Chapter 8 Origins of Life

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 DNARNAprotein 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