A New History of Life (23 page)

Biological turnover and genetic diversity during the Cambrian explosion. The classical Cambrian explosion interval spans the Tommotian, Atdabanian, and Botomian stages of the Siberian platform. The turnover shows the number of genera that either arose or disappeared during the particular stage. (From Bambach et al., “Origination, Extinction, and Mass Depletions of Marine Diversity,”
Paleobiology
30 (2004): 522–42)

Modern coral reefs have been called the “rainforests of the oceans” because they share with rainforests the trait of high species diversity and abundance in small areas, and that is often the shared first impression—that there is so much life. But from there the comparisons largely cease. In a rainforest, or any forest, most of the life to be found is plant life. Reefs, on the other hand, are composed almost exclusively of animals. On any reef there are indeed large numbers of leafy, bush-shaped forms that are plant
like
. Yet virtually all are formed by animals, from soft corals to sponges to lacy bryozoans. One could argue that the verdant green of photosynthesizing plants covering great swaths of our planet’s continents are the most obvious evidence, if seen from space, that our planet is a world of life. But there is an entirely different kind of biological signal that can be seen from space—this one in the seas. It is the presence of tropical marine coral reefs, best illustrated by the Great Barrier Reef, which lines more than a thousand miles of eastern Australia’s coastline. But there are many more reefs than the Great Barrier Reef, magnificent as it is. The equatorial seas are filled with numerous coral atolls, fringing reefs, and the vast pale-green lagoons that these wholly biological structures enclose. These reef systems are parts of a very ancient kind of ecosystem, one that predates forests and even land animal life of any kind. They remain one of the most diverse of all ecosystems, and are essentially long-lived superorganisms that pop up again after every mass extinction and planetary die-off of the last 540 million years.

A hallmark of the coral reef environment is the abundance of movement virtually everywhere, for the flitting and schooling of fish, to the never-ceasing wave action on the reef, to the waving and billowing soft corals, pulsing and undulating in the active water movement that characterizes the reef environment. Every coral reef is home to
fish—many fish, of many sizes, shapes, and behaviors. Some school, some lurk, some swim in solitary splendor, and some—the omnipresent sharks—simply patrol. And it is not just the vertebrate members of these diverse communities that are seemingly always on the move. Closer inspections show that an amazing diversity of invertebrates is seemingly in constant motion as well—if usually slower than the fish. Smaller reef shrimp dance from coral to coral, while crabs large and small can be seen in their constant foraging. Snails, slower yet, perambulate according to some plan known only to them, and the gastropods to be found on any reef are diverse as well: there are large carnivorous species, such as the beautiful tritons, as well as equally large but herbivorous conch shells. Under the coral rubble, at least during the day, a cornucopia of the beautiful cowries huddle or slowly feed on tiny bits of algae, while the ferocious cone shells move among them, searching for most of their kind’s normal prey—small worms. Some, however, such as the textile cones, are fish eaters, and use a highly modified tooth, shaped like a harpoon and dipped in poison, to spear fish and then consume them whole. Turgid sea cucumbers move on the sediment—or just beneath it, constantly ingesting massive amounts of sand at one end and constantly expelling large sandy pellets from the other. There they share the upper foot of white coral sand with heart urchins. Other echinoderms are there as well, from a variety of predatory starfish to the placidly perched—but also swimming—comatulid crinoids. Color—and especially motion by a great and diverse assemblage of species. The coral reef ecosystems of today are filled with motion and color and there is every reason to suspect they have always been so.

Reefs, in fact, are very ancient evolutionary inventions,
¹
and their rise to prominence mirrors the rise to high diversity that followed the Cambrian explosion. In a way a hydrogen bomb is a good analogy. Thermonuclear fusion and the enormous explosion that ensues only take place in the immense heat of an atomic explosion. That is how a hydrogen bomb works: an atomic (fission) bomb with plutonium is ignited, which in turn creates sufficient heat and pressure to start the fusion reaction—and fusion explosion. In similar fashion the Cambrian explosion of diversity was the heat and fuel that led to the far greater
Ordovician diversification, and one of the most important of the products of this huge run-up in species numbers was the invention of the coral reef.

