Shadows of Forgotten Ancestors (21 page)

Even when the incompleteness of the fossil record is taken into account, the diversity or “taxonomic richness” of life on Earth is found to have been steadily increasing, especially in the last 100 million years.
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Diversity seems to have peaked just as humans were really getting going, and has since declined markedly—in part because of the recent ice ages, but in larger part because of the depredations of humans, both intentional and inadvertent. We are destroying the diversity of beings and habitats out of which we emerged. Something like a hundred species become extinct each day. Their last remnants die out. They leave no descendants. They are gone. Unique messages, painstakingly preserved and refined over eons, messages that a vast succession of beings gave up their lives to pass on to the distant future are lost forever.

More than a million species of animals are now known on Earth, and perhaps 400,000 species of eukaryotic plants. There are at least thousands of known species of other organisms, non-eukaryotes, including bacteria. Doubtless we have missed many, probably most. Some estimates of the number of species range beyond 10 million; if so, we have even glancing acquaintance with less than 10% of the species on Earth. Many are becoming extinct before we even know of their existence. Most of the billions of species of life that have ever lived are extinct. Extinction is the norm. Survival is the triumphant exception.

We’ve sketched the changes on the Earth’s surface at the end of the Permian Period, some 245 million years ago; they resulted in the most devastating biological catastrophe so far displayed in the fossil record. Perhaps as many as 95% of all the species then living on Earth became extinct.
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Many kinds of filter-feeding animals attached to the ocean floor, beings that had for hundreds of millions of years characterized life on Earth, disappeared. Ninety-eight percent of the families of crinoids became extinct. We don’t hear much about crinoids these days; sea lilies are their surviving remnant. Wholesale extinctions also occurred among the amphibians and reptiles that had settled the land. On the other hand, sponges and bivalves (like clams) did comparatively well in the late Permian extinction—one consequence of which is that they are still plentiful on Earth today.

Following mass extinctions it typically takes 10 million years or more for the variety and abundance of life on Earth to recover—and then, of course, there are different organisms around, perhaps better adapted to the new environment, perhaps with better long-term prospects, or perhaps not. In the millions of years following the end of the Permian Period, volcanism subsided and the Earth warmed. This killed off many land plants and animals that had been adapted to the late Permian cold. Out of this set of cascading climatic consequences, conifers and ginkgoes emerged. The first mammals evolved from reptiles in the new ecologies established after the Permian extinctions.

Of all the species of animals alive at the end of the Permian, only about twenty-five of them, it is estimated, have left any descendants at all; ten of which account for 98% of the contemporary families of
vertebrates, which comprise about forty thousand species.
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The rate of evolutionary change is full of fits and starts, blind alleys and sweeping change—the latter driven often by the first filling of a previously untenanted ecological niche. New species appear quickly and then persist for millions of years. In only the last 2% or 3% of the history of life on Earth, the extravagant diversification of the placental mammals has produced

For the vast bulk of Earth history, until just recently, not one of these beings had existed. They were present only potentially.

Think of the genetic instructions of a given being, perhaps a billion ACGT nucleotide pairs long. Randomly change a few nucleotides. Perhaps these will be in structural or inactive sequences and the organism is in no way altered. But if you change a meaningful DNA sequence, you change the organism. Most such changes, as we keep saying, are maladaptive; except in rare instances, the bigger the change, the more maladaptive it is. For all of mutation, gene recombination, and natural selection put together, the continuing experiment of evolution on Earth has brought into being only a minute fraction of the range of possible organisms whose manufacturing instructions could be specified by the genetic code. The vast bulk of those beings, of course, would be not merely maladapted, not just freaks, but wholly inviable. They could not be born alive. Nevertheless, the total number of possible functioning, living beings is still vastly greater than the total number of beings who have ever been. Some of those unrealized possibilities must be, by any standard we wish to adopt, better adapted and more capable than any Earthling who has ever lived.

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Sixty-five million years ago most of the species on Earth were snuffed out—probably because of a massive cometary or asteroidal collision. Among those killed off were all the dinosaurs, which had for nearly 200 million years—from before the breakup of Gondwanaland—been
the dominant species, the ubiquitous masters of life on Earth. This extinction event removed the chief predators of a small, fearful, cowering nocturnal order of animals called the mammals. If not for that collision—a late step in the tidying up of interplanetary space of the remaining worlds on eccentric orbits—we humans and our primate ancestors would never have come to be. And yet, if that comet had been on a slightly different trajectory, it might have missed the Earth entirely. Perhaps, in its many relays around the Sun, its ices would all have melted and its rocky and organic contents slowly spewed as fine powder into interplanetary space. Then all it would have provided for life on Earth would have been a periodic shower of meteors, perhaps admired by some newly-evolved, curious, large-brained reptile.

On the scale of the Solar System, the extinction of the dinosaurs and the rise of the mammals seem to have been a very near thing. The causality corridor, figuratively speaking, was only inches wide. Had the comet been traveling a little slower or faster or headed in a slightly different direction, no collision would then have occurred. If other comets that in our real history missed the Earth had been on slightly different trajectories, they would have hit the Earth and killed off life in some different epoch. The cosmic collision roulette, the extinction lottery, reaches into our own time.

