by Bill Bryson
A sleeker, more cartoon-like view of Anomalocaris shows how much room for interpretation ancient fossils often give. (Credit 21.10)
So the Burgess Shale specimens weren’t so spectacular after all. This made them, as Fortey has written, “no less interesting, or odd, just more explicable.” Their weird body plans were just a kind of youthful exuberance—the evolutionary equivalent, as it were, of spiked hair and tongue studs. Eventually the forms settled into a staid and stable middle age.
But that still left the enduring question of where all these animals had come from—how they had suddenly appeared from nowhere.
Alas, it turns out the Cambrian explosion may not have been quite so explosive as all that. The Cambrian animals, it is now thought, were probably there all along, but were just too small to see. Once again it was trilobites that provided the clue—in particular, that seemingly mystifying appearance of different types of trilobite in widely scattered locations around the globe, all at more or less the same time.
On the face of it, the sudden appearance of lots of fully formed but varied creatures would seem to enhance the miraculousness of the Cambrian outburst, but in fact it did the opposite. It is one thing to have one well-formed creature like a trilobite burst forth in isolation—that really is a wonder—but to have many of them, all distinct but clearly related, turning up simultaneously in the fossil record in places as far apart as China and New York, clearly suggests that we are missing a big part of their history There could be no stronger evidence that they simply had to have a forebear—some grandfather species that started the line in a much earlier past.
And the reason we haven’t found these earlier species, it is now thought, is that they were too tiny to be preserved. Says Fortey: “It isn’t necessary to be big to be a perfectly functioning, complex organism. The sea swarms with tiny arthropods today that have left no fossil record.” He cites the little copepod, which numbers in the trillions in modern seas and clusters in shoals large enough to turn vast areas of the ocean black, and yet our total knowledge of its ancestry is a single specimen found in the body of an ancient fossilized fish.
“The Cambrian explosion, if that’s the word for it, probably was more an increase in size than a sudden appearance of new body types,” Fortey says. “And it could have happened quite swiftly, so in that sense I suppose it was an explosion.” The idea is that, just as mammals bided their time for a hundred million years until the dinosaurs cleared off and then seemingly burst forth in profusion all over the planet, so too perhaps the arthropods and other triploblasts waited in semi-microscopic anonymity for the dominant Ediacaran organisms to have their day. Says Fortey: “We know that mammals increased in size quite dramatically after the dinosaurs went—though when I say quite abruptly I of course mean it in a geological sense. We’re still talking millions of years.”
Incidentally, Reginald Sprigg did eventually get a measure of overdue credit. One of the main early genera, Spriggina, was named in his honour, as were several species, and the whole became known as the Ediacaran fauna after the hills through which he had searched. By this time, however, Sprigg’s fossil-hunting days were long over. After leaving geology he founded a successful oil company and eventually retired to an estate in his beloved Flinders Range where he created a wildlife reserve. He died in 1994 a rich man.
A tyrannosaurus expresses understandable alarm at the arrival of a lethal meteor in this irresistibly dramatic but misleading interpretation of the last instant before impact of the rock that wiped out the dinosaurs. Travelling faster than a bullet, an incoming meteor would be moving much too swiftly to be seen, much less to provoke alarm. (Credit 22.1)
GOODBYE TO ALL THAT
When you consider it from a human perspective, and clearly it would be difficult for us to do otherwise, life is an odd thing. It couldn’t wait to get going, but then, having got going, it seemed in very little hurry to move on.
Consider the lichen. Lichens are just about the hardiest visible organisms on Earth, but among the least ambitious. They will grow happily enough in a sunny churchyard, but they particularly thrive in environments where no other organism would go—on blowy mountaintops and Arctic wastes, wherever there is little but rock and rain and cold, and almost no competition. In areas of Antarctica where virtually nothing else will grow, you can find vast expanses of lichen—400 types of them—adhering devotedly to every wind-whipped rock.
