A view of non‐analog worlds

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 δ13 C and δ34 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 δ13 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 δ13 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 theme that is reiterated throughout the book. As usual, there are not only unanswered and unanswerable questions about details of such a hypothesis, but alternative scenarios. Knoll presents all sides without firmly placing his foot in any camp, allowing the reader to ponder the merits of each. One way or another, cellular life forms began to play a role on Earth. And, that role is to begin changes on the planet that cannot be undone. The Oxygen Revolution is designed to lead the reader up the stratigraphic column, back towards the Cambrian, with an examination of the Gunflint Chert in Ontario. This is the place where it all started with the collection of an unusual carbonaceous rock by Stanley Tyler in the 1950s that was sent to Elso Barghoorn at Harvard for examination. There was life before the “Dawn of Life” in the Cambrian, first changing our view of pre-Phanerozoic history (Barghoorn and Tyler, 1965). There is now relatively good age constraint on this sequence that is somewhat older than 1878 ± 2 Myr, based on a volcanic bed near the top of the section. Stromatolites in these rocks are different from those in other Precambrian areas, appearing more like sinters adjacent to mineral-charged springs than algal mounds formed within the tidal zone. And, unlike other microfossil assemblages, these fossils appear most similar to extant iron-metabolizing bacteria that are not common in today's oceans. At present, iron-loving bacteria are restricted in their habitats to unique chemical environments. In contrast, these bacteria are a persistent feature in the Archeoproterozoic (2.1–1.8 Byr ago) and before, indicating the near absence of free oxygen during this interval of time. Knoll not only presents the long-held arguments and evidence to support a low oxygen planet, but also the arguments of the opposition who contend that an oxygen-rich environment was in place much earlier than 2.2 Byr (Ohmoto, 1996; Canfield, 1998). Here, again, Knoll gives the reader a well-designed and illustrated set of arguments explaining the anomalous δ13 C, δ32 S, and δ34 S biogeochemical markers found in the record. His conclusion is that although oxygen may have been locally abundant in the Archean, it wasn't until the Proterozoic that it was sufficient in both the atmosphere and hydrosphere to impact global environmental and biological systems. No paleontologist can miss a trip to Russia. The complex geology spread across much of the northern hemisphere provides endless opportunities to untapped resources if only you can get there from here. Knoll brings the reader back to Siberia where cyanobacteria dominate the carbonates and associated rocks in the Great Wall along the Kotuikan River. But the problem encountered in these rich fossil assemblages is how to differentiate the true biological signature of diversity. How can we say, with certainty, that the organisms seen here, some 400 Myr later than the iron-rich Gunflint cherts, are the same as those documented in older rocks based solely on morphology? Can the same morphotype have had different physiological requirements and pathways at different points in Precambrian time? And, what about convergence? Knoll discusses population stasis, Sewall Wright's ideas on the adaptive landscape, and Karl Niklas' models (Niklas, 1994) and concludes that when only one functional demand must be satisfied to ascend to your adaptive peak, then a single peak it is. “Bacteria can be famously single-minded” (p. 115). The stromatolites and primary sedimentary structures found in the Great Wall section are different from those in older rocks. Here we see for the first time crinkly laminated algal mats, teepee structures, sheets of ooids, and stromatolite domes. The systematic diversity also is different. But, rather than ascribing it to evolutionary changes in the biota, he demonstrates that environmental change is the root cause of the differences seen in these rocks. These reflect changes in the ocean, resulting in a new array of sedimentary textures recording episodic changes in carbonate chemistry. And locked away with these sedimentary textures and bacterial remains might be some of the first traces of eukaryotes. The Origin of Eukaryotic Cells begins with an outline of the early 20th Century hypothesis by Merezhkovsky (Khakhina, 1992) of endosymbiosis followed by the now textbook account of Margulis' theory. The relationships established between the various symbionts can be tracked by where different algal groups sit on the tree of life. The acquisition first of a mitochondrial symbiont allowed for metabolic stabilization within the cell, whereas molecular evidence indicates that photosynthetic symbiosis must have occurred at least half a dozen times (e.g., Delwiche, 1999). These iterative relationships spread photosynthesis through the eukaryotic domain. But, there's always a catch. There are organisms constructed of nucleated cells without either mitochondria or chloroplasts; the example given is Giardia. Knoll relates the stories of the pioneering molecular studies focused on eukaryotic phylogeny and the quandry posed by these organisms, and then presents the argument that their mitochondria were lost (although retaining mitochondrial genes) during adaption to an anaerobic habitat. In addition to molecular biology identifying nuclear genetic material of proteobacterial origin in the early basal branches within the tree of life, he the recent hypothesis of and they that The to all then, was a between a archaea, and for and a of both and anaerobic The organic into the in and for the of new organic As changing the available in by the lost their and evolved a proteins were to in cellular along with genetic or resulting in a new cellular organization. This evolutionary may explain the of an unusual found only in anaerobic environments and for anaerobic the et al., As new are those found to be across and those restricted to lead to interpretations that there may be more than just in the towards the eukaryotic It is in that the reader is introduced to the fossil of a much preserved in the of Myr than the Great Myr is nearly the from the Cambrian to the present and times the the of the at the boundary This section a volcanic bed with an age of ± Myr which is by the Here, both algae and and of and In some the preserved form a Knoll the necessary to identify such remains as and then the processes that preserved these This is because evidence for the biology of Neoproterozoic and algae just found before the Cambrian, to cells by a cells in and other cellular in that reflect are all present and of these are the in the provide only a of to But there are preserved and Knoll these in the remainder of the chapter that the of these are recorded in cherts from at And, their are the that this must have from