Lecture 25 _March 11_ History of Life post

Lecture 25 _March 11_ History of Life post - Biology 171...

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3/9/11 1 Lecture 25: Friday March 11, 2011 Biology 171 History of Life on Earth Announcements Next Week in Discussion: Speciation in Cichlids Text Reading Chapter 27 (479-492) Telling Time & the Age of the Earth Pre-Cambrian Life Cambrian Explosion Post-Cambrian Diversification Mass-Extinction events Why We Care Given that there is a single tree of life, we can’t really understand ourselves unless we understand our evolutionary history If we think that it’s important to conserve or manage biodiversity, we should have some understanding of that biodiversity The strong links between biology and the non- living world during evolutionary history should remind us to be careful with the planet Universal Tree of Life based on DNA sequences For Ribosomal RNA Animal, plants and fungi represent a tiny fraction of the genetic diversity of life on earth Two strains of the bacterial species Escherichia coli are genetically more different than a human and a platypus
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3/9/11 2 Telling Time in Evolutionary History How old is the earth? How can we estimate the age of certain taxa that exist now, or exist only in the fossil record? We often date rocks and fossils by studying radioisotopes ( 14 C, 40 K, 238 U) in the rocks or in the fossils themselves Molecular clocks use DNA or RNA sequence divergence to estimate the timing of clade divergence Radiometric Dating Radioactive isotopes have unstable nuclei and form “daughter isotopes” while releasing energy The rate at which daughter isotopes are formed is constant The ratio of parent to daughter isotopes can therefore be used to date a particular material 14 C has a half-life of 5730 years 238 U has a half-life of 4.5 billion years Decay of 14 C over time How Old Is the Earth? Meteorites formed 4.58 Ga (billions of years ago), and the Moon formed 4.51 Ga. Earth must be about the same age, but no direct radiometric dating is possible because Earth was initially molten. About 3.5 Billion YA Events at plate boundaries. At some plate boundaries, such as oceanic ridge, the plates separate, and molten rock wells up in the gap.
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