For a long time, people couldn’t understand how they did it. Because lichens grew on bare rock without evident nourishment or the production of seeds, many people—educated people—believed they were stones caught in the process of becoming plants. “Spontaneously, inorganic stone becomes living plant!” rejoiced one observer, a Dr. Hornschuch, in 1819.
Closer inspection showed that lichens were more interesting than magical. They are in fact a partnership between fungi and algae. The fungi excrete acids which dissolve the surface of the rock, freeing minerals that the algae convert into food sufficient to sustain both. It is not a very exciting arrangement, but it is a conspicuously successful one. The world has more than twenty thousand species of lichens.
Like most things that thrive in harsh environments, lichens are slow-growing. It may take a lichen more than half a century to attain the dimensions of a shirt button. Those the size of dinner plates, writes David Attenborough, are therefore “likely to be hundreds if not thousands of years old.” It would be hard to imagine a less fulfilling existence. “They simply exist,” Attenborough adds, “testifying to the moving fact that life even at its simplest level occurs, apparently, just for its own sake.”
It is easy to overlook this thought that life just is. As humans we are inclined to feel that life must have a point. We have plans and aspirations and desires. We want to take constant advantage of all the intoxicating existence we’ve been endowed with. But what’s life to a lichen? Yet its impulse to exist, to be, is every bit as strong as ours—arguably even stronger. If I were told that I had to spend decades being a furry growth on a rock in the woods, I believe I would lose the will to go on. Lichens don’t. Like virtually all living things, they will suffer any hardship, endure any insult, for a moment’s additional existence. Life, in short, just wants to be. But—and here’s an interesting point—for the most part it doesn’t want to be much.
This is perhaps a little odd, because life has had plenty of time to develop ambitions. If you imagine the 4,500 million years of Earth’s history compressed into a normal earthly day, then life begins very early, about 4 a.m., with the rise of the first simple, single-celled organisms, but then advances no further for the next sixteen hours. Not until almost eight-thirty in the evening, with the day five-sixths over, has the Earth anything to show the universe but a restless skin of microbes. Then, finally, the first sea plants appear, followed twenty minutes later by the first jellyfish and the enigmatic Ediacaran fauna first seen by Reginald Sprigg in Australia. At 9:04 p.m. trilobites swim onto the scene, followed more or less immediately by the shapely creatures of the Burgess Shale. Just before 10 p.m. plants begin to pop up on the land. Soon after, with less than two hours left in the day, the first land creatures follow.
Thanks to ten minutes or so of balmy weather, by 10:24 the Earth is covered in the great carboniferous forests whose residues give us all our coal, and the first winged insects are evident. Dinosaurs plod onto the scene just before 11 p.m. and hold sway for about three-quarters of an hour. At twenty-one minutes to midnight they vanish and the age of mammals begins. Humans emerge one minute and seventeen seconds before midnight. The whole of our recorded history, on this scale, would be no more than a few seconds, a single human lifetime barely an instant. Throughout this greatly speeded-up day, continents slide about and bang together at a clip that seems positively reckless. Mountains rise and melt away, ocean basins come and go, ice sheets advance and withdraw. And throughout the whole, about three times every minute, somewhere on the planet there is a flashbulb pop of light marking the impact
of a Manson-sized meteor or larger. It’s a wonder that anything at all can survive in such a pummelled and unsettled environment. In fact, not many things do for long.
Remnants of a lost age: petrified logs in Petrified Forest National Park in Arizona. (Credit 22.2)
Perhaps an even more effective way of grasping our extreme recentness as a part of this 4.5-billion-year-old picture is to stretch your arms to their fullest extent and imagine that width as the entire history of the Earth. On this scale, according to John McPhee in Basin and Range, the distance from the fingertips of one hand to the wrist of the other is Precambrian. All of complex life is in one hand, “and in a single stroke with a medium-grained nail file you could eradicate human history.”
Fortunately, that moment hasn’t happened, but the chances are good that it will. I don’t wish to interject a note of gloom just at this point, but the fact is that there is one other extremely pertinent quality about life on Earth: it goes extinct. Quite regularly. For all the trouble they take to assemble and preserve themselves, species crumple and die remarkably routinely. And the more complex they get, the more quickly they appear to go extinct. Which is perhaps one reason why so much of life isn’t terribly ambitious.
