plants so different?

The critically defining feature of flowering plants is that their seeds develop within a tight enclosure: the ovary. Somehow, the flowering plant ovule (or a group of ovules) became enclosed within one or a few leaf-like structures. These leaf-like devices (called carpels) formed the ovary, within which the ovules are protected, pollinated, and nourished. As the ovary matures into a fruit, each fertilized ovule becomes a seed. This new device—the ovary ripening into a fruit—is one of the glories of our modern world. The mature ovary gives us the juicy tomato, the tasty papaya, the nutritious avocado, the woody coconut, and the pod that houses peas. Enclosed within additional floral tissues we have the apple, the cherry, the pumpkin, and the watermelon. In a few, the floral axis gets involved, forming the strawberry and tropical cherimoya, while the inflorescence axis helps produce a pineapple. In other lineages the ovary wall remains thin and hard, a simple encasement for the seed. This group includes our most important source of nourishment: the cereal grains of the grass family (Poaceae). Low in moisture, high in nutrition, and easy to store, cereal grains allowed humans to build grand civilizations. Wheat, rice, maize, barley, sorghum, rye, oats, and the millets continue to power the human enterprise today. The seeds of legumes or pulses (Fabaceae), with higher protein contents, also played a central role in sustaining human communities. These include peas, lentils, chickpeas, beans of many kinds, soy, and peanuts. Some societies have used tubers, corms, and roots as primary food sources. These include potatoes, yams, sweet potatoes, cassava, carrots, turnips, and others. All these food plants are Angiosperms! And just as the flowering plants have powered the human enterprise, they are the primary energy sources for a majority of terrestrial ecosystems. By diversifying so grandly themselves, flowering plants allowed other terrestrial lineages to diversify as well.

What was it that gave Angiosperms the ability to prosper in such a grand variety of shapes and sizes? The answer to this question involves another unique aspect of flowering plants—called double fertilization. After the pollen grain splits open, germinating on the stigma of the flower, it forms a pollen tube. This microscopic tube grows toward the ovules, carrying two or three nuclei. Upon reaching the egg apparatus inside the ovule, one of these nuclei will unite with the egg nucleus to effect fertilization. So far, nothing special: not much different from sperm nucleus joining egg cell in so many other plants and animals. But there's more: a second nucleus from the pollen tube also gets into the act! By joining with two free nuclei within the egg apparatus, the ovule begins to build endosperm—the tissue that will nourish the developing embryo within the seed. Called “double fertilization and triple fusion” this advance in plant complexity fostered a major expansion of biodiversity.

THE ADVANTAGE OF DOUBLE FERTILIZATION

Does double fertilization, yielding a triploid endosperm with three sets of chromosomes, make much difference? For the developing embryo and seed, probably not. Nevertheless, double fertilization was a key factor in Angiosperm success. To understand the importance of double fertilization, we need to look at that other grand crowd of seed plants, the Gymnosperms. These include pines, cedars, redwoods, and other conifers: cycads, gingkoes, and the strange Gnetopsids. Gymnosperm means naked seed, because the seed is not enclosed within an ovary. Angiosperm means hidden seed, since it forms within the ovary. Before the arrival of flowering plants, Gymnosperms dominated terrestrial landscapes, and they still do in the grand conifer forests of America's Pacific Northwest and Rocky Mountains, Podocarpus forest just beneath the summit of Mt. Kenya, or the vast needle-leaf forests of the northern taiga. But despite a long history, they number less than a thousand living species, and all are woody. This is an important point: there is no such thing as an herbaceous Gymnosperm! In striking contrast, Angiosperms have herbs scattered all across their many lineages, from lilies and grasses to petunias and an awful lot of weeds. What explains the ability of flowering plants to make so many weeds?

Gymnosperms just aren't very clever. Most of them produce seeds and endosperm in advance of fertilization. Not smart! Wind-pollinated Gymnosperms depend on frequent winds and closely grouped populations to effect fertilization; when these parameters are lacking, fertilization may fail. Producing energy-rich seeds that cannot sprout—because they weren't fertilized—is a huge waste of energy.17 Larger woody plants can afford this waste; little plants cannot! To repeat: most Gymnosperms produce energy-rich seeds before fertilization has taken place. That's not the way Angiosperms operate. What double fertilization does is to initiate endosperm production only after fertilization has occurred. Thanks to double fertilization, precious resources will not be wasted on seeds that cannot sprout. In addition, tight coupling of fertilization with endosperm initiation speeds up seed production. This is why flowering plants have been able to produce so many little short-lived herbs and why, after three hundred million years, the Gymnosperms have never built a weed.

