History of Life on Earth
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
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