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| Unit 4: Demos |
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Earth formation hypothesis (1a) Protobionts, coacervate droplets, proteinoid
microspheres (3a) Timeline of life Modes of attack, infection: plant
viruses v. bacteriophages v. animal viruses (6a) Anti-viral drugs, why don't viruses respond to antibiotics? Centers for Disease Control and Prevention BSE information The evolution of complex biochemical pathways |
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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