A New History of Life Read Online Free PDF Page B

multicellular life), there is a surprising disagreement about exactly when this seminal biological innovation happened. Over fifty years ago, geologists realized that some of the oldest stream-deposited sedimentary rocks on Earth contained rounded grains of the common mineral called pyrite (fool’s gold) as well as another mineral containing tiny amounts of uranium (the mineral named uraninite). These minerals are extremely unstable in the presence of oxygen (like iron, they quickly rust), and are never found in our open, oxygenated oceans and land areas unless completely cut off from contact with our normal, oxygenated atmosphere. That led to the initial concept that the atmosphere contained very little oxygen until some time near the end of Archean time, perhaps as late as 2.5 billion years ago or even later. Most of the geological community agrees that even at these dates, oxygen concentrations in the atmosphere were so low that both pyrite and uraninite grains could exist on land and in the sea without rusting, and in fact in rocks created as late as 2.5 billion years ago we find both pyrite anduraninite in abundance, telling us that at those dates the amount of oxygen in air and sea would have been nil. Yet by 2.4 billion years ago both kinds of minerals disappear from rocks created underwater or on land. Does this mean that cyanobacteria thus evolved only after 2.5 to 2.4 billion years ago? This has spurred a profound debate of great importance to understanding the history of life.
    How to solve this quite important question was to take years of research. The disagreement centered on whether the cyanobacteria evolved around 2.5 billion years ago, or perhaps 1 billion years earlier, nearer 3.4 billion years ago, and hence around almost as soon as life on Earth appeared in the first place. In the late 1990s the then-novel use of chemical fossils, also known as biomarkers, seemingly solved the problem: Australian geologists found what they concluded to be clear biomarker evidence that there had to be something creating oxygen in shallow oceans during the Late Archean (before 2.5 GA) time interval. They reported trace levels of biomarkers in Archean rocks that—at least in the modern biosphere—require molecular oxygen in the biosynthetic pathways; a class of organic molecules called sterols are a prime example.
    This discovery was singular enough that we will paraphrase the abstract from the paper itself: Molecular fossils (biomarkers) from 2,700-million-year-old sedimentary strata found from cores taken from an ancient part of the deep and old Australian sedimentary rock record indicate that when these ancient strata were actually deposited, they were in an environment shared by photosynthesizing bacteria called cyanobacteria, putting far back in time the oldest-known occurrence of these tiny, oxygen-producing microbial plants. But even more surprisingly, a second kind of biomarker called steranes found in the sampled strata provided persuasive evidence that not only the prokaryotic life forms were present, but that eukaryotes were there too—a group whose first fossils come from strata as much as a billion years younger than the age of the rock cores of this study.
    This paper, printed in the prestigious journal Science , hurled a revolutionary new finding at the scientific world for two reasons—the presence not only of photosynthesis producing oxygen at a very earlydate, but also the even more surprising discovery of one of the three great groups, or domains, of life, the Eukarya (the other being Bacteria and Archaea, both microbial and dominantly single celled) in the old rocks too. All this evidence came from cores extracted from deep in the Earth. The take-home point: both photosynthetic bacteria and eukaryotes existed far earlier than previously thought, all the way back to 2,700 million years ago. This electrifying paper in one fell swoop rewrote scientific history, and the history of life as well.
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