Free Life on a Young Planet by Andrew H. Knoll

about living cyanobacteria, including their phylogenetic relationships.Add the good fortune that they are easily preserved and include species that can be recognized by form alone, and it becomes clear that cyanobacteria make an excellent flagship for paleontological studies of early life.
    Although most fossils in the Spitsbergen cherts are undoubted or probable cyanobacteria, rare specimens of relatively large microfossils (greater than 100 microns), some shaped like miniature vases ( plate 2f ) and others studded with spines, provide tantalizing glimpses of a different biology. These fossils are limited to sediments that washed into tidal channels from offshore, suggesting that if we continue our environmental transect seaward, we might discover a diversity of Proterozoic life barely hinted at by the cherts.
    During our field expeditions I collected many samples of black shales that accumulated in quiet subtidal environments beyond the ooid shoal. (Like the cherts, these shales are black because they contain organic matter.) Mesmerized by the chert biotas, I didn’t do much with our shale samples, but when Nick Butterfield (now, like Harland, at Cambridge University) came on board as a graduate student, I suggested that he take a look at them to get some firsthand experience with Precambrian rocks.
    Microscopic fossils are common in shales of all ages, packed cheek by jowl with clay minerals that inhibit decay. The mineral fabric of these rocks can be dissolved in strong acid, leaving organic remains to be mounted on glass slides and studied by optical or electron microscope. Conventional preparations of Spitsbergen shales yield conventional fossils, but Nick developed a set of unconventional procedures that enabled him to identify and gently free fragile remains. His painstaking work revealed a paleontological treasure trove. Plenty of cyanobacteria occur in these shales; neither then nor now are these microbes limited to tidal flats. But the Spitsbergen shales also contain diverse fossils of eukaryotic organisms, recognizable by their distinctive shapes. The vaseshaped fossils that washed onto the tidal flat are there, as are the large cells decked with spines. More exciting, however, the shales contain multicellular algae, the remains of small seaweeds that formed lawns on the shallow seafloor. Some of these fossils closely resemble green algaethat can still be seen today ( plate 2e ). Others, however, have no close modern counterparts. Like trilobites and dinosaurs they are extinct, consigned by selection or catastrophe to the dustbin of (natural) history.
    The Spitsbergen fossils are abundant and beautifully preserved, they are distributed over a range of sedimentary environments, and they include both prokaryotes and eukaryotes. On the other hand, they occur only in limited horizons of black chert and shale. The true ubiquity and diversity of late Proterozoic life is revealed by other biological indicators, most conspicuously stromatolites , the wavy-laminated, domed, and candelabrum-like structures seen in Akademikerbreen rocks ( figure 3.3 ).
    Stromatolites are the predominant features of carbonate rocks formed in Precambrian oceans. Stromatolitic buildups are uncommon today, but examples from places like the Bahama Banks and, especially, remote Shark Bay in Western Australia show how they form. Communities of microorganisms spread across sediment surfaces, weaving together to form coherent mats. Cyanobacteria (and, sometimes, algae) at the mat surface trap and bind fine particles supplied by waves and currents. As a veneer of mud or sand accumulates, these populations grow upward, reestablishing the mat at the sediment surface. Deeper in the mat, bacteria consume dead cells, changing local chemistry in a way that causes carbonate crystals to form. The processes of colonization, trapping and binding, and carbonate precipitation are discontinuous, but endlessly repetitive, with the result that fine layers of