Chapt 26 - CHAPTER 26 - Early Earth and the Origin of Life...

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Introduction Life is a continuum extending from the earliest organisms through various phylogenetic branches to the great variety of forms alive today. The diversification of life on Earth began over 3.8 billion ago. Geologic events that alter environments have changed the course of biological evolution. For example, the formation and subsequent breakup of the supercontinent Pangea had a tremendous impact on the diversity of life. Conversely, life has changed the planet it inhabits. The evolution of photosynthetic organisms that release oxygen into the air had a dramatic impact on Earth’s atmosphere. Much more recently, the emergence of Homo sapiens has changed the land, water, and air on a scale and at a rate unprecedented for a single species. Historical study of any sort is an inexact discipline that depends on the preservation, reliability, and interpretation of past records. The fossil record of past life is generally less and less complete the farther into the past we delve. Fortunately, each organism alive today carries traces of its evolutionary history in its molecules, metabolism, and anatomy. Still, the evolutionary episodes of greatest antiquity are generally the most obscure. A. Introduction to the History of Life One can view the chronology of the major episodes that shaped life as a phylogenetic tree. Alternatively, we can view these episodes with a clock analogy. 1. Life on Earth originated between 3.5 and 4.0 billion years ago For the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular. The Earth formed about 4.5 billion years ago, but rock bodies left over from the origin of the solar system bombarded the surface for the first few hundred million years, making it unlikely that life could survive. No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago. The oldest fossils that have been uncovered were embedded in rocks from western Australia from 3.5 billion years ago. The presence of these fossils, resembling bacteria, would imply that life originated much earlier. This may have been as early as 3.9 billion years ago, when Earth began to cool to a temperature at which liquid water could exist. 2. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago Prokaryotes dominated evolutionary history from about 3.5 to 2.0 billion years ago. The fossil records supports the hypothesis that the earliest organisms were prokaryotes. Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea. Representatives from both groups thrive in various environments today.
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Chapt 26 - CHAPTER 26 - Early Earth and the Origin of Life...

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