FLOWERING PLANTS: GREATER COMPLEXITY YIELDS GREATER DIVERSITY

Flowering plants have achieved greater morphological complexity than any other plant lineage, and they've done this in many ways. In chapter 3 we mentioned how chromosome doubling can result in forming new species: this same process has played a major role in flowering plant diversification as well. Angiosperms have a remarkably resilient genome; adding to, or doubling, their genomes results in few negative consequences. Within the flowering plants, individual genome sizes vary over a more than two-thousand-fold range! At the bottom end sits Genlisea in the bladderwort family with a 63.4 Mbp (mega base pairs) genome, while Paris japonica in the lily alliance has a genome around 130,000 Mbp. Compare this with mammals having a five-fold range between their smallest and largest genomes, or birds with a meager two-fold range.

Best of all, flowering plants elaborated yet another clever tactic: advertising! Many flowers embellish their sexual parts with a bright whorl of colorful petals, enticing aromas, and sugary nectar. A great variety of animals respond to this enticement to fill their gas tanks, carrying pollen from flower to flower. Colorful flowers probably originated in different lineages and at different times over the history of the flowering plants. The Magnolia flower has a long central axis bearing spirals of stamens and ovaries (rather like leaves on a stem). However, the magnolia flower is very different from either the cup-like flowers of the laurels, the three-parted flowers of the lilies, or the five-parted flowers in wild roses and many others.

Animal pollination was a major advance in our planet's ability to support high species numbers. Wind pollination is effective for species that are well represented in the local landscape. With lots of other individuals of your species nearby, some of your pollen is likely to reach a stigma for pollination. However, if your numbers are few and scattered across the landscape, wind pollination becomes unlikely. There is no way in which wind pollination could sustain a modern rain forest with three hundred different tree species within a few acres. In a modern rain forest, many species are represented by only one or two individuals per acre—not enough to make wind-pollination work effectively. Whether imbedded in a species-rich rain forest, or scattered across desert thorn bush, brightly colored flowers are visible to their pollinators. Only sentient agents can travel from one bright orange blossom through dense underbrush or across a rocky landscape to find another orange bloom. Pretty flowers (most of them) offer something important to their animal visitors: sweet energy or nutritious pollen. By making rarity less likely to result in extinction, animal-mediated pollination gave the Angiosperms a big advantage. Starting with perhaps only a few hundred species 120 mya, and with over 260,000 species today, flowering plants have grandly expanded ecosystem complexity over the last hundred million years.

Let's return to the structural variety of Angiosperms. Their shapes range from cacti that look like spiny barrels to many-branched shrubs, tall trees, elegant palms, and weedy herbs. This profusion has transformed the world. Unlike a conifer forest with its many narrow spires, and thanks to wood with greater tensile strength, Angiosperm forests have canopies with widely spreading branches. This means that there is more to grow on (if you are an epiphyte) or swing from (if you are a monkey). Also, flowering plants have more specialized vascular tissue, both in the water-carrying xylem and the food-dispersing phloem. Leaves and leaf-architecture vary widely among flowering plants. Slender leaf-stalks (petioles) not only allow the blade to orient itself for greater exposure to the Sun but also provide flexibility in gale-force winds. In addition, leaf-stalks can form abscission layers at their base, to jettison the leaf at the end of a growing season. While not unique to flowering plants, deciduous leaves allow trees and shrubs to survive in strongly seasonal environments. Also, many Angiosperm leaves have high vein-densities, allowing them to transpire more moisture, while capturing more carbon dioxide and photosynthesizing faster.18 By pumping more moisture from the soil into the high canopy—where most of it is evaporated—flowering plants sustain the rain forest. This is why cutting down a rain forest changes local climate so drastically.

Angiosperm variety has resulted in a spectacular range of differing fruits and seeds. Sizes range from coconut palms and cannonball trees (whose falling fruit might crack your skull) to the powder-like seeds of orchids. Many fruits provide succulent tissues that are eagerly devoured by animals: from the many-seeded watermelon to the single-seeded avocado. Others lack fleshy tissues but harbor nutrients, as in the cereal grains. Many fruits lack food value but are built to travel with the wind, outfitted with winged extensions or cottony fluff. Others have hooked hairs to latch onto passing animals and clothing (the inspiration for Velcro). Together, fruits and seeds help disperse flowering plants far and wide.

