History of Life on Earth How long has life been on Earth? What are the relationships between organisms as time moved forward? BioH - Ch 19 1

Early Earth 2

Early Earth Atmosphere Hydrogen Cyanide Carbon Dioxide Carbon Monoxide Nitrogen Hydrogen Sulfide Water 3

Experimental Evidence The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller (then a graduate student) showed that the discharge of electric sparks into a mixture of H 2, CH 4, and NH 3, in the presence of water, led to the formation of a variety of organic molecules, including several amino acids. 4

At the end of one week, Miller observed that as much as 10-15% of the carbon was now in the form of organic compounds. Two percent of the carbon had formed some of the amino acids which are used to make proteins. Perhaps most importantly, Miller's experiment showed that organic compounds such as amino acids, which are essential to cellular life, could be made under the conditions that scientists believed to be present on the early earth. Millers Experiment 5

Agents of Metabolism 6

Origin of Prokaryotic cells The next step in evolution was the formation of macromolecules. The monomeric building blocks of macromolecules have been demonstrated to polymerize spontaneously under plausible prebiotic conditions. Heating dry mixtures of amino acids, for example, results in their polymerization to form polypeptides. But the critical characteristic of the macromolecule from which life evolved must have been the ability to replicate itself. Only a macromolecule capable of directing the synthesis of new copies of itself would have been capable of reproduction and further evolution. 7

Next Steps Only nucleic acids are capable of self- replicating through complimentary base- pairing. These self-replicating units may have been enclosed in a double layer of phospholipids. This would be considered the first cell. Because cells originated in a sea of organic molecules, they were able to obtain food and energy directly from their environment. But such a situation is self-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. 8

Evolution of Metabolism The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism are highly conserved in present-day cells. All cells use adenosine 5- triphosphate (ATP) as their source of metabolic energy to drive the synthesis of cell constituents and carry out other energy-requiring activities, such as movement (e.g., muscle contraction). Generation of metabolic energy, using glycolysis - the anaerobic breakdown of glucose to lactic acid to produce ATP. Photosynthesis utilizes energy from sunlight to drive the synthesis of glucose from CO 2 and H 2 O, with the release of O 2 as a by-product. The O 2 released by photosynthesis is used in oxidative metabolism, in which glucose is broken down to CO 2 and H 2 O, releasing much more energy than is obtained from glycolysis. 9

Endosymbiotic Theory 10

Common view of evolution of life forms See text: Pages

Where does this lead? 12