A short history of nearly everything - Bill Bryson [169]
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 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 vertebrates—which is to say, the first land animals from which we would derive—are something of a mystery. This is