2 nucleotides are the monomers

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2. Joining of small molecules that formed macromolecules
RNA could have provided the template for DNA, which is a more stable genetic material Exact knowledge about the origin of life may not exist BUT there is overwhelming  evidence that explains the history of life  Key events in lifes history include the origins of single-celled and multi-celled organisms as well as  the colonization of land The study of fossils has helped establish a geologic record Four eons divide the geologic record: 1. Hadean 2. Archaean 3. Proterozoic 4. Phanerozoic The Phanerozoic (last half-billion years) encompasses multicellular eukaryotic life The Phanerozoic is divided into 3 eras: Paleozoic, Mesozoic, Cenozoic Table 25.1 Major boundaries between geologic divisions correspond to extinction events in the fossil reord The oldest knows fossils are stromatolites Stromatolites date back 3.5 BYA Prokaryotes were the sole inhabitants of earth from 3.5 to about 2.1 billion years ago Photosynthesis & the Oxygen Revolution Most atmospheric oxygen (O2) is of biological origin By about 2.7 BYA, o=O2 began to accumulate in the atmosphere and rusted iron-rich terrestrial rocks “O2 revolution” led to extinction of many prokaryotes The first Eukaryotes Oldest fossils of eukaryotic cells date back to 2.1 BYA
Endosymbiotic theory proposes that mitochondria and plastids (chloroplasts & related organelles) were formerly small prokaryotes living within larger host cells An endosymbiont is a cell that lives within a host cell Serial endosymbiosis hypothesis suggests that mitochondria evolved before plastids through a sequence of endosymbiotic events

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