So much variety in growth forms, fruits and seeds makes clear why other plants and animals have increased their numbers in concert with the expansion of the flowering plants. Modern ferns expanded their numbers shortly after the flowering plants began diversifying.19 Ants also began proliferating around 100 mya.²⁰ Beetle lineages feeding on flowering plants have many more species than do related beetles that do not use flowering plants as their food source. Epiphytic plants and small animals have also increased their numbers as Angiosperm-dominated forests expanded.²¹

Though these radiations began around 100 mya, the flowering plants have continued innovating in more modern times. Grasslands and savanna woodlands are among the most extensive modern biomes. They appear to have begun expanding about 30 mya, as indicated by the increasingly deep (hypsodont) teeth of large grazing herbivores. Since grasses do not fossilize easily, we infer the expansion of grasslands by the increased length of teeth among grazers. Silica crystals in grass cells wear down the teeth of grazers; these herbivores responded by growing longer teeth over evolutionary time. Grasses can regrow from their base after both fire and grazing. (This is why you can mow your lawn but not your petunias.) Decreasing temperatures over the last thirty million years have lowered rainfall and increased fires over many regions, allowing grasslands to expand.

More recently, a number of grass lineages have pioneered a new kind of photosynthesis, called C₄ photosynthesis. Not only do these grasses seem to have a competitive advantage in hot dry environments but some burn hotter. In this way, they erode woodlands more aggressively, widening their domain with each successive fire. From our own point of view, expanding grasslands with their large nutritious grazers provided the setting in which we tripled our brain volume over the last three million years! All told, flowering plants have been the primary drivers in building today's terrestrial biodiversity.22

RULERS OF THE LAND: THE TETRAPOD VERTEBRATES

As we've already noted, land vertebrates evolved in freshwater rivers and estuaries. In fact, much of their preparation for becoming creatures of the land developed while they were still really fish. Living in small streams and estuaries, these fish faced the challenge of moving through water obstructed by fallen logs and aquatic vegetation. Negotiating such obstacles required the help of their front fins. Turns out the ancestors of land vertebrates had fins arising from projections on the side of their bodies. With a lobe-like base, the front pair of fins had greater freedom of motion to push their way forward. Soon, the base of the fin became elongated—the origin of arms. Later, these arms were articulated with elbows, and they developed digits for traction. Living in waters laden with silt and poor in oxygen, these animals had already developed lungs to gulp air—augmenting the work of their gills. Abandoning the buoyancy of water, a rib cage supported larger lungs for a more dynamic life on land, as their hindquarters developed a second pair of legs. But what might have driven such a unidirectional trajectory? Most likely, hunger! The shores of shallow streams were home to tasty prey; none of the early land vertebrates were vegetarians. Around 360 mya, four lateral fins were being transformed into four legs, which is why most land vertebrates are tetrapods.

Vertebrates had a number of important advantages over their invertebrate neighbors. By carrying one's architectural support inside, there wasn't the problem of having to shed a thick outside armature when growing larger. Also, an internal skeleton allowed for a greater variety of muscle-attachments and enhanced flexibility. Bony fishes had elaborated the many-parted backbone, providing both rigidity and some flexibility. By propping up this backbone with two pairs of limbs, land vertebrates developed what William K. Gregory called “the bridge that walks.” The tetrapod backbone resembles a suspension bridge; anchored fore and aft by shoulders and hips. From a waddling amphibian of 340 mya, with arms and legs splayed out sideways (as were the fins), legs became positioned under the body for more efficient movement in more advanced lineages. With the vertebral column as central axis, evolution fashioned a variety of designs, from giant herbivorous dinosaurs to graceful antelopes and the speedy cheetah. By losing legs and multiplying vertebral elements, Mother Nature also fashioned slithering snakes. Becoming two-legged (bipedal) was another innovation, allowing carnivorous dinosaurs, their descendants the birds, and human hunters to become so successful.

A strong internal architecture allowed land vertebrates to diversify into a variety of forms, from frogs to pheasants. With spacious lungs, a diaphragm to pump air, and a strong four-chambered heart, land vertebrates could grow large and remain active. Fishy scales were transformed into a variety of dermal coverings, most elegantly in the feathers of birds, and comforting in the fur of mammals. For over 150 million years, both carnivorous and vegetarian dinosaurs ruled terrestrial environments. During that time our own lineage, the mammals, remained small and mostly nocturnal. Being active at night made hearing an important aspect of survival. Three bones in the jaws of reptiles became transformed in early mammals and are now part of the inner ear, allowing mammals to hear a wide range of frequencies. Better hearing, in turn, gave rise to larger and more versatile brains, as warm-blooded mammals processed more information, more quickly.

