000200010270083524_Ch2 - 8140606_CH02_p020-089.qxd 7:47 PM...

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20 About 4.5 billion years ago the planet Earth coalesced from clumps of debris floating through space after the Big Bang. For another billion years Earth’s surface was a harsh place; asteroid bombardment and volcanic eruptions were con- stantly remodeling the face of the planet. Yet it was during this tumultuous period that life on Earth began. Some re- searchers believe that organic molecules arose from a pri- mordial soup of methane, ammonia, and water, energized by atmospheric electrical discharges. Others believe that the first organic molecules arose from chemical reactions of products of deep-sea volcanoes. Regardless of the origins of the first small organic molecules, the pathway to living or- ganisms required the formation of larger macromolecules with the capacity for catalysis and self-replication. At some point around 4 billion years ago, these purely chemical processes produced the earliest life-form, the progenote. The progenote was likely a chemoautolithotroph, capable of surviving without oxygen and living on inorganic sources of energy and carbon. The closest living relatives to the progenote are likely the archaea. These modern prokaryotes can survive in the harshest environments that now exist on Earth, such as sulfuric hot springs and deep-sea vents. The progenote was the ancestor to all organisms on the planet and, as a result, it is likely that many of the ubiquitous biological features arose in the progenote. The dependence on water, the role of nucleic acids, the use of only 20 amino Chemistry, Biochemistry, and Cell Physiology C H A P T E R 2 8140606_CH02_p020-089.qxd 10/11/08 7:47 PM Page 20 Pearson Learning Solutions Not For Resale Or Distribution
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21 acids in proteins, and the basic pathways of intermediary metabolism are shared attributes of all living organisms. Within the first billion years, the progenote gave rise to three distinct types of organisms: eubacteria, archaea, and eukaryotes. Each lineage diversified independently over the next 3 billion years. The two prokaryote lineages, eubacteria and archaea, remained single-cell organisms with little in- tracellular organization. In contrast, the ancestral eukary- otes experienced evolutionary changes that resulted in the production of membranous, subcellular compartments, thereby increasing intracellular organization. This began when the earliest eukaryotes found a way to package their DNA into a membrane-bound compartment: the nucleus. Later, around 3 billion years ago, a eukaryote engulfed a bac- terium that resembled a modern purple bacterium. Although the purple bacterium was probably ingested as food, it devel- oped a symbiotic relationship with its host, replicating with the host cell. Over time, the bacterial endosymbiont lost its capacity to exist outside the cell, and the host cell became re- liant on the metabolic contributions of the endosymbiont, the ancestor of mitochondria. By 2 billion years ago the diverse groups of protists were established. The protists include or- ganisms like the euglena (with features of both animals and
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