The first reefs—and by reef, we mean a wave-resistant, three-dimensional structure built by organisms—date all the way back to the earliest Cambrian period. They were not coral reefs, but composed of archaic, now long-extinct sponges called Archeocyathids.
²
Coral reefs are slightly younger; the first of these are found in the Ordovician period, and they really increase in distribution, size, and diversity by the Devonian period. They remained a rather constant and ecologically recognizable ecosystem until the end of the Permian—when the reefs and so much else were decimated by the Permian mass extinction.

Let us imagine that we were capable of going back into time and take a dive on a Paleozoic coral reef, one that is 400 million years in age. At first glance there is a surprising similarity to the reefs of today. Corals dominate the reefs; they are the bricks of the 3-D structures that are reefs, and like a brick house, are held together by many kinds of biological mortar, mostly encrusting species that serve to cement and bind the many heads and coral fronds into enormous and complex ramparts and foundations of limestone. But on closer view, the 400-million-year-old corals can be seen to be entirely different in basic appearance, and certainly in taxonomic composition. The massive coral heads are composed of a family that while building overall shapes similar to the corals of today are actually very different in their finer morphology: these are the tabulate corals, and these filled the same niches as are held today by scleractinian corals, the common corals of our modern reefs. Between these broadly branching and hemispherical tabulate coral colonies are other “framework builders,” other bricks in the wall. Many of these are stromatoporoids, a strange, carbonate-producing sponge still living today, but never in the sizes or diversity of the Paleozoic era. Scattered among these two massive inhabitants are a second kind of coral, solitary in nature, called rugose corals, solitary species that look like the horns of a bull, but in this case the pointed end of the “horn,” the calcium carbonate skeleton of these rugose
corals, are cemented to the substrate, and the widest end, facing up, is the seat for a single broad sea-anemone-looking animal.

Like our modern scleractinian corals, no matter how large, and composed of how many of the small, tentacled bodies that are the basic body plan of a coral, the tabulates were a single “individual”—at least genetically. But in fact all corals, surely then as well as now, are vast
colonies
of tiny sea-anemone-like polyps, each a ring of poison-tipped tentacles surrounding a small central mouth. But unlike a seashore rock covered with a smattering of the small, common sea anemones (which are solitary polyps) found the world over, each of these tiny polyps linked to others around it by a thin sheet of tissue. Every part of these sometimes-vast colonies is genetically identical. But this is not just one animal. In fact, any coral supports a vast and diverse assemblage of plants within its tissues. Throughout both the coral’s polyp-to-polyp connecting tissue, as well as in the polyp itself, are untold numbers of tiny plants—single-celled dinoflagellates that live in symbiotic bliss with the corals. It is a great deal for both: the tiny plants get the four things they most want: light, carbon dioxide, nutrients (phosphates and nitrates), and protection within the coral flesh, protection from the many organisms that would love to dine on a tasty if tiny plant.

The Cambrian period came to an end because of a mass extinction, one that affected many of the more successful members of what has come to be known as the Cambrian fauna—sea life composed of such early animal entrants in the overall history of animal life as trilobites, brachiopods, and many of the very exotic arthropods of the Burgess Shale, such as
Anomalocaris
(although in 2010 a new fossil deposit of Ordovician age has yielded the youngest
Anomalocaris
of all, and thus perhaps the Late Cambrian mass extinction was kinder to some of the odd Burgess Shale fauna than previously considered). This particular extinction has long been known, but it is not listed as “major,” in that
less than 50 percent of marine forms died out. This acted like gasoline on the open fire of diversification, perhaps, in that those forms that were less adaptive died out, opening the way for new innovation and new species in the same manner that ridding a garden of weeds leads to a rapid proliferation of new growth from the nonweeded.

