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| Unit 4: Demos |
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Earth formation hypothesis (1a) Abiotic formation, accumulation of small organics (1c) Formation of organic polymers from the ancient oceans (1c) Snowball Earth (optional) More Snowball Earth (optional) Protobionts, coacervate droplets, proteinoid microspheres (3a) Scientists Debate RNA's Role at Beginning of Life on Earth (optional) Was DNA invented by viruses? (optional) Modes of attack, infection: plant viruses v. bacteriophages v. animal viruses (6a) How do viruses leave host cells? (6c) Retroviruses & Why is HIV / AIDS so deadly? (6b) (supplemental) Anti-viral drugs, why don't viruses respond to antibiotics? The fight against bacetrial resistance to antibiotics (optional, from scitech, the Cornell science and technology magazine) US Food & Drug Administration BSE FAQ Viroids (optional) Making Sense of Mad Cow Disease (optional) BSE and vCJD: Instant Expert (optional) The evolution of complex biochemical pathways 13e/13f: Which came first, cyclic or non-cyclic photophosphorylartion? Why was the evolution of autotrophic pathways necessary for life to continue? What is meant by the term oxygen revolution? Note: nonoxygenic photosynthesis is equivalent to cyclic photophosphorylation. Enodsymbiosis and the origin of Eukaryotes (optional) |
Timeline of Life
The oldest chemical evidence of life appears at ~3.8 billion years ago (bya). The oldest fossils of identifiable organisms date from 3.5 bya and come in the form of mats of fossilized bacteria called stromatolites. The first eukaryotes to appear in the fossil record look like single-celled algae and appear around 2.2 bya, following the accumulation of oxygen in the atmosphere that was brought about by photosynthesizing cyanobacteria beginning around 2.7 bya. The fossil record suggests that eukaryotic life was single-celled for as much as a billion years, with the first multicellular eukaryotes (algae) appearing 1.2 bya. Animals do not appear in the fossil record until the late Precambrian, ~600 million years ago (mya). These macroscopic lifeforms (plants, fungi, and animals) would move onto land just 100 million years later (500 mya). As the diagram below shows, this Cambrian period (which began ~540 mya) was a time of rapid diversification of animals. Most modern phyla appeared in the early Cambrian.
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click to enlarge Fossils from the last 3.5 billion years show a relatively abrupt transition from body plans of single cells to a rich diversity of animal-body architectures. The first multicellular animals appear about 570 million years ago, shortly before the beginning of the Cambrian, and examples of them are shown on the left of the reconstruction below. They did not have mineralized skeletons, but were instead soft-bodied creatures resembling sea pens or jellyfish. These were joined about 35 million years later by the animals represented in the center of the reconstruction—shelled invertebrates, including clams, snails and arthropods such as trilobites. Soon after, echinoderms such as starfish appeared, followed by chordates, the lineage that gave rise to humans and all other vertebrates. All of the basic architectures of animals were apparently established by the close of the Cambrian explosion. Subsequent evolutionary changes, even those that allowed animals to move out of the sea onto land, involved only modifications to those basic body plans.
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