A New History of Life Read Online Free PDF Page A

chapter.
    There is a long-standing puzzle associated with these BIF sediments—in order to be distributed so widely and gently, the iron has to have been dissolved in water—and that means it should have been in the greenish reducing form called ferrous iron . On the other hand, for it to have been precipitated out means that it was rusted into the red ferric form, which is not soluble in water at all: it simply falls out of water as particles, rather than dissolving in water, as a cube of sugar would. The problem is oxygen: ferrous iron reacts instantly with free molecular O 2 to form the red ferric state. Any iron or iron mineral that is bright red in color tells us that the iron has undergone this chemical change, which we commonly call rust, and that almost always requires molecular O 2 . How could oxygen levels in the ocean waters be low enough to allow the iron to stay in the green soluble form, and yet then be available to make it rust? This was a long and perplexing scientific mystery.
    Over fifty years ago, one of the important figures in Precambrian paleobiology, Preston Cloud of the University of California at Santa Barbara, hypothesized that the oxygen needed to turn dissolved ferrous iron into the rusted, particular ferric iron in the oceans came from a group of primitive photosynthetic microbes known as the blue-green algae, which are now called the cyanobacteria. 3 This is the only organism on Earth that ever learned how to perform the life-giving process of oxygenic photosynthesis, which is literally the ability to cleave a water molecule and liberate its oxygen atom. Some of their descendants were enslaved by other organisms, and now serve us all as the green light-gathering organelles in plants and other algae. Every plant on Earth now has tiny “capsules” that evolved from those first cyanobacteria, but are now “endosymbiosis” slaves doing the bidding of the multicellular plant. Preston Cloud envisioned a floating “oxygen oasis” of these first tiny photosynthesizers, the cyanobacteria, each excreting tiny amounts of oxygen, and over hundreds of millions of years radically changing the nature of not only life on Earth, but the chemistry of our planet’s oceans, atmosphere, and even rock cover. With eachtiny trace of new oxygen liberated into the ancient Archean sea, tiny flakes of rust would then settle to the ocean bottom, slowly but inexorably accumulating to make the banded iron formations.
    Molecular oxygen is one of the most toxic compounds around. Anyone who takes antioxidants along with their vitamin supplements knows that they fight cancer—and cancer is usually caused by oxygen wrecking delicate cell chemistry at the wrong time, in the wrong place, and changing it into a new zombie-like killer cell as a result. Antioxidants are not just an advertising slogan. Oxygen is a cell wrecker, cell changer, and often cell killer because of its chemical ferociousness. So how could the organisms producing this poison survive as soon as the oxygen molecules were liberated?
    This now leads to a classic “chicken and the egg” problem: any early life form that evolved a system to release O 2 without having protective antioxidant enzymes would have killed itself, so the systems to control oxygen would have had to evolve first. But all of the oxygen in our atmosphere is produced by oxygenic photosynthesis, so there should not have been any oxygen before this to drive the evolution of the protective enzymes! Thus, there must have been some nonbiological source that produced trace amounts of molecular oxygen and then exposed primitive cells to it in an environment where they could gradually evolve enzyme systems to protect them from this poison, in a way analogous to how we protect ourselves from killer diseases by exposing ourselves to tiny amounts of the disease when we are very young, letting our body gradually build up defenses.
    But where did this early oxygen “vaccine” come from if not from