It was also as if the biological world discovered entirely new ways for animals and plants to make a living, as well as finding entirely new places to live: areas that were poorly populated in the Cambrian, such as brackish water and freshwater, as well as both deeper and shallower areas in the sea, right into the surf zones themselves, became ripe for colonization by animals. Many of these were still sedentary, spending their entire lives sitting in one place, filtering the ever-richer and more nutritious marine plankton. But species numbers and biomass numbers alike climbed.
³

Many kinds of animals were present in the Ordovician that had not yet evolved in the Cambrian, and many of these appeared soon after the end of the Cambrian mass extinction. The result was an assemblage of animals that is markedly different than most of the Cambrian faunas. Trilobites are still there, but compared to the Cambrian oceans, when they may have been the most commonly encountered animal at most depths, they were overwhelmed in numbers as well as numbers of species of animals with shells—the brachiopods and more than a few mollusks as well. The biggest winners were animals that had evolved an entirely novel way of living—animals that were colonial. While colony formation was something that had been used by other biology far simpler in body plan, including many kinds of plants, microbes, and protozoa, in the Ordovician the leitmotif of colonial life dominated and drove the relentless diversification that is the hallmark of the Ordovician: corals, bryozoans, and new kinds of sponges among many others.

The reasons for this great diversification go back to oxygen.
⁴
Our view is that the true effects of oxygenation in the sea can be seen from this point. Here, then, we will make an interpretation that historians do, one that is as yet still new enough in science that it cannot be considered as hard truth, but one that has enormous explanatory power. It
also lets us look, quite appropriately at this point in the book, at an overview of animal diversification. We will argue that it was oxygen levels more than any other factor that has left us with the diversity curve of animals through time, results that are hard science and accepted.

The Ordovician period can be regarded as the second part of the two-part initiation of animal diversity on Earth, with the first being the Cambrian explosion,
⁵
and in both cases, rising oxygen was the driver. Like the Cambrian, it was a time when new species as well as new kinds of body plans appeared at a faster rate than was characteristic of more recent times. This high rate of evolution and innovation was partly in response to filling up the world with animals for the first time. The history of life in the Cambrian was a filling of the seas with many experiments. The post-Cambrian history was one where many of these early and clearly primitive and inefficient evolutionary designs were replaced in what became a rocketing increase in biodiversity as competition ruthlessly killed off the less fit. Evolution became a means of exploring the engineering excellence of body plans.

The history of biodiversity, which can be thought of as the assembly and numbers of the various categories of organisms (especially animals, because they leave the most abundant and recognizable fossils), was first presented by the English geologist John Phillips, who is also credited with subdividing the geological time scale through the introduction of the concepts of Paleozoic, Mesozoic, and Cenozoic eras. Phillips, who published his monumental work in 1860 both defining these new eras and discerning the largest-scale pattern of evolutionary change that can be found in the fossil record, recognized that major mass extinctions in the past could be used to subdivide geological time, since the aftermath of each such event resulted in the appearance of a new fauna as recognized in the fossil record. But Phillips did far more than recognize the importance of past mass extinctions and define new geological time terms: he proposed that diversity in the past was far
lower than in the modern day, and that the rise of biodiversity has been one of wholesale increases in the number of species, except during and immediately after the mass extinctions. His scheme recognized that mass extinctions slowed down diversity, but only temporarily. Phillips’s view of the history of diversity was completely novel. Yet a century passed before the topic was again given scientific attention.

In the late 1960s, paleontologists Norman Newell and James Valentine again considered the problem of exactly when and at what rate the world became populated with species of animals and plants.
⁶
Both wondered if the real pattern of diversification was of a rapid increase in species following the so-called Cambrian explosion of about 530 to 520 million years ago (using the revised dates, not those favored in the 1960s), followed by an approximate steady state. Their arguments rested on the importance of preservation biases in older rocks. Perhaps the pattern of increasing diversification through time seen by Phillips was in reality the record of
preservation
through time, rather than the real evolutionary
pattern
of diversification. According to this argument, the change of species is reduced in ever-older rocks, so that sampling bias is the real agent producing the so-called diversification he saw. This view was soon after echoed by paleontologist David Raup in a series of papers
⁷
that forcefully argued that there are strong biases against older species being discovered and named by scientists, since older rocks experience more alteration through recrystallization, burial, and metamorphism; entire regions or biogeographic provinces have been lost through time (therefore reducing the record for older rocks); and there is simply more rock of younger age to be searched.