A view of non‐analog worlds

Abstract

I'll readily admit that I had promised to finish this book review before the beginning of the current academic year, having read it earlier this summer during our first field season. But, somehow the northern hemisphere summer has passed quickly and the leaves are now beginning to turn color in the northeast; it's only mid-September. Our concept of time goes back towards the dawn of civilization when early societies used a variety of astronomical means to track the seasons. The earliest Assyrian calendar was based on the lunar cycle, but such a means of keeping time necessitated the introduction of additional months in leap years to even things out. And, even though the early Romans tried to standardize their concepts of time defining months as having either 29 or 31 days (30 was an unlucky number then), the addition of an extra month every second year (i.e., Mercedonius) was necessary. In 45 BC, Caesar reformed the calendar which became known as the Julian calendar, comprised of months of either 30 or 31 days and a leap year. But, under this scheme, the date of the vernal equinox drifted. It wasn't until the Council of Trent (1545–1563) when Pope Gregory XIII authorized the reformation of the calendar that the Gregorian Calendar was conceived and adopted where every fourth year became a leap year (except for century years not divisible by 400). Even then, the Gregorian calendar wasn't universally adopted until 1918 when the Russians changed from their long-held Julian calendar. Why spend so much time talking about time and our concepts of time? Why not just get to the book review? Since the discovery of radioactivity by Becquerel and the Curies and the development of geochronological techniques within the last 100 yr, geoscientists have recognized that time is as immense as the concept of the universe is to physicists. This concept of Deep Time has radically changed the way in which we view our planet and the timing of the abiotic and biotic processes involved in its evolution. The first radioisotopic scale was published in 1934, and advances in the identification and proof of a variety of decay series allowed for independent mineralogical assemblages from the same rock to be assessed, thereby confirming the numerical age of crystallization. It is this fact, that Earth has a very very long, almost incomprehensible, historical record, that Andy Knoll has outlined elegantly in Life on a Young Planet. We are grounded in the life and times that surround us; but we are asked in this book to abandon our perspectives and concepts of the time governing our lives to consider evidence and processes operating on scales without personal reference. The challenge is rewarding. Andy Knoll is Fisher Professor of Natural History at Harvard University where he studied with Elso Barghoorn at Precambrian "ground zero." His interdisciplinary investigation of the biological, chemical, and physical aspects of early life has resulted in the development of an Earth Systems approach to chronicling our planet, culminating in his election to the U.S. National Academy of Sciences early in his career. This interdisciplinary approach is reflected in the three precepts governing the presentation of the first three billion years of evolution on Earth. The first precept is the narrative history of life in Deep Time, where disparate facts are synthesized to provide a coherent picture for the reader. As he states, "contemporary biological diversity is the product of nearly 4 billion years of evolution. We are part of this legacy" (p. 3). Understanding what came before may help us to understand and change our personal reference. Science is not conducted in a vacuum. It is the result of personal interactions in various parts of the world over one's short, geologically instantaneous career. Knoll's second goal is to present this history as human enterprise. And, lastly, the synthesis. What grand themes can we identify during the evolutionary history of early life on our planet, and what might we expect to encounter elsewhere in the universe? The book is comprised of 13 chapters that take the reader through basic concepts necessary to provide the geological and biological groundwork, setting the stage for an understanding of more complex and synthetic discussions that make up the remainder of the text. Precambrian rocks are not evenly distributed across the globe, nor are the same parts of this long history found on every continent. For a geoscientist this means only one thing—a good travel agent. The reader first is brought to Siberia to learn about the Precambrian–Cambrian boundary above which a dramatic change in biological diversity is recorded whereupon a short discussion of Darwinian evolution and Punctuated Equilibrium ensues. Systematics, phylogeny, and physiology are used to present the argument that bacteria rule the world and that we have evolved into their world, not vice versa. Prokaryotic metabolisms