Read Online A New History of Life by Peter Ward - Free Book Online Page B

onset of the Greek word “cryo,” all right: a great freeze. The result of the Proterozoic snowball Earth episodes was that there must have been a great mass extinction in the oceans, and perhaps on land as well. Their populations crashed during the snowball Earth episodes—when all or very nearly all of the Earth’s surface was covered by ice or snow—but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic.
    It is a given that any young aspiring paleontologist will gravitate to dinosaurs above all fossils. Since professional paleontologists always start as fossil-mad kids, in fact the supposedly less exciting fossil groups attract far less attention, even among the eventual professionals. Few indeed are the young scientists wanting to study tiny microfossils. And yet some of the most important of all scientific questions can be answered with them. So it is with larger questions about the history of life, as the acritarchs and other microfossils of a billion years ago are rich in question-answering information, and only recently have provided whole new insights into what was, in fact, a hugely important time period in the history of life, beginning at a billion years ago.
    From 2 to 1 billion years ago, the microfossils of Earth were simple and long ranging through the rock record. They must have been formed by both prokaryotic and small (compared to later) eukaryotes of the single-celled variety, such as the still-living protozoans. But about a billion years ago a strange thing happened. The formerly unornamented microfossils began to acquire ornamentation.
    The increased spinosity of acritarchs, starting about 1 billion years ago but then continuing through the Cambrian period, could have several causes. First, spines on a small sphere would increase surface area to volume relationships, and thus slow the rate of settling of these tiny spheres in the ocean. Many planktonic species extant today use this method of staying high in the water column rather than sinkingonto a deep bottom and sure burial under the constant snowfall of sediments that typifies most ocean bottoms. But a second use of spines is defense against predators. Perhaps the oceans of a billion years ago began to harbor an ever-greater rogues’ gallery of carnivores (or for acritarchs, these might technically be considered herbivores). In any event, eaten is eaten, no matter what one calls the eater. Yet the new work of Knoll and his group now show that spiny microfossils became ever more diverse and abundant soon after the end of the last snowball Earth, some 635 million years ago, but then utterly disappeared about 560 million years ago, a time when the evolution of animals was well under way. In the next chapter we will see how an understanding of what we might call the Ediacaran revolution is importantly fleshed out by the record of spiny microfossils, as well as by their disappearance. We will return to the story of these spiny microfossils in the next chapter.

    Changes in the morphology of acritarchs, enigmatic microfossils that were composed of a number of distinct kinds of small, marine, floating organisms. Notice the change from Proterozoic (A), which are smooth, to the very spiky forms of the late Neoproterozoic (B) and Cambrian (C).
THE END OF THE BORING BILLION
    Here is a view of a shallow sea bottom, some 1 billion years ago: Kelp-like plants and green algae wave in the currents, as do shimmering mats of rainbow-hued microbial life, multicolored sheaths of the softest chiffon covering all of the sunlit portions of the bottom. 5 Stromatolites peak out from the bottom sheaths, large to small domesand hummocks punching upward out of the microbial sheaths. The water is thick with life, single celled to multicellular. There is nary an animal anywhere on the planet. But a genetic and atmospheric clock is ticking down toward catastrophe and icy crèche.
    In the oceans a revolution was brewing a billion