History of Life on Earth

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Chapter 25: History of Life on Earth
Synthesis of Organic Compounds on Early Earth
* The Earth probably formed about 4.6 billion years ago, and was bombarded with rocks and other material until about 3.9 billion years ago. * The Earth then cooled, allowing for the formation of oceans. Scientists hypothesize the general atmosphere, or at least some regions, were naturally reducing environments, meaning that they added electrons to compounds. * Activation energy provided by lightening or UV radiation may have been able to create organic compounds and amino acids, as demonstrated by a number of modern experiments. Abiotic Synthesis of Macromolecules

* Experiments have been done in which amino acid solutions in hot sand have formed polymers, but not true proteins. These polymers may have functioned as basic catalysts of some kind, however. Protobionts

* Cells have genetic material in the form of DNA and RNA, which they are also capable of replicating. Nothing like this has been generated spontaneously in lab experiments. * However, early structures called protobionts have had some of the capabilities associated with life. Experiments have spontaneously create protobionts, which are simple sphere of membrane that can perform simple metabolic and reproductive functions. * Note: phospholipids spontaneously form a bilayer, like the membrane that surrounds cells, so that part of the puzzle is easy to solve. Self-Replicating RNA and the Dawn of Natural Selection

* Simple RNA structures called ribozymes can carry out basic chemical reactions and are even capable of replicating themselves. * As ribozymes replicated themselves (with errors) protobionts could have developed internal collections of slightly different enzymes that formed a rudimentary metabolic system. The RNA in these early “cells” may have served as a template for the eventual creation of a DNA genome, which would have reduced the number of errors made during replication. The Fossil Record

* The fossil record gives a glimpse of life on Earth during different time periods and provides clues for evolutionary research. However, the fossil record also has significant gaps, but some are being filled by new discoveries. How Rocks and Fossils are Dated

* Fossils appear in individual sediment layers, which tell us the order that they were formed in but not an actual age in years. Scientists use radiometric dating to determine absolute ages. * Radiometric dating is based on the fact that some radioactive elements have predictable half-lives, or periods in which half of the substance will decay. If you know how much of a certain radioactive element an organism has when it died, such as carbon-14, you can determine its age by measuring the amount of carbon-14 remaining today. * Older fossils are harder to date, but you can at least guess based on the age of fossils in the surrounding layers. The Origin of New Groups of Organisms

* The presence of certain bones, different kinds of teeth and other characteristics can help researchers make inferences about what an animal may been like while it was alive. Changes between similar fossils of different time periods also show the pace of evolutionary change. The First Single-Celled Organisms

* Scientists have found fossilized stromolites that are thought to have lived 3.5 billion years ago – the earliest organisms discovered to date. Stromolites are mounds of prokaryotes that bind to their kin and other inorganic material. Photosynthesis and the Oxygen Revolution

* 2.7 billion years ago, there were probably cynobacteria in the ocean that used photosynthesis for energy and released oxygen in the process. The oxygen that these bacteria released would have eventually begun reacting with iron, and finally escaped into the atmosphere as a gas. * This buildup of oxygen actually killed many prokaryotes, and provided a strong selective force in favor of cells that could use oxygen in...
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