Chapter 8 – Origins of Life
8.1 Life as a Geologic Force Rather than originating in a single event, life may have arisen through a series of steps, each building upon the complexity established by the previous mechanism During its evolution, life may have undergone evolutionary selection for certain metabolic pathways over others o Some of these pathways (such as photosynthesis) would eventually start to act as a geologic force by exchanging matter and energy with physical and chemical systems (such as the rock and hydrologic cycles) 8.2 Early Theories of the Origin of Life: Spontaneous Generation and Panspermia The theory of spontaneous generation arose from still earlier concepts regarding the distinction between living and nonliving things o This theory holds that living entities are generated by the power of nature, and new living forms are constantly being generated from nonliving ones The results of Pasteur’s microbiological experiments demonstrated that bacteria could only arise from other bacteria o This resulted in the demise of the spontaneous generation theory The ongoing quest to understand the origin of life led to panspermia, that life came from chemical compounds or even bacterial spores from space (brought by meteorites and comets) o Panspermia shares some aspects of the steady state theory of the universe 8.3 What Is Life? Basic Traits of Life o A living organism is an open system that exchanges matter and energy with the surrounding environment o Furthermore, living organisms are surrounded by a cell membrane, which acts as a barrier to separate the cell contents from the surrounding environment o Living systems are also able to reproduce themselves from nonliving matter and energy taken from their environment o Living systems are more complex than nonliving (they consist of many more compartments) o Living systems store and process information and transmit it to their offspring o Living systems can evolve Composition of Life o Life is based on carbon; many molecules of living organisms are based on long chains of carbon atoms o Carbon is prominent because it formed early during nucleosynthesis, so it is abundant in the universe; it is also versatile, forming one or more covalent bonds with hydrogen, nitrogen and sulfur o Nitrogen resembles carbon in that it is abundant (the main component of Earth’s atmosphere), forms multiple covalent bonds with other elements,
© 2013 Jones and Bartlett Learning, LLC and thus can form a variety of complex compounds (such as amino acids and proteins) o This is also true of sulfur and phosphorous; these elements may have been involved in prebiotic energy storage and transfer o Hydrogen is the most abundant (and was the first) element in the universe; it forms weak bonds with other molecules, making it ideal for many biochemical reactions 8.4 Chemical Evolution Early Theories o Darwin and Huxley were early supporters of chemical evolution: that simple precursors could be assembled to produce life o Two basic approaches to chemical evolution are bottom-up (building blocks are synthesized and then assembled) and top-down (existing biological systems and biochemical pathways are extrapolated backward to simpler components and systems) o Miller and Urey’s experiment lent support to earlier ideas regarding the synthesis of organic molecules under conditions present on the early Earth (the bottom-up approach); later studies indicated that different (less reducing) conditions actually existed o Recent research has focused on other mechanisms or sites where prebiotic compounds might have been concentrated o Clay minerals may have been involved in concentrating substances and catalyzing their reaction o Life could have developed in one of a number of different settings, such as hot springs o Many life forms in modern hot springs are extremophiles (survive under conditions hostile for most life forms) and/or chemoautotrophic (rather than use light to manufacture carbon compounds, they obtain energy from chemical reactions) Hydrothermal Vents and the Pyrite World o Deep sea vents are a promising site for the origin of life because they incorporate a number of different aspects of early evolution into a single hypothesis o Early Earth was warm, so spreading centers would have been abundant o The depth of hydrothermal vents would have made them less affected by Hadean and Archean impacts o The most primitive organisms are thermophiles o Hydrothermal vents are characterized by reducing conditions o Sulfide minerals such as iron pyrite occurred at hydrothermal events; cell membranes, early metabolism, and the genetic code are all suggested to have originated on the surface of pyrite or other sulfide minerals The RNA World o The Central Dogma of biology is DNARNAprotein o The RNA world scenario holds that RNA and protein synthesis appeared first, and DNA evolved later as a means of storing and protecting
© 2013 Jones and Bartlett Learning, LLC information, with RNA remaining as the intermediary between DNA and protein. o However, the RNA world requires the evolution of some form of primitive metabolism before the RNA world could have appeared Autocatalysis o In each of these “worlds” life may have driving its own formation through the process of autocatalysis, in which reactants and products organize themselves into a more complex system o Autocatlysis is a type of self-organizing system; this type of behavior has been observed repeatedly in laboratory settings, so there is no reason to believe it does not occur in nature 8.5 Origin of Eukaryotic Cells The earliest cells were prokaryotic (lacking a nucleus) Eukaryotic cells (protists and higher organisms) have a distinct membrane- bounded nucleus and may have organelles o Eukaryotes may have originated through endosymbiosis, the engulfment of one prokaryotic cell by another cell o Animal-like eukaryotic cells arose through acquisition of mitochondria, and plant-like eukaryotic cells by engulfment of photosynthetic bacteria, which became the forerunners of chloroplasts 8.6 Precambrian Fossil Record and Molecular Clocks The fossil record for the Precambrian is poor, making the question of when the first prokaryotes and eukaryotes arose very difficult to address o Results from protein sequencing suggest that the various groups of prokaryotes and eukaryotes diverged no earlier than 2.0-2.5 billion years ago (a billion years later than what the fossil record suggests) o Scientists are attempting to use molecular clocks compare the sequences of RNA and proteins of prokaryotes and eukaryotes to answer this question
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