Early Earth and the Origin of Life Chapter 26

Introduction to the History of Life Life on Earth originated between 3.5 and 4 billion years ago Stromatolytes: banded domes of sediment that form around motile microbes resembling spherical/filamentous prokaryotes, found in rocks 3.5 billion years old

Major Episodes in the History of Life Prokaryotes were the only game in town for about 2 billion years: Bacteria/Archaea diverged about 2-3 b.y.a. Photosynthetic prokaryotes produced enough oxygen to make earth an aerobic atmosphere about 2.5 BYA. Eukaryotes around from about 1.7 BYA Plants evolved from green algal protists during the precambrian era (Oldest land plants 475 mya) Fungi and animals evolved from different heterotrophic eukaryotes during the precambrian era (oldest fossils 700 mya)

Prebiotic Chemical Evolution and the Origin of Life Life began between 4 and 3.5 bya. Life was thought to have begun from nonliving materials organizing into molecular aggregates Abiotic synthesis of organic monomers was hypothesized by Oparin and Haldane, and proven in the lab by Miller and Urey. Organic polymers can be constructed in the lab by replicating early earth conditions (Fox created proteinoids)

More Prebiotic Chemical Evolution Protobionts: aggregates of abiotically produced organic molecules, probably predated living cells. It’s possible they developed metabolism, homeostasis, and more. Experiments with coacervates suggest the feasibility. RNA was probably the first genetic material, which was the last piece of the puzzle for living things. Cech’s discovery of ribozymes suggests RNA may have been replicating in the prebiotic world.

RNA and Darwinian Evolution The Membrane bound Protobiont (hypothetically): -Incorporated genetic information -Selectively accumulated monomers -Used enzymes programmed by genes to make polymers, and carry out chemical rx’s -Grew and split, passing on gene copies This variation would be subject to natural selection

DNA on the Scene RNA was the original template to make DNA Because DNA is more stable, it would have been selected for over RNA RNA’s role would be changed to become an intermediate in translation

The Origin of Life…..Theories Panspermia: Some organic compounds may have reached earth via meteorites/comets Most researchers think life formed in shallow water or in moist sediments Others think life occurred on the sea floor using energy/chemicals from deep sea vents (Also atmospheric conditions were harsh) Rebek constructed a simple organic compound which served as a template for its own replication

Major Lineages of Life

Alternative Classification Systems 6 Kingdom System: Prokaryotes are broken into Archaebacteria and Eubacteria 3 Domain System: Larger than kingdoms: Bacteria, Archaea, and Eukarya 8 Kingdom System: Bacteria, Archaea, Archaezoa, Chromista, Protista, Plantae, Fungi, Animalia