So any time life does something bold it is quite an event, and few occasions were more eventful than when life moved on to the next stage in our narrative and came out of the sea.
Land was a formidable environment: hot, dry, bathed in intense ultraviolet radiation, lacking the buoyancy that makes movement in water comparatively effortless. To live on land, creatures had to undergo wholesale revisions of their anatomies. Hold a fish at each end and it sags in the middle, its backbone too weak to support it. To survive out of water, marine creatures needed to come up with new load-bearing internal architecture—not the sort of adjustment that happens overnight. Above all and most obviously, any land creature would have to develop a way to take its oxygen directly from the air rather than filter it from water. These were not trivial challenges to overcome. On the other hand, there was a powerful incentive to leave the water: it was getting dangerous down there. The slow fusion of the continents into a single land mass, Pangaea, meant there was much less coastline than formerly and thus less coastal habitat. So competition was fierce. There was also an omnivorous and unsettling new type of predator on the scene, one so perfectly designed for attack that it has scarcely changed in all the long aeons since its emergence: the shark. Never would there be a more propitious time to find an alternative environment to water.
Plants began the process of land colonization about 450 million years ago, accompanied of necessity by tiny mites and other organisms which they needed to break down and recycle dead organic matter on their behalf. Larger animals took a little longer to emerge, but by about 400 million years ago they were venturing out of the water, too. Popular illustrations have encouraged us to envision the first venturesome land dwellers as a kind of ambitious fish—something like the modern mudskipper, which can hop from puddle to puddle during droughts—or even as a fully formed amphibian. In fact, the first visible mobile residents on dry land were probably much more like modern woodlice, sometimes also known as pillbugs or sow bugs. These are the little bugs (crustaceans, in fact) that are commonly thrown into confusion when you upturn a rock or log.
For those that learned to breathe oxygen from the air, times were good. Oxygen levels in the Devonian and Carboniferous periods, when terrestrial life first bloomed, were as high as 35 per cent (as opposed to nearer 20 per cent now). This allowed animals to grow remarkably large remarkably quickly.
Thanks to their practice of drawing oxygen from the atmosphere, marine plankton, like the diatoms shown here, enable geochemists to work out oxygen levels on Earth in prehistoric times. (Credit 22.3)
And how, you may reasonably wonder, can scientists know what oxygen levels were like hundreds of millions of years ago? The answer lies in a slightly obscure but ingenious field known as isotope geochemistry. The long-ago seas of the Carboniferous and Devonian swarmed with tiny plankton which wrapped themselves inside tiny protective shells. Then, as now, the plankton created their shells by drawing oxygen from the atmosphere and combining it with other elements (carbon especially) to form durable compounds such as calcium carbonate. It’s the same chemical trick that goes on in (and is discussed elsewhere in relation to) the long-term carbon cycle—a process that doesn’t make for terribly exciting narrative but is vital for creating a habitable planet.
Eventually in this process all the tiny organisms die and drift to the bottom of the sea, where they are slowly compressed into limestone. Among the tiny atomic structures the plankton take to the grave with them are two very stable isotopes—oxygen-16 and oxygen-18. (If you have forgotten what an isotope is, it doesn’t matter, though for the record it’s an atom with an abnormal number of neutrons.) This is where the geochemists come in, for the isotopes accumulate at different rates depending on how much oxygen or carbon dioxide is in the atmosphere at the time of their creation. By comparing the ancient rates of deposition of the two isotopes, geochemists can read conditions in the ancient world—oxygen levels, air and ocean temperatures, the extent and timing of ice ages and much else. By combining their isotope findings with other fossil residues that indicate other conditions such as pollen levels and so on—scientists can, with considerable confidence, recreate entire landscapes that no human eye ever saw.