At the depth in the fossil record above which there are no more dinosaurs, there is, worldwide, a telltale thin layer of the element iridium, which is abundant in space but not on the Earth’s surface. There also are tiny grains bearing the signs of a collosal impact. This evidence tells us of a high-speed collision of a small world with the Earth which distributed fine particles worldwide. The remains of the impact crater may have been discovered in the Gulf of Mexico near the Yucatan Peninsula. But something else is found in this layer as well: soot. Planet-wide, the time of this great impact was also the time of a global fire. The debris from the impact explosion, spewed out into the high atmosphere and falling back through the air all over the Earth—a continuous meteor shower filling the sky—illuminated the ground far more brightly than the noonday Sun. Land plants everywhere on Earth burst into flames, all at once. Most of them were consumed. There is an odd causal nexus connecting oxygen, plants, giant impacts, and world-immolating fire.

There are many ways in which such an impact could have extinguished
long-established and, if we may call them that, self-confident forms of life. After the initial burst of light and heat, a thick pall of impact dust enveloped the Earth for a year or more. Perhaps even more important than the world fire, the lowered temperatures, and a planet-wide acid rain was the absence for a year or two of enough light for photosynthesis. The primary photosynthesizing organisms in the oceans (then as now covering most of the Earth) are little one-celled plants called phytoplankton. They are especially vulnerable to lowered light levels because they lack major food reserves. Once the lights get turned out their chloroplasts can no longer generate carbohydrates from sunlight, and they die. But these little plants are the principal diet of one-celled animals that are eaten by larger, shrimp-like creatures, that are eaten by small fish, that are in turn eaten by large fish. Turn off the lights, wipe out the phytoplankton, and the entire food chain, this elaborate house of cards, collapses. Something similar is true on land.

The beings of Earth depend on one another. Life on Earth is an intricately woven tapestry or web. Yank out a few threads here and there, and you can’t be sure whether that’s all the damage you’ve done, or whether the whole fabric will now unravel.

Insects and other arthropods are the principal agents by which dead plants and animal excrement are cleaned up. Scarabs—the dung beetles identified with the sun god and worshipped by the ancient Egyptians—are specialists in waste management. They collect the nitrogen-rich animal excrement accumulating on the surface of our planet and transport this fertilizer down where the plant roots are. Some sixteen thousand beetles have been counted on a single fresh elephant pat in Africa; two hours later the pat was gone.
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The Earth’s surface would be very different (and very messy) without dung beetles and their like. In addition, the microscopic feces of mites and springtails are major constituents of the soil humus from which the plants grow. Animals then eat the plants. We live off each other’s solid wastes as well.

Other inhabitants of the soil kill off the young plants. Here is Darwin’s account of a little experiment he did to illustrate the hidden ferocity lurking just beneath the placid surface of a country garden:

Some plants provide food for specific animals, in turn, the animals act as agents for the sexual reproduction of the plants—in effect, couriers taking sperm from male plants and using it for artificial insemination of female plants. This is not quite artificial selection, because the animals are not much in charge. The currency these procurers are paid in is usually food. A bargain has been struck. Maybe the animal is a pollinating insect, or bird, or bat; or a mammal to whose furry coat the reproductive burrs adhere; or maybe the deal is food supplied by the plants in exchange for nitrogenous fertilizer supplied by the animals. Predators have symbionts that clean their coats or scales or pick their teeth in exchange for leavings. A bird eats a sweet fruit; the seeds pass through its digestive tract and are deposited on fertile ground some distance away: another business transaction consummated. Fruit trees and berry-bearing bushes often take care that their offerings to the animals are sweet only when the seeds are ready to be dispersed. Unripe fruit gives bellyaches, the plants’ way of training the animals.

The cooperation between plants and animals is uneasy. The animals cannot be trusted; given a chance, they’ll eat any plant in sight. So the plants protect themselves from unwelcome attention with thorns, or by producing irritants, or poisons, or chemicals that make the plant indigestible, or agents that interfere with the predator’s DNA. In this endless slow-motion war, the animals then produce substances that disable these adaptations by the plants. And so on.

The beasts and vegetables and microbes are the interlocking parts, the gear train, of a vast, intricate and very beautiful ecological machine of planetary proportions, a machine plugged into the Sun. Pretty nearly, all flesh is sunlight.

Where the ground is covered with plants perhaps 0.1% of the sunlight is converted into organic molecules. A plant-eating animal saunters by and eats one of these plants. Typically the herbivore extracts
about a tenth of the energy in the plant, or about one ten-thousandth of the sunlight that could, with 100% efficiency, have been stored in the plant. If the herbivore is now attacked and eaten by a carnivore, about 10% of the available energy in the prey will wind up in the predator. Only one part in a hundred thousand of the original solar energy makes it to the carnivore. There are no perfectly efficient engines, of course, and we expect losses at each stage in the food chain. But the organisms at the top of the food chain seem inefficient to the point of irresponsibility.
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A vivid image of the interconnection and interdependence of life on Earth was provided by the biologist Clair Folsome, who asks you to imagine what you would see if all the cells of your body, flesh and bones, were magically removed:

And, Folsome stresses, any other plant or animal on Earth, under the same dispensation, would reveal a similar “seething zoo of microbes.”
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A biologist from some other solar system, in an unblinking examination of the teeming lifeforms of Earth, would surely note that they are all made of almost exactly the same organic stuff, the same molecules almost always performing the same functions, with the same genetic codebook in use by almost everybody. The organisms on this planet are not only kin; they live in intimate mutual contact, imbibing each other’s wastes, dependent on one another for life itself, and sharing the same fragile surface layer. This conclusion is not ideology, but
reality. It depends not on authority, faith, or special pleading by its proponents, but on repeatable observation and experiment.