other and evolved to this by the Proterozoic physical evidence abounds for the as as biogeochemical evidence for and evidence for appear at this time? The answer is The Proterozoic Group ± Myr) in Australia an eukaryotic different from those in the Grand or both as and molecular fossils The are up to in that have from their that often These features are similar to those in and this of one's during its life, is not a of means that by were in the and had evolved a organization. And that feature brings us to and the of genetic variation within and Knoll proposes the hypothesis as the for eukaryotic that little allowing for of between organisms, and genetic I be writing this review and you be We all be for this evolutionary the of in the then, the beginning of a to the The again, is This is because the Proterozoic us of a number of global that appear to play a role in the evolutionary These oxygen within both the atmosphere and sea but a environment in the deep marine life in a of the of available of can their and would have been But, when is present in the column, it can be used in available for metabolic Knoll that during these diversity and of be preserved in where into the ocean, of that could be used to free up The second may be even more to changing global environments But, this is until Chapter a short in rocks at the very of the Proterozoic Myr) some of the first evidence for fossils and is These though, nor can be into extant (although in are to and most have been into a group the 1998). These fossils have a variety of forms comprised of and Knoll the and with these your picture these and other what are these Knoll that all these forms were to the and may have algae as in the remainder of the chapter is to the of evolutionary that can be documented in the fossils the of in the whereupon Knoll the possible the reader is brought to the there is a good fossil the innovation of and its evidence in where the are with the and fossils that in a variety of and But these are Proterozoic in assemblages that the “Dawn of are to be at least it's necessary to to the Chapter Cambrian brings the reader back to Siberia along the Kotuikan River. The evolutionary relationships are and biology is The tree is beginning with the in the and the This group is presented in some the primary into and and the of within the The is and as to what with the that the Myr of time over which as and evolved really be a Can And Knoll contends that because these years more than billion years of biological (p. it really about the same way that the last from the boundary to might just not really for these evolutionary must be found at the of biology and of the is to and the role that play in the of But, there is in the the between paleontological evidence and molecular et al., 1996; Knoll and 1999). This concludes the Knoll if it is to of molecular change based on into the to this have earlier than the physical but recent using genetic in leaves a between the and the The only three to the of early genetic our to read from genetic are and the fossil evidence is and both are but different The conclusion is that problem remains it and to be the theme the evidence for ecological change in the In and the Proterozoic sedimentary indicates a global age at this time. The global age just but Knoll that and occurred at least and chemical evidence for what has been is presented to the reader et al., and the δ13 C is This hypothesis has now been for nearly a and the details of the as as the under are The that occurred over this Myr interval are used to set the the for the biological found in Cambrian out the problem associated with the and of sea and the of within a global that would be under the of the hypothesis, and the problem of the to his arguments result in alternative that can be Knoll proposes the variation of the the biological preserved in the carbonates indicates that even though there were for the Proterozoic marine biota, the eukaryotic tree through ecological The are in space and appearing in the fossil the last of Knoll's To there is no evidence for in any older part of the sedimentary record. Hence, if ecological the eukaryotic tree during times and allowed for the of the eukaryotic during good are the found only the last Knoll it is the of the between and available oxygen and then discusses how probably the The reader is brought back to the and presented evidence for oxygen just the of the global A second of independent evidence for this is by the when all is and points to the that evolution in the was by The last is the Why is there such a at the Cambrian The answer may have to with a in the at the once again, for the biosphere. Knoll that can explain both the stratigraphic and the between the and Cambrian and that the evolutionary the between and development (although there are who would the role may have and their are The of the first Byr of life on but may not the that life elsewhere in our In the were when they had biological evidence in a The the no Knoll the that that year and what has been There is no argument over the from but must be before the hypothesis evidence of life in this rock can be These are that the organisms must have in within the and the biological has for nearly 4 What the reader has in the then, must be to this argument because this where the Knoll to the carbonate of the that the that these as for mineral-charged The may provide to the physical but not the biological And even the organic biomarkers are of are in and not but there is no evidence that they are biological in What these biomarkers though, is that if life existed on it may have molecular Knoll the with an examination of the interpreted and details the over whether such 100 be biological in there life on Knoll on what we and to about these life interpretations must be based on three the or chemical found in these rocks make in of known biological processes as to a abiotic we all of the possible forms life may take in how would we an biological The biology may be but the and place on what any can the of evidence presented to support the of life, only the for the unusual of in the carbonate has To some the features found in these only could be the result of biological the But, as such structures have been synthesized in the and the support for a biological origin is leaves only the last to a travel and This to The to the book is And, as to the to an it to that Life on a Young provides the undergraduate and and their across a array of the Byr of evolutionary innovation and stasis, and the for are into a of Earth a to which in are introduced early in their and the biology often are not to deep time have little or no concept of the This is even more this book be read by Why might it be for to what about the very deep it's because the short to this part of our history about as as it take you to read the just of are more than of And those organisms that the and our much than But, as Knoll so all that came we evolved into their we be as we are the of and time.

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