We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time T S Eliot
Despite an almost bewildering diversity of form and function, all cells share a common core of molecular features.. The reciprocal observation is equally striking. In spite of their fundamental unity of molecular structure, organisms display extraordinary variation in size, shape, physiology, and behavior. Life's unity and diversity are both remarkable in their own ways; together they comprise the two great themes of comparative biology. (page 17, emphasis added)
Animals may be evolution's icing, but bacteria are the cake (page 19)
abbreviations used in this article:
CO2: carbon dioxide, involved in the photosynthesis / respiration cycle. It is also a greenhouse gas, trapping heat (infrared radiation) and warming the earth.
Mya: million years ago
geological periods discussed:
Cambrian: 543 Mya to 485 Mya. During the "Cambrian Explosion", "large" - multicellular - plants and animals appeared. "Modern" - oxygen breathing- forms of life proliferated in the seas and soon spread to land in the following order: plants, arthropods (insects and their kin) and, finally, our own ancestors, the vertebrates (marine tetrapods)
Proterozoic (also called the pre-Cambrian): about 2.5 billion to 543 million years ago. Aside from colonial microbial stromatolites which formed coral-like structures in water, life was microbial. See two stromatolite photos below.
Most definitely a nerd book. The intended readership are both science literate general readers and workers in the earth and life sciences. To appreciate this text fully you need a decent background - self-acquired or school learned - in the earth sciences, paleontology and evolutionary theory in particular. A basic grounding or working knowledge in biology (ecology) and (bio)chemistry helps. On the plus side, the author's style is remarkably readable. Essentials are generally well explained, allowing the novice to tread into deeper waters than they otherwise would be capable of handling.
The subject of Life on a Young Planet is the appearance and evolution of early life on earth before the Great Cambrian Explosion, about 600 million years ago (Mya), when "large" (visible to the naked eye) plants and animals wildly proliferated and "the world as we know it" was born. On young earth, life was microscopic: bacteria, one celled animals and plants.
How old is life? The first microfossil traces of life are hotly contested. Mineralogical processes can produce microfossil like structures. Finding "biomarkers", chemical signatures of life (or of its decomposition products), in association with putative microfossils can, in some circumstances, provide a strong case that one is dealing with real microfossils. Today, claims of microfossils 3.7 years old are being made. If confirmed, they would indicate an extraordinary early emergence of life. The solar system was born 4.5 billion years ago and the earth itself is only about 4 billion years old!
The modern tree of life, based on genetic similarity. Modern multicellular animals and plants are found in the upper right hand corner among the "more evolved" Eukaryota. Click on image to enlarge.
Everything you wanted to know about Archaea - the extremophiles - but were afraid to ask:
A modern testate amoeba, which lives in a mineral shell which is secreted by this one celled "animal" (technically, a protist, neither animal or plant). Testate amoebas are Eukaryota, "more evolved" lifeforms, and lie on the rightmost branch of the Tree of Life along with animals and plants. See above diagram. They are one of earliest animal-like life forms dating back at least 750 Mya, well before the Cambrian Explosion when modern multicelled animals and plants proliferated, diversified and flourished. Testate amoeba are still important members of contemporary aquatic and soil ecosystems!
A fossil testate amoeba shell unearthed by author Knoll. The white bar on the lower left of the photo is 25 microns (millionths of a meter). The shell is thus about 100 microns in length, the width of the average human hair. It was secreted about 720 to 635 Mya in the pre-Cambrian.
Video of living testate amoeba. The shell is composed of sand grains cemented together with silicic acid secreted by the amoeba.
Over time early microbes developed communal "biofilms". Later, biofilms would evolve into the tissues of higher multicellular organisms like plants and animals). Stromatolites are pseudo-corals consisting of layers of mineral matter (sand grains or fine sediments) alternating with photosynthesizing microbial mats. Stromatolites may achieve impressive dimensions and some stomatolite fossils have been dated at a credible 3.5 billion years. Interestingly, living stromatolites are found in some seas today.
living stromatolites, Shark Bay, Australia
pre-Cambrian stromatolite fossil from Bolivia
Life on a Young Planet argues that life is a co-dependent, co-evolving network of energy flows, chemical cycles and living organisms, a living tissue or fabric which includes the body of the earth itself, its seas and atmosphere. The health of individual organisms and species ultimately depends upon the integrity of the whole fabric, a lesson humanity is painfully learning today because of our disruption of vital ecological systems and the chemical equilibria of the planet.
