Shadows of Forgotten Ancestors Read Online Free PDF Page B

imagine, are microscopic, discovered only by painstaking scientific analysis. Someare. But some of the most ancient traces left by life on Earth are easily visible to the untrained naked eye—although the beings that made them were microscopic. Often meticulously preserved, they’re called stromatolites; not unusual are examples the size of a basketball or a watermelon. A few are half the length of a football field. Stromatolites are
big
. Their age is read from the radioactive clocks in the ancient basaltic lava in which they are embedded.
    They still grow and flourish today—in warm bays, lagoons, and inlets in Baja California, Western Australia, or the Bahamas. They’re composed of successive layers of sediment generated by mats of bacteria. The individual cells live together. They must know how to get on with the neighbors.
    We glimpse the earliest lifeforms on Earth and the first message conveyed is not of Nature red in tooth and claw, but of a Nature of cooperation and harmony. Of course, neither extreme is the whole truth; and, examining modern stromatolites more closely, we find single-celled microbes freely swimming in and around the mats. Some of them are busily devouring their fellows. Perhaps they too were there from the beginning.
    Some stromatolite communities are photosynthetic; they know how to convert sunlight, water, and carbon dioxide into food. Even today, we humans are unable to build a machine that can perform this transformation with the efficiency of a photosynthetic microbe, much less a liverwort. Yet 3.6 billion years ago the stromatolitic bacteria could do it.
    Exactly what happened between the time of the first seas, rich in organic molecules and future prospects, and the time of the first stromatolites is beyond our present ability to reconstruct. Stromatolite-forming microbes could hardly have been the first living things. Before there were colonial forms, there must, it seems, have been individual, free-living, one-celled organisms. And before that, something even simpler. Perhaps before the first photosynthetic organisms, there were little beings that could eat the organic matter littering the landscape: Eating food seems to be a great deal less demanding than manufacturing it. And those little beings themselves had ancestors … and so on, back to the earliest molecule or molecular system able to make crude copies of itself.
    Why did colonial forms develop so early? Maybe it was because ofthe air. Oxygen, generated today by green plants, must have been in short supply before the Earth was covered by vegetation. But ozone is generated from oxygen. No oxygen, no ozone. If there’s no ozone, the searing ultraviolet light (UV) from the Sun will penetrate to the ground. The intensity of UV at the surface of the Earth in those early days may have reached lethal levels for unprotected microbes, as it has on Mars today. We are concerned—and for good reason—that chlorofluorocarbons and other products of our industrial civilization will reduce the amount of ozone by a few tens of percent. The predicted biological consequences are dire. How much more serious it must have been to have no ozone shield at all.
    In a world with deadly UV reaching the surface of the waters, sunblock may have been the key to survival—as it may become again. Modern stromatolite microorganisms secrete a kind of extracellular glue that helps them to stick together and also to adhere to the ocean floor. There would have been an optimum depth, not so shallow as to be fried outright by unfiltered UV, and not so deep that the visible light is too feeble for photosynthesis. There, partly shielded by sea-water, it would have been advantageous for the organisms to put some opaque material between themselves and the UV. Suppose, in reproducing, the daughter cells of one-celled organisms did not separate and go their individual ways, but instead remained attached to one another, generating—after many reproductions—an