The principal reason oxygen levels were able to build so robustly throughout the period of early terrestrial life was that much of the world’s landscape was dominated by giant tree ferns and vast swamps, which by their boggy nature disrupted the normal carbon recycling process. Instead of completely rotting down, falling fronds and other dead vegetative matter accumulated in rich, wet sediments, which were eventually squeezed into the vast coal beds that sustain much economic activity even now.
The heady levels of oxygen clearly encouraged outsized growth. The oldest indication of a surface animal yet found is a track left 350 million years ago by a millipede-like creature on a rock in Scotland. It was over a metre long. Before the era was out some millipedes would reach lengths more than double that.
With such creatures on the prowl, it is perhaps not surprising that insects in the period evolved a trick that could keep them safely out of tongueshot: they learned to fly. Some took to this new means of locomotion with such uncanny facility that they haven’t changed their techniques in all the time since. Then, as now, dragonflies could cruise at over 50 kilometres an hour, instantly stop, hover, fly backwards, and lift far more, proportionately, than any flying machine humans have come up with. “The U.S. Air Force,” one commentator has written, “has put them in wind tunnels to see how they do it, and despaired.” They, too, gorged on the rich air. In Carboniferous forests dragonflies grew as big as ravens. Trees and other vegetation likewise attained outsized proportions. Horsetails and tree ferns grew to heights of 15 metres, club mosses to 40 metres.
The first terrestrial vertebrates—which is to say, the first land animals from which we would derive—are something of a mystery. This is partly because of a shortage of relevant fossils, but partly also because of an idiosyncratic Swede named Erik Jarvik, whose odd interpretations and secretive manner held back progress on this question for almost half a century. Jarvik was part of a team of Scandinavian scholars who went to Greenland in the 1930s and 1940s looking for fossil fish. In particular they sought lobe-finned fish of the type that presumably were ancestral to us and all other walking creatures, known as tetrapods.
An illustration of Ichthyostega, traditionally portrayed as one of the first animals to make the transition from sea to land—though quite how at home it would have been on land is a matter of some debate. Surface living provided great opportunities but also formidable challenges. (Credit 22.4)
Most animals are tetrapods, and all living tetrapods have one thing in common: four limbs, each of which ends in a maximum of five fingers or toes. Dinosaurs, whales,
birds, humans, even fish—all are tetrapods, which clearly suggests they come from a single common ancestor. The clue to this ancestor, it was assumed, would be found in the Devonian era, from about 400 million years ago. Before that time nothing walked on land. After that time lots of things did. Luckily the team found just such a creature, a metre-long animal called an Ichthyostega. The analysis of the fossil fell to Jarvik, who began his study in 1948 and kept at it for the next forty-eight years. Unfortunately, Jarvik refused to let anyone else study his tetrapod. The world’s palaeontologists had to be content with two sketchy interim papers in which Jarvik noted that the creature had five fingers on each of four limbs, confirming its ancestral importance.
Jarvik died in 1998. After his death, other palaeontologists eagerly examined the specimen and found that Jarvik had severely miscounted the fingers and toes—there were actually eight on each limb—and failed to observe that the fish could not possibly have walked. The structure of the fin was such that it would have collapsed under its own weight. Needless to say, this did not do a great deal to advance our understanding of the first land animals. Today three early tetrapods are known and none has five digits. In short, we don’t know quite where we came from.
But come we did, though reaching our present state of eminence has not, of course, always been straightforward. Since life on land began, it has consisted of four megadynasties, as they are sometimes called. The first consisted of primitive, plodding but sometimes fairly hefty amphibians and reptiles. The best-known animal of this age was the Dimetrodon, a sail-backed creature that is commonly confused with dinosaurs (including, I note, in a picture caption in the Carl Sagan book Comet). The Dimetrodon was in fact a synapsid. So, once upon a time, were we. Synapsids were one of the four main divisions of early reptilian life, the others being anapsids, euryapsids and diapsids. The names simply refer to the number and location of small holes found in the sides of their owners’ skulls. Synapsids had one hole in their lower temples; diapsids had two; euryapsids had a single hole higher up.
