By Jack Kallmeyer, email@example.com
LIFE ON A YOUNG PLANET, THE FIRST THREE BILLION YEARS OF EVOLUTION ON EARTH by Andrew H. Knoll. Princeton, NJ: Princeton University Press, 2003. $39.50 cloth, $18.95 soft cover; 275 pp; illustrated with numerous black & white charts, drawings and photographs plus eight color plates; 21 pages list works for further reading organized by pertinent chapter; index.
Life on a Young Planet fills a void missed in most books available about evolution in which little is said about Pre Cambrian life especially in the detail presented here. The Pre Cambrian encompasses the first 4 billion years of earth history. Most of this is ignored by many as the fossil evidence used to support evolutionary theory is perceived to be missing or scant. Knoll shows the educated reader that fossils are not the only evidences for evolution of life on this planet.
Knoll moves to sites around the world and back again as he discusses not only the early evolution of life on earth but also its origins from nothing ex nihilo.
Life on a Young Planet begins with a brief historic look at interpretation of the Cambrian explosion when life on earth went from apparently nonexistent to abundant. Knoll relates Darwin's dilemma with this situation. Darwin knew that organisms evolved from previously living ancestors but with the sudden appearance of complex life in the Cambrian, and no apparent evidence of predecessors, he felt this caused problems for his theory of descent. Darwin knew that for evolution to work as he thought there had to be a long succession of ancestral forms prior to the Cambrian. The Pre Cambrian became one of the big gaps in the fossil record.
From this introduction Knoll moves to the relatedness of all life forms and the construction of the diagrammatic tree of life. The tree of life is rooted in the prokaryotes including bacteria and archaea. As Knoll points out, although we like to think of ourselves as the most successful beings on earth it is in fact the prokaryotes that excel. Here we are introduced to genetics, RNA, DNA, the biologically based cycles of carbon dioxide, nitrogen and sulphur, and the various trophic (food source) schemes of the prokaryotes. Most important to know are the byproducts of organisms living, respiring, eating, excreting and dying. All of these functions leave chemical signals in the rock record. These signals are used throughout Life on a Young Planet for geochemical proofs of life's existence at times when fossil evidence is problematic or missing altogether.
In Knoll's chapter "Life's Signature in Ancient Rocks" he begins his introduction with some of the younger of the early life forms around 600 to 800 mya (million years ago) near Spitsbergen, Norway. Most significant are the Cyanobacteria and their preservation and implications for the paleoenvironment. Modern analogues are employed to compare and contrast the past and present. Knoll employs the chemistry of the organisms to explain preservation and the geochemical signals that demonstrate the existence of biological forms alongside the actual fossils. This discussion goes so far as to describe the chemical micro environment around decaying cells. This important section precedes later ones wherein the chemical signs of life are present but biological forms are not preserved as fossils. Key terms are introduced including "fractionation" a term used to describe the preferential uptake of one atomic isotope versus another by respiring or feeding organisms. One of the examples given is the propensity of photosynthetic life forms to use CO2 containing the lighter Carbon isotope 12 C versus the heavier 13 C. A similar fractionation occurs with Sulfur reducing organisms as they prefer Sulfates containing lighter isotopes of Sulfur.
From the youngest of the Pre Cambrian life forms, Knoll jumps to the oldest in the Warrawoona Group of Western Australia. These sediments and volcanic rocks are 3,500 mya. Before speaking of the life forms Knoll discusses the proofs of the antiquity of these deposits and the age of the earth. Here the reader is introduced to absolute radiometric dating. When radiometric dating is brought up, most people think of Carbon 14 dating. Carbon 14 (14C) is limited to about 100,000 years with the most modern techniques. Deep time requires the use of the radioactive isotopes of Potassium and Uranium. The details are well explained including the cross checks and finer accuracy of current methods.
As Knoll points out, the evidence of Warrawoona life forms is contentious. Not all scientists interpret the structures preserved in the cherts the same way. Some of the structures appear to be Stromatolites and even bacteria. The cherts are thought to be evidence of hydrothermal formations. Geochemical evidence through Carbon isotopic data seemingly indicates that photosynthetic organisms existed at this time but even that is not a total certainty.
One of the more demanding chapters to the casual reader is "The Emergence of Life." Here Knoll delves into the processes whereby life could form ex nihilo. This is necessarily a complex chapter and I will be unable to reduce it to a few key explanations. The structure and chemistry of RNA and DNA are described as well as their self replication ability. Most interesting is the fact that RNA can, in essence, form from nothing or, better put by Knoll as he references the work of Harvard's Walter Gilbert, "As Gilbert saw it, RNA was the information rich molecule that could form by self organization and catalyze its own replication" (Life on a Young Planet, p. 79). Knoll continues with the development of cell walls and organelles, cell metabolism, and protein formation and function. In reading this chapter, you will see that there are indeed ways to produce life from chemistry under the right conditions. But, after presenting the latest research and thoughts on the formation of life even Knoll has to conclude that, "We are not close to solving the riddle of life's origins." He further states however that, "Most [chemists and molecular biologists] now believe that life's origin . . . involved chemistry that was both probable and efficient . . . " ( Life on a Young Planet, p. 88).
