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 couldnt wait to get going, but then, having gotten 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 goon 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 lichenfour hundred types of themadhering devotedly to every wind-whipped rock.
For a long time, people couldnt understand how they did it. Because lichens grew on bare rock without evident nourishment or the production of seeds, many peopleeducated peoplebelieved they were stones caught in the process of becoming plants. Spontaneously, inorganic stone becomes living plant! rejoiced one observer, a Dr. Homschuch, 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 that 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 weve been endowed with. But whats life to a lichen? Yet its impulse to exist, to be, is every bit as strong as oursarguably 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 dont. Like virtually all living things, they will suffer any hardship, endure any insult, for a moments additional existence. Life, in short, just wants to be. Butand heres an interesting pointfor the most part it doesnt 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-billion-odd years of Earths history compressed into a normal earthly day, then life begins very early, about 4A.M., with the rise of the first simple, single-celled organisms, but then advances no further for the next sixteen hours. Not until almost 8:30 in the evening, with the day five-sixths over, has 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 9:04P.M.trilobites swim onto the scene, followed more or less immediately by the shapely creatures of the Burgess Shale. Just before 10P.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 11P.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 one even larger. Its a wonder that anything at all can survive in such a pummeled and unsettled environment. In fact, not many things do for long.
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 inBasin 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 hasnt happened, but the chances are good that it will. I dont 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 isnt terribly ambitious.
So anytime 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 architecturenot 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 landmass, Pangaea, meant there was much, much less coastline than formerly and thus much 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 eons 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 that 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 fishsomething like the modern mudskipper, which can hop from puddle to puddle during droughtsor even as a fully formed amphibian. In fact, the first visible mobile residents on dry land were probably much more like modern wood lice, 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 percent (as opposed to nearer 20 percent now). This allowed animals to grow remarkably large remarkably quickly.
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 that 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. Its the same chemical trick that goes on in (and is discussed elsewhere in relation to) the long-term carbon cyclea process that doesnt make for terribly exciting narrative but is vital for creating a livable 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 isotopesoxygen-16 and oxygen-18. (If you have forgotten what an isotope is, it doesnt matter, though for the record its 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 these ancient ratios, the geochemists can cunningly read conditions in the ancient worldoxygen levels, air and ocean temperatures, the extent and timing of ice ages, and much else. By combining their isotope findings with other fossil residuespollen levels and so onscientists can, with considerable confidence, re-create entire landscapes that no human eye ever saw.
The principal reason oxygen levels were able to build up so robustly throughout the period of early terrestrial life was that much of the worlds 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 three feet 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 tongue shot: they learned to fly. Some took to this new means of locomotion with such uncanny facility that they havent changed their techniques in all the time since. Then, as now, dragonflies could cruise at up to thirty-five miles an hour, instantly stop, hover, fly backwards, and lift far more proportionately than any human flying machine. 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 fifty feet, club mosses to a hundred and thirty.
The first terrestrial vertebrateswhich is to say, the first land animals from which we would deriveare 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.
Most animals are tetrapods, and all living tetrapods have one thing in common: four limbs that end in a maximum of five fingers or toes. Dinosaurs, whales, birds, humans, even fishall 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 three-foot-long animal called anIchthyostega . 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 study his tetrapod. The worlds paleontologists had to be content with two sketchy interim papers in which Jarvik noted that the creature had five fingers in each of four limbs, confirming its ancestral importance.
Jarvik died in 1998. After his death, other paleontologists eagerly examined the specimen and found that Jarvik had severely miscounted the fingers and toesthere were actually eight on each limband 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 dont 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 bookComet ). 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 to be 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.
Over time, each of these principal groupings split into further subdivisions, of which some prospered and some faltered. Anapsids gave rise to the turtles, which for a time, perhaps a touch improbably, appeared poised to predominate as the planets most advanced and deadly species, before an evolutionary lurch let them settle for durability rather than dominance. The synapsids divided into four streams, only one of which survived beyond the Permian. Happily, that was the stream we belonged to, and it evolved into a family of protomammals known as therapsids. These formed Megadynasty 2.
Unfortunately for the therapsids, their cousins the diapsids were also productively evolving, in their case into dinosaurs (among other things), which gradually proved too much for the therapsids. Unable to compete head to head with these aggressive new creatures, the therapsids by and large vanished from the record. A very few, however, evolved into small, furry, burrowing beings that bided their time for a very long while as little mammals. The biggest of them grew no larger than a house cat, and most were no bigger than mice. Eventually, this would prove their salvation, but they would have to wait nearly 150 million years for Megadynasty 3, the Age of Dinosaurs, to come to an abrupt end and make room for Megadynasty 4 and our own Age of Mammals.