Without question, the history of organic evolution has been marked by the emergence of new and more complex body plans. The elaboration of multicellular organisms, more than 550 mya was a major leap forward. Later, the land biota gave rise to ever-more species and lifestyles within terrestrial habitats of ever-greater complexity. But it wasn't all honey and roses; serious extinction events have also marked the history of life.

EXTINCTIONS: CHAPTER ENDINGS IN THE HISTORY OF LIFE

Early geologists trying to piece together the history of our planet found evidence for sudden changes in fossil faunas over short intervals of time, and they used these changes to demarcate different periods in the geological record. Continuing discoveries did not bridge these major discontinuities. The Cambrian explosion of complex animals was real; these animals were not preceded by smaller versions of simpler creatures. A sudden decline of diversity in the late Ordovician was not an artifact of insufficient exploration or fewer outcrops. For earth scientists, drastic biotic changes were useful, helping define chapters in life's long history. Extinction events were used to separate time periods. Of all these abrupt changes, paleontology's biggest extinction ended the Permian period, about 250 mya. It is estimated that over 80 percent of marine species became extinct during this prolonged calamity—the Permo-Triassic Extinction. Huge outpourings of lava in Siberia at exactly this time, poisoning air around the globe, seem the likely cause.

A second major punctuation mark in the history of life, the End-Cretaceous Extinction, took place quite suddenly 65 mya. All around the world, and at what appeared to be the same moment in time, the fossil record changed. Dinosaurs, the grandest animals that ever roamed the surface of our planet, suddenly went missing.23 Only one dinosaur lineage, the feathered birds, enlivens our modern world. Giant reptiles of the sea—the mosasaurs, plesiosaurs, and ichthyosaurs—also vanished. Foraminifera, minute marine animals, collapsed in great numbers, and only slowly recovered. Not as severe as the Permo-Triassic extinction, the end-Cretaceous event was more recent in time and seemed more abrupt. With less evidence lost to erosion over time, this extinction could be studied in greater detail.

Why so sudden a demise for so many extraordinary animals? This question remained a back corner of evolutionary studies until a small group of scientists proposed a very dramatic scenario. In the June 6, 1980, issue of Science magazine, Luis Alvarez and his associates claimed that an impact of extraterrestrial origin had wiped out the dinosaurs! You don't get much bolder than that in the halls of academic science. As evidence, the Alvarez team announced their discovery of an excess of iridium in the fine-grained clay layers marking the end of the Cretaceous period. Their evidence came from sediments in northern Italy, Denmark, and New Zealand, where this specific time period was clearly preserved. What they discovered was a sudden increase in iridium concentration at the very end of the Cretaceous. Knowing that iridium is much more common in meteorites than it is in the Earth's outer crust, the Alvarez group claimed the “iridium anomaly” was evidence for a devastating impact by a large asteroid!

The asteroid-impact hypothesis initiated a frenzy of activity, both by those hoping to support the idea and those trying to demolish it. Further work in Scandinavia, the western United States, and Australia made clear that the anomaly was indeed worldwide and had apparently occurred at precisely the same moment in time. The discovery of shocked tektites (bits of glass and fused particles produced by a high-energy impact) were also found around the world at this particular moment in time. With the largest tektites found in and around the Gulf of Mexico, and using deep-core samples from oil-drilling operations, scientists have identified the buried crater itself, the Chicxulub structure, beneath the edge of Mexico's Yucatan peninsula.24 Over time, a clear consensus was reached: the iridium anomaly is the signature of a catastrophic encounter with an asteroid.

Extensive volcanism, with huge amounts of lava erupting in India at around the same time, also stressed biotas at this time. But the so-called “bolide impact” seems to have been crucial. Exactly how a meteorite between 2 and 6 miles (0.8–10 km) in diameter might affect the Earth on impact continues to be the subject of speculation. Smashing into the Yucatan's limestone rocks would have sent a charge of pulverized rock high into the atmosphere. This is the event that blanketed our planet with a layer of fine dust, minute tektites, and the iridium anomaly. A globe-encircling cloud of fine dust would have had several harrowing effects. Scattering sunlight, the dust cloud would have darkened the sky and lowered temperatures everywhere. Some indication of the effect of high-flying dust can be inferred from the recent eruption of Mt. Pinatubo in the Philippines, which caused the average temperature in the northern hemisphere to drop by about 1°C (1.8°F) for most of a year. Though we will never know exactly how long the end-Cretaceous dust-cloud persisted, it changed the history of life.

