BIOS
W3L2 An Explosion of Life.pdf

262 263 when did the phyla evolve according to the

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263 When did the phyla evolve? According to the fossils, the earliest members of nearly all major animal lineages appear suddenly in the fossil record However, researchers can also use “molecular clocks” to estimate the divergence times of phyla What is a molecular clock? 264 Molecular Clock Hemoglobin amino acid sequences Zuckerkandle & Pauling, 1965 265
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Age of phyla from molecular clocks When researchers used “molecular clocks” to estimate the divergence times of phyla first recovered in the fossil record, they inferred that the phyla are much older than the Cambrian! 266 Slow fuse or megatonnage? Did the phyla evolve much earlier, then radiate? = slow fuse Perhaps most phyla existed in tiny, morphologically simple, larval-like forms, and the explosion is an explosion in morphological complexity? 267 Slow fuse or megatonnage? Or, are the molecular clocks wrong? = megatonnage If the molecular clocks are wrong, the Cambrian explosion is an explosion in the origination of phyla 268
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What caused the Cambrian Explosion? 1. A sudden increase in atmospheric O 2 -- Due to photosynthesis by algae & bacteria -- Higher O 2 allowed for higher metabolism and much larger body sizes 269 What caused the Cambrian Explosion? 2. The origin of hard parts: co-evolutionary dynamics drove the evolution of complexity in body plans. 270 What caused the Cambrian Explosion? 1. A sudden increase in atmospheric O 2 2. The origin of hard parts: co-evolutionary dynamics drove the evolution of complexity in body plans. 3. Extinction: The end of the Proterozoic may be marked by an extinction event. Adaptive radiations often follow extinction events 271
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The End 272
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