Prebiotic Evolution Chapter 18
Until about 300 years ago people believed in the idea of spontaneous generation –that life comes from non-living material. 1668 Francesco Redi proved that magots did not come from rotting meat. Could microorganisms arise spontaneously? 1864 Lois Pasteur finally disproved this idea.
How did life arise on Earth? 1. “panspermia” – life arrived here from outer space • Not a testable hypothesis 2. Prebiotic evolution – life arose from non-living matter
For prebiotic evolution to take place, conditions on the early Earth must have been very different • The atmosphere contained virtually no free oxygen • There was no life on Earth • Life had 300 million years to get its act together
The early Earth’s atmosphere probably had hydrogen gas, ammonia, methane, and traces of carbon dioxide and hydrogen sulfide. • No rust • No ozone – UV radiation may have been a driving force • Heat from the earth and lightning also provided energy to make chemical bonds
Organic molecules could have come from comets, asteroids or meteors. • Either way, oceans may have been full of organic molecules. • More complex molecules needed more help to form, probably on clay and metal ions at the edges of oceans. • Can make peptides of up to 50 amino acids in length.
Glucose, ribose, deoxyribose and other sugars form from formaldehyde (CH2O) when exposed to UV radiation. • Adenine forms from hydrogen cyanide (HCN) and was probably the first base to form. • Molecules had to come together in an organized fashion.
Polysaccharides form coacervates in solution. • Membrane-like boundary • Act like cells • Absorb molecules and release products • Form new spheres by budding
First information carrying molecule • Was probably RNA since it does not need enzymes or a primer • RNA can act as an enzyme
The first cells • Were heterotrophs feeding off organic molecules in the oceans • Used fermentation – pathway common to all cells.
Cyanobacteria were the first to produce free oxygen • Mitochondria came after • Endosymbiotic theory • Endosymbiosis • Mitochondria, chloroplasts, flagella, nucleus • Spirochete • Thermoplasma acidophilum