More important, the asteroid hypothesis helps us understand why some creatures survived and some didn't. The hypothesis implies that effects on animal life in the ocean would be especially severe. Minute phytoplankton near the ocean surface carry on photosynthesis, and, if this source of energy is diminished, life in the entire water-column collapses. Unlike forest and swamp, there are no thick layers of decaying vegetation at the bottom of the sea. Analyses of geological cores off the shore of New Jersey have found debris from the end-Cretaceous impact above deposits containing the microscopic shells of many minute marine organisms. Above the debris layer, nearly all the microscopic species are gone, topped by thousands of years of deposition before the fauna is fully restored. These cores, more than a thousand miles from the impact site, bear witness to the near-instantaneous death of marine planktonic creatures.

On land, most insect groups and some mammal lineages managed to survive the extinction event. But the dinosaurs were wiped out. In contrast, those larger animals that could endure longer time periods without food, such as crocodiles, turtles, snakes, and lizards, survived. (Here is strong evidence that dinosaurs were dynamic warmer-blooded creatures, needing lots of food on a regular schedule.) Several distinctive bird lineages died out, but many survived to become today's bird fauna. More significantly, since the impact occurred in the Northern Hemisphere, it may be that extinction was less severe in the Southern Hemisphere. Land plants show some losses after the impact, but spore and pollen diversity soon recovered. Overall, there is little doubt: though dinosaur numbers may have been declining in the face of volcanism in the late Cretaceous, the final blow was a catastrophic impact of extraterrestrial origin.

BIODIVERSITY EXPANDS WITH EVOLUTIONARY TIME

Despite extinctions large and small, the history of life on planet Earth seems to be one of expanding numbers and increasing complexity. For larger life forms, it all began rather tentatively with the enigmatic Ediacarans. These leaf-like and pancake-like beings are the first larger biological organisms found in the fossil record, 560 mya. They were followed by a short period of small shelly fossils, together with a few trails and burrows. Then suddenly, around 540 mya, a variety of larger fossil forms are evident. The Cambrian explosion had begun, transforming the oceans into a zoo of differing creatures. Animal families diversified further, and their numbers continued to rise during the Ordovician. A severe extinction ended the Ordovician and ushered in the Silurian (443 mya). Moderate ups-and-downs in family diversity continued over time, but then came the catastrophic Permo-Triassic extinction (250 mya).25 Nevertheless, plants and animals recovered and began diversifying once again. By around 150 mya, biodiversity had surpassed the numbers of species alive before the great extinction event.

Grand extinctions in the history of life allow geologists to sequentially divide the fossil record. In fact, the two largest extinctions allow us to divide animal history into three grand epochs. The first is called the Paleozoic (560 to 250 mya) and was terminated by the end-Permian extinction. The Mesozoic (250 to 65 mya) was the second act, terminated by the end-Cretaceous extinction. The third epoch, the Cenozoic, began with the end-Cretaceous extinction and is arbitrarily terminated at the end of the last ice age.²⁶ Following the three eons described at the beginning of this chapter, this last 560 million year eon is called the Phanerozoic.

Major extinctions have done more than punctuate the history of life. Mammals triumphed only after the dinosaurs departed, and they dominate land surfaces today. The destruction of entrenched faunas created new ecological opportunities. Niles Eldredge has argued that extinctions played a significant role in the progressive evolution of animal life, as old faunas were devastated and replaced by new faunas.²⁷ Plant history, however, is very different, with major extinctions playing a lesser role. Surviving in the soil as seeds or spores, plant lineages have managed to make it through both major and minor calamities. The grand extinctions did cause extensive ecological upheaval; millions of years were necessary before ecological recovery was complete after the Permian period.28 However, nothing in the history of plants is similar to the radiation of mammals once the dinosaurs were gone.²⁹ Surely this is due to more intense competition and predation among animals: a world “red in tooth and claw”!