An interesting example of co-dependence and co-evolution are the Eukaryota, the domain which includes dominant modern life-forms like plants and animals. It is now believed that the Eukaryota arose from the symbiotic co-dependence of single celled organisms. It is believed that the mitochondria - the sub-cellular "organelles" which our bodies' cells use to burn sugars with oxygen to liberate vital energy - were once free living organisms. Several lines of evidence support this conclusion, including the fact that mitochondria contain their own DNA (genetic code) and replicate independently when our bodily cells replicate.
From the micro-level to the macro-level, the co-dependence and co-evolution of life is evident. Plants absorb carbon dioxide (CO2) from the air and water from the soil. Using the energy of visible light rays from the sun, sugars, starches and other energy rich molecules are synthesized from CO2 and water during photosynthesis. Oxygen is released as a by-product. Photosynthesized energy rich hydrocarbons (sugar, starch, oil, fat) form the basis of the food web. Herbivores eat plants to incorporate energy rich hydrocarbons to power their metabolisms. Carnivores eat herbivores to incorporate energy rich molecules incorporated in the herbivores bodies. Through co-dependent co-evolution, efficient carbon cycling was established over time: the photosynthesis / respiration cycle. Plants absorb CO2, synthesize energy rich hydrocarbons and release oxygen during photosynthesis. Animals respire the oxygen released by plants to burn energy rich hydrocarbons, releasing CO2 in the process. A rough equilibrium between photosynthesis and respiration has been established over time allowing all organisms to flourish. Too much oxygen, and our forests would burn up in planetary conflagrations while too little would prevent the emergence of animal life.
But co-dependent co-evolution runs even deeper. Both plants and animals are Eukaryota. When these organisms die most of their organic matter is quickly recycled by decomposers: insects, worms, fungi, bacteria.. But not all organic matter is recycled. In water, organic matter may precipitate to the bottom as sediment before it decomposes. Such environments are often anoxic, contain little or no oxygen. Now, Eukaryota like plants and animals require oxygen and are therefore incapable of extracting energy from buried sediments (and, in the process, recycling vital bio-elements back into the environment, in this case, sea or lake water). Only anaerobic prokaryota - micro-organisms not requiring oxygen for metabolism - are capable of processing buried organic matter to liberate essential bio-elements like nitrogen and sulfur. In effect, without the recycling activity of the "lowly" prokaryota, we "more evolved" Eukaryota would disappear as vital bio-elements were buried and removed from biospheric circulation! The "web of life" is much more than a fuzzy New Age or mystical meme. It is a literal, physical reality upon which our very lives depend..
As prof Knoll puts it so eloquently,
"Life on a Young Planet records that moment in time when biologists and Earth scientists began working together to build an integrative picture of our planet's history. We now know that we can only understand life's evolutionary trajectory by embedding it in the chronicle of Earth's dynamic environmental history. That is the lesson of deep Earth history, and it is also a lesson for today, when human technology has emerged as an environmental force of geological prominence. Whether we look outward, searching for a second example of life, or forward, hoping to navigate wisely through an era of mounting global change, the record of Earth and life through time provides a fundamentally important catalog of experience that can help guide our actions." (page XV, emphasis added)
As is so often the case with the best science writing, much of the charm of this book comes from it's evolutionary approach to science itself. The clash of competing theories, the fruitful alliances between disciplines, the slow unearthing of critical clues.. all these are grain for Prof Knoll's mill. For those interested in life's early history and deep ecology and who possess the requisite science background, this book is a must read. Even novice earth science readers should find it a challenging and stimulating read (and don't forget the extensive suggested reading list for aid..) Definitely recommended.
"For scientists, unanswered questions are like Everests unclimbed, an irresistible lure for restless minds" (page 224)