form the fundamental ecological circuitry of life, underpinning everything that occurs in the biosphere. From here, the reader is brought to the Tree of Life and Woese's (1987) domains, focusing on the often overlooked diversity in the Archaea and Bacteria. Knoll proposes that such a tree provides not only an understanding of relationships between organisms, but also serves as a proxy to the environmental history of Earth because particular groups are restricted ecologically in space throughout time. This idea, in itself, provides biologists and paleobiologists with new insights into the linkages between various biogeochemical cycles. Knoll's quest moves to Spitsbergen in the following chapter where Neoproterozoic rocks (800–600 Myr) preserve vestiges of life and the environments in which this life thrived before the Cambrian explosion. The 20 000 foot-thick section is used to explain basic sedimentology associated with marine environments and that Hutton's concept of uniformitarianism—the present is the key to the past—is understood to be about Earth processes rather than direct substitutable examples of the here-and-now into the deep past. These processes are reflected both in the abiotic sedimentary and the biotic paleontological record, and Knoll sets out to describe the microfossil assemblages within these rocks. To which group are they most likely comparable? The answer comes in a well-presented comparison of the morphology and life strategies (including mat-building and ooid boring) of extant cyanobacteria and the Neoproterozoic fossils. But, to cinch the comparison, the reader is brought to equivalent-aged rocks in the Grand Canyon where molecular signals (biomarkers) of archaea, bacteria, proterozoa, and algae have been identified. A brief discussion of the stable isotopes 13 C and 34 S is presented to alert the reader to cosmopolitan biological fingerprints throughout Earth history. The Archean (3.5 Byr) Warrawoona Group in western Australia provides the backdrop for an introduction to the laws governing relative and radioisotopic-dating techniques, including recent advances using the ion microprobe. All of these fundamental concepts and techniques are found universally in nearly all introductory-level undergraduate courses in geology. But their presentation is necessary not only for the casual reader, but also for those professionals who have had little or no exposure to modern geological principles. It is in Australia that Knoll begins to interject his ideas and interpretations on the earliest evidence of life, and throughout the book the reader is treated to balanced, thought-provoking arguments on what we know, what we think we know, and what we really don't know. Although the Warrawoona Group has been interpreted to preserve the earliest evidence of life, Knoll demurs based on the work of van Kranendonk. Van Kranendonk (Brasier et al., 2002 ; Garcia-Ruiz et al., 2003) has mapped these cherts as hydrothermal in origin, forming beneath the sea floor, not on the sea floor. And, following a visit to the Natural History Museum in London as part of the research conducted while writing the book, Knoll re-examined thin-section specimens only to find the fossil structures to be minerogenic that may be draped by organic films. Bill Schopf (Schopf, 1993 ; Schopf et al., 2002 ) would contend otherwise, fueling the protocols of science. There is 13 C evidence indicating that an early biosphere existed when these sediments were deposited, but skepticism abounds about the resiliency of biomarkers as far back as the Archean. Other localities are examined—the Barberton Mountain Land in Africa and the Akilia Island off the coast of southwest Greenland—and discussed as to whether or not early evidence of biological systems are preserved and whether or not 13 C signatures within these rocks can be the result of chemical fractionation. If so, Knoll contends that these earliest biosignatures may not be as reliable as presently believed. Hence, if life was forged by the same physical and chemical processes that shaped the crust and the ocean, what features distinguish life forms? Chapter 5, The Emergence of Life, acquaints the reader with introductory biology, beginning with the earliest experiments of Stanley Miller and Harold Urey and then onto basic cellular organization. Within that context, the reader is introduced to the functions of both DNA and RNA and the difficulties these particular compounds would have faced to originate de novo. But once RNA was synthesized under one or more different possible catalytic reactions, evolution may have governed the trajectory of life. Evolution, though, wasn't perfect, and with mistakes in replication probably commonplace, a pool of natural variation was available on which selection could act. And, with selection pressures acting on both the RNA and the proteins synthesized by their activity, a "protobiological" merger could have followed, forming the first innovation by alliance, a t