In contrast, the botanical trajectory across time is marked by new structural innovations. A vascular system allowed plants to become erect; a tubular cambium produced wood and tall trees; pollen allowed seed plants to reproduce in drier habitats. Finally, flowering plants gave rise to a more diverse, dynamic, and nutritious vegetation. Some years ago, paleobotanist Norman Hughes estimated the increasing numbers of vascular plant species over the last 300 million years.³⁰ Vascular plants, you'll recall, include the flowering plants, conifers, cycads, ferns, and fern allies. These are the major players in building three-dimensional terrestrial vegetation. Hughes suggested that there were only about 500 species of vascular plants worldwide during the Carboniferous period, around 300 mya. By 150 mya, his estimate rose to 3,000 species. At the end of the Cretaceous, 65 mya, Hughes’ estimate rises to 25,000 species. Today's total of vascular plants numbers more than 275,000 species. Though Hughes’ estimates were highly speculative, they imply an extraordinary increase in land plant diversity over the last 300 million years. More important, Hughes’ numbers suggest an expansion from 3,000 to perhaps 300,000 species over just the last 150 million years! While ferns and mosses also expanded their numbers over the last 150 million years, it is the flowering plants that account for much of the recent escalation.³¹ And with more flowering plants came more beetles, more ants, more birds, more mammals.

Though nowhere near as numerous as flowering plants, birds and mammals have fostered a more subtle kind of amplification in species numbers, and they did this thanks to what biologists call parental investment. Birds and mammals are complex, intelligent, high-energy creatures. They are expensive, both to produce and to maintain. Young birds and baby mammals cannot be left to fend for themselves; they need high-quality food in regular servings. That's what parenting is all about. Unlike most amphibians and reptiles, neither birds nor mammals can let their newborn offspring fend for themselves. In fact, diversification of passerine birds over the last 25 million years may have been spurred by their ability to build sturdy nests in secluded sites, protecting their young in this way.32 By having to provision their young, both birds and mammals have expanded biodiversity.

Both the young eagle and the young lion will become active hunters only after coming close to their adult size and weight. Little lions do not chase rabbits; young eagles cannot leave the nest until their wings are fully formed. What this means is that both lion and eagle become active players in their ecosystem only when they can play the same roles that their parents do. Since the lion pride provides for the growing cubs, young lions will not begin to hunt until nearly fully grown. The same holds true for virtually all birds and mammals. Compare this with a tyrannosaur. Little tyrannosaurs probably had to hunt little prey, gradually growing to their awesome adulthood over more than twenty years. This meant that tyrannosaurs hunted a wide variety of prey over their life span, and there simply wasn't as much room in the landscape for other, smaller, predators. Paleontological evidence indicates that ecosystems dominated by the dinosaurs were relatively poor in animal species. Compare that scenario with Africa's savanna today, where many predators all share the same landscape. Thanks to intense parental investment, bird and mammal species are neatly restricted to their ecological roles, allowing for finer niche division among a variety of similar species. Prolonged parenting in birds and mammals has grandly enhanced animal diversity.

An overview of our planet's extraordinary history makes something very clear: Terrestrial plant and animal life has increased in both complexity and species numbers over time. Putting all the data together leads to a simple but profound conclusion—contemporary biodiversity is higher than it has ever been before.33 Unfortunately, since becoming fossilized is more likely within the sea, a huge majority of fossil data is that of marine animals, not life that lived on land. Marine fossils also record increasing family and generic diversity over time.³⁴ Though earlier estimates of expanding marine diversity have recently been scaled back, a clear evolutionary trend appears to be evident.³⁵ An analysis focused on marine ecosystems themselves concluded that these have become more complex over the last 250 million years.³⁶ Contradicting these studies, another analysis suggests that marine fossil diversity has not increased greatly over the long haul; the increase in species is simply a reflection of more abundant fossil exposures over more recent geological history.³⁷ Apparently, the question of increasing marine diversity has not been resolved. Perhaps all the “parking spaces” were taken long ago and the only thing of note is having old species replaced by new species?³⁸ Such a scenario may be reasonable for marine life, but it stands in utter contrast to what's been happening on land. The terrestrial record is incontrovertible: species richness has increased enormously over the last 400 million years.³⁹

Here is one of history's great enigmas! Why, after each and every extinction event, did animal and plant numbers not only regain their previous numbers but add even more? Why have land plants become both more complex in structure and more numerous in species over the last 400 million years? Next, let's examine the forces responsible for greater diversity and complexity in life over time.