In chapter 6, "The Oxygen Revolution", we are brought up to date on the origins of our oxygen rich atmosphere. The increase in atmospheric oxygen has primarily been linked to the evolution and success of the photosynthetic Cyanobacteria. Banded iron formations were once thought to signal the beginning of worldwide expansion of the oxygen producing Cyanobacteria the oxygen causing iron rich waters to precipitate iron oxide in the form of hematite. These formations had a fairly well defined formation period in geologic time ending, as the thinking went, when all the iron was precipitated from the earth's waters. At this point, the atmospheric oxygen levels rose to higher levels allowing for the evolution of more advanced life forms. Apparently this is no longer a completely accepted process. Geochemical analysis has shown that oxygen levels rose prior to the end of Banded Iron Formation and indeed, Banded Iron Formations occur in younger rocks when oxygen was abundant. Knoll takes the reader through the analysis now accepted for the oxygenation of the Earth. Since Cyanobacteria seem to be the basis for this oxygen revolution, Knoll devotes the entire next chapter to the abundance and evolution of these organisms which he calls "Life's Microbial Heros."
Subsequent chapters describe the origins and evolution of the Eukaryotic cells from which all "higher" life forms developed. Knoll presents the most current thinking on cell development. These are the more familiar cells with nuclei, mitochondria and chloroplasts. There is much evidence that the internal cell structures developed from symbiotic relationships between bacteria. For example, the chloroplasts of plants appear to be co opted Cyanobacteria. The mitochondria of animal cells has its own DNA separate from the cell nucleus supporting the thinking that it originated as a symbiotic bacterium.
Fossil evidence of the Eukaryotes appears around 600 mya in the Doushantou Formation in China. There is a gap in datable rock units around this time and it is possible that the Eukaryotes evolved as much as 300 million years earlier. The fossil evidence includes probable reproductive spores and multicellular algae. The most spectacular fossils from this Formation are small bundles of cells that have been recognized as animal eggs and embryos undergoing cell division. Geochemical evidence from this time indicates that the oceans were still not fully oxygenated, having hydrogen sulfide at depth similar to the stratification seen in the modern Black Sea. This has implications for the evolution of eukaryotic algae as their survival would necessarily have been near shore.
Knoll next takes us into somewhat more familiar multicellular life with the Ediacara fauna first described from Australia. There remains much discussion as to what these strangely shaped impressions might have been. Some resemble modern sea pens, some resemble trilobite precursors, some resemble jellyfish and others are preserved as trace fossil trails. The fauna has now been found at numerous sites of the same age around the world. From Ediacara Knoll moves to the first calcified fossil evidence from the Nama Group in Namibia. These too are Pre Cambrian and problematic to some degree as their exact affinity is not absolutely clear. The Nama Group contains massive algal and Cyanobacterial reefs which, in themselves, contain fossils of more strange animals. Knoll points out that although these organisms were animals and preceded the Cambrian explosion, the animals themselves were not the evolutionary ancestors of advanced Cambrian forms.
Finally Knoll takes us back to the Cambrian and a discussion of the timing of evolutionary sequences as revealed in the rock record. Paleontological data and biological genetic divergence data do not agree as to the timing of the divergence of major groups of animals within the Cambrian. Knoll suggests that there is more room for study in this area between paleontology and biologists.
Knoll discusses the driving forces that allowed the "explosion" of life in the Cambrian. The fossil record shows the evolution of life from simpler forms beginning around 3,500 mya to more complex forms just before the Cambrian. Even at that, there remains a significant jump in diversity across the Pre Cambrian Cambrian division. Knoll coins the term "permissive ecology" as an evolutionary force. At the end of the Pre Cambrian the earth was subject to massive glaciation processes that were much more extensive than the more familiar recent ice ages. These ice ages (some say there were as many as four episodes) were truly global in extent with even some opinions favoring very little open water at all the "Snowball Earth" hypothesis. The cause? One theory postulates that tectonic forces disrupted the Carbon cycle dropping atmospheric CO2 levels and cooling the planet. This idea is supported by geochemical data. There are other ideas about the mechanism and extent of this glaciation but by any cause the result would have been a huge ecological shock to most life forms. Knoll believes that this glaciation and probable mass extinction event provided the subsequent permissive ecological environment that allowed rapid diversification of surviving organisms thus the Cambrian Explosion.
Knoll ends with a short section about astrobiology. He discusses the Martian meteorite found in Antarctica and the fanfare that surrounded the statements by NASA that they had found evidence of biological processes in carbonate minerals within it. Independent research eventually discredited the extraterrestrial origin of the "evidence". Knoll goes on to discuss the potential for finding life elsewhere in the universe.
READABILITY Advanced High School and up with studies in Biology,
Chemistry, Physics and a bit of Geology. May be most suitable for undergraduates
with Biology Chemistry, Physics and Geology studies.
ON THE UPSIDE Excellent coverage of Pre Cambrian evolution with multidisciplinary approaches to causality. Good photographs of early life forms.
ON THE DOWNSIDE Unfortunately, if the reader isn't familiar with
Biology, Chemistry, Physics and elementary Geological processes he will be
unable to grasp much of the material in this book.
OVERALL RATING Excellent. An important book covering the accumulated knowledge of the origins and early evolution of life on earth.