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 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-billion-odd 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 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 9:04 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 one even larger. It’s 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 in
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 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 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 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 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 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. 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 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 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 these ancient ratios, the geochemists can cunningly 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-pollen levels and so on-scientists 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 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 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 haven’t 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
