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Origins and Evolution of Life: Discovering an Astrobiological Connections between Stars and Cells,
This review is from: First Life: Discovering the Connections between Stars, Cells, and How Life Began (Hardcover)
"The history of science suggests that a continuous, focused effort to try to understand a problem, ... We will never know exactly how life began on early Earth, but we will know life can begin on a suitable planetary surface, because we will watch life emerge when just the right set of conditions come together." -- David Deamer
Scientists who study the origin of life strive to discover the chemical reactants and physical conditions that ignited the first forms of life on planet earth. One question they wrestle with peculiarly is how complex molecules such as amino acids, proteins, lipids, and DNA originated. All of these molecules are carbon based and are quite complex. Certainly, there was a ready supply of carbon on early Earth in the form of carbon dioxide and methane, but the synthesis process, from simple to complex, is still under debate. A popular origin-of-life proposition is that complex biological compounds assembled by chance, out of an organic broth, on the early Earth's surface. This proto bio-synthesis culminated in one of these bio-molecules being able to produce replicas of itself. The first laboratory tests conducted in response to this question was that known as the Miller-Urey experiment, simulated early Earth's atmospheric conditions and resulted in a spontaneous formation of organic compounds including amino acids. This evidence that complex organic molecules could have resulted from basic chemical reactants, cannot account for all complex amino molecules necessary for life, not even all 20 basic amino acids for living organisms. Despite hard efforts, scientists failed to create all the molecules needed for life in laboratory simulations of early Earth conditions.
The inability of scientists to synthesize the multitude of molecules represented by mundane life, today has stimulated the search for other explanations. Have scientists made incorrect predictions of what the conditions were truly like on early Earth? It may be chemically impossible to obtain all the molecules necessary for life starting from the simple menu of reactants at the conditions provided by early Earth! When researchers ran into the problem of how these cellular building blocks might be organized, it was suggested that a replicator was initially required. However, since DNA, the current 'reigning replicator', requires an extensive array of protein components in order to replicate, it was speculated that RNA could catalyze its own replication, resulting in the 'RNA world' hypothesis. Many researchers, therefore, think that RNA, a DNA's cousin, may have been the first complex molecule from which life evolved. RNA carries genetic information like DNA, but it can also conduct chemical reactions as proteins do. In addition, numerous spontaneously-produced inhibitors block pre-biotic chemistry, mandating the use of pure compounds. Robert Shapiro, a New York University chemist, concluded that, "The majority of origin-of-life scientists who still support the RNA-first theory either accept this concept or feel that the immensely unfavorable odds were simply overcome by good luck."
Deamer's thesis diverges from the standard RNA-world concept. He focuses not on the generation of a naked RNA-like polymer, but rather on the formation of a bubble-like membrane organism that stores and transports cellular products, digesting metabolic wastes within the cell, or vesicle, enclosed by a complex fatty membrane, which prevents leakage. Vesicles with similar properties have been formed in the laboratory from certain fatty acids. Deamer holds that the spontaneous formation of vesicles, into which RNA could be incorporated, was a vital step in life's origin. Unluckily, his theory retains the unlikely generation of self-replicating polymers as RNA. Nevertheless, Deamer's insight collapses the synthetic proofs put forward in numerous papers supporting the RNA world. He ends "First Life," by calling for the construction of a new set of biochemical simulators that match more closely the conditions on the early Earth. Unfortunately, the chemicals that he suggests for inclusion are selected from modern biology, and may have not existed in ancient geochemistry.
Instead of complex molecules, life started with small molecules interacting through a closed cycle of reactions, Shapiro argues. These reactions would produce compounds that would feed back into the cycle, creating an ever-growing reaction network. All the interrelated chemistry might be contained in simple membranes, or what a physicist calls 'garbage bags'. These might divide just like cells do, with each new bag carrying the chemicals to replicate the original cycle. Accordingly, genetic information could be passed down, and the system could evolve by creating more complicated molecules that would enact the reactions better than the small molecules. "The system would learn to make slightly larger molecules," Shapiro argues. This origin of life based on small molecules is called 'metabolism first'. Responding to critics who say that small-molecule chemistry is not methodical enough to produce life, introducing the concept of an energetically favorable 'driver reaction' that would act as a constant engine to run the various cycles.
The never ending controversy of how the universe originated seems to be a virtual standoff seeing that neither view can offer empirical proofs. The "origin of life" mystery is often in a full swing conflict between replicator-first and metabolism-first theories. Proponents of each hypothesis debate how each other's theories cannot possibly work in the natural environment of prebiotic earth. Currently available data indicate that the origin of life is extremely unlikely to have occurred through prebiotic chemistry before the advent of life on the early earth. Deamer takes the reader from the vivid and unpromising chaos of the Earth, billions of years ago to the present, to his laboratory, where he contemplates the prospects for generating synthetic life. He introduce us to astrobiology, a new discipline that studies the origin and evolution of life on Earth, relating it to the birth and death of stars. The adventure starts with planet formation, and interfaces between minerals, water, and atmosphere, and the physical chemistry of carbon compounds. Even after all those decades, the evidence in favor of a naturalistic causes for the origin of life has not significantly improved.
Origins and Evolution of Life: An Astrobiological Perspective (Cambridge Astrobiology)