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2/4/13 Evolution Biol 4802 Lecture 5, Chapter 5! n Topics for today n Archean n Proterozoic n Paleozoic – Cambrian explosion History Of Life On Earth! Chapter intro figure 1 2/4/13 History Of Life On Earth! n Common themes: n Climates and land mass distributions have changed over time n Composition of life on Earth is also constantly changing n Several “mass extinction” events have occurred n Diversification rates are generally quite high after mass extinctions n Extinct taxa are often replaced by unrelated but ecologically similar taxa Before Life Began! n “Big Bang” – around 14 billion years ago universe originated via explosion of exceptionally hot, dense particle Before Life Began! n Solar system (including Earth) is about 4.6 billions years old 2 2/4/13 Before Life Began! n Earth’s early atmosphere – lot of water vapor, little oxygen n Extensive volcanic activity released additional gases – carbon dioxide, hydrogen sulfide, etc. n Also methane, ammonia, other gases Miller-Urey Experiment! n Test for what sorts of chemicals would form under environmental conditions of early Earth n Conditions produced: n More than 20 different amino acids n Sugars n Precursors to purines and pyrimidines n Precursors to other biomolecules Miller-Urey Experiment! n Miller-Urey conditions weren’t completely realistic – need to replicate output from volcanoes n Hydrogen sulfide, sulfur dioxide, carbon dioxide Now get even greater variety of biomolecules and precursors n Other experiments have produced lipidbased cell membrane-like substances n 3 2/4/13 The Problem of Replication! n Currently, we have: n DNA, which can store and transmit genetic information, but can’t perform biological tasks n Proteins, which can perform a wide variety of biological tasks, but can’t replicate n How do we solve this chicken-or-egg problem? Ribozymes! n Small RNA molecules that have selfcatalytic properties Sydney Altman, Thomas Cech Ribozymes! n Small RNA molecules that have selfcatalytic properties – including splicing, chemical modification, and…replication!! n So RNA can act as both genetic material (information containing) and biological catalyst (enzyme) 4 2/4/13 “RNA World”! First replicating “organisms” were selfreplicating RNAs n Selection favored RNAs that were most efficient and accurate at self-replication n Fig. 5.1 “RNA World”! Adsorption of nucleotides to clay particles may have facilitated polymerization n Clay particles can also catalyze formation of lipid envelope n “RNA World”! n Earliest self-replicating “organisms” were aggregations of molecules (RNA, protein, lipid) n Self-replicating “organisms” probably arose multiple times, but just a single origin to all extant life n Initially no phenotype to self-replicating molecules except replication speed/ accuracy 5 2/4/13 Where Did Proteins Come In?! n As cofactors to ribozymes? n May also explain origin of genetic code Fig. 5.2 Why is There Little Evidence of Early Life?! Earth was still very volatile n Ongoing bombardment by meteors n Erosion, volcanoes, plate tectonics n Complex aggregations of molecules generally don’t leave a fossil record n Fig. 4.1 But There is Some Evidence! n 13C : 12C ratio in atmosphere is ~1 : 99 n Living organisms prefer 12C n 3.7 BYO graphite deposits in Greenland have isotopic signature of life 6 2/4/13 But There is Some Evidence! n Fossilized dividing cells from rocks in South Africa 3.26 byo fossils of dividing cells Modern dividing cells Knoll and Barghoorn 1977 Geological Time! n If the history of the Earth were represented as a single year: n First evidence of life in late March marine animals in late October n Extinction of dinosaurs and diversification of mammals on December 26 n Human/chimp divergence ~11:00 AM on 12/31 n Common Era (AD) begins at 11:59:47 PM on 12/31 n Everything from US Civil War to present is in last second of the “year” n First Precambrian Life! Time before Paleozoic Era (~550 MYA and earlier) n Includes: n n Archean (origin of life, photosynthesis) n Proterozoic (origin of eukaryotes, multicellularity) 7 2/4/13 First Life Forms! Stromalites – layers of microorganisms that build up layers of sediment n First life probably looked a lot like modern stromalites on the coast of Australia n Fig. 5.3 Photosynthesis! n First organisms were anaerobic – energy production occurs in absence of oxygen n Oxygen actually harmful to some anaerobes Aerobic energy production is much more efficient n Photosynthesis evolved ~3 BYA in some prokaryotes n n Gradually oxygen levels in atmosphere built up, allowing for evolution of efficient aerobic respiration Evolution of Domains of Life! For ~2 billion years, all life was prokaryotic n Single-celled eukaryotes evolved ~1.5 BYA n Fig. 2.1 Fig. 5.4 8 2/4/13 Deep Roots of Tree of Life are Somewhat Uncertain! n Different genes tell different stories… Freeman and Herron 2004 Fig. 2.1 Horizontal Gene Transfer! Likely very common among early prokaryotes n Still common in bacteria today n Perna et al. 2001 link on web Another View of the Tree of Life! Endosymbiosis (formation of mitochondria and chloroplasts) Fig. 2.1 9 2/4/13 Evolution of Multicellularity! For another 800-900 million years, all life was single-celled n Oldest multicellular fossils date to 550-575 MYA (Ediacaran fauna) n Related to sea anemones “Problematica” – related to ?? Fig. 5.7 Cambrian Explosion!! Cambrian Period represents start of Paleozoic Era n During ~20 MY period, almost all phyla and classes of marine animals suddenly appeared in the fossil record n Fig. 5.8 Burgess Shale! Giant mudslide off coast of Canada 505 MYA; successive mudslides created Burgess Shale n Discovered in 1909, understood in 1970s n 10 2/4/13 Burgess Shale! n Included earliest known jawless vertebrates (e.g. lampreys) Fig. 5.9 Burgess Shale! n Most current body plans arose during Cambrian Cambrian Explosion! Fig. 5.11 11 2/4/13 What Caused the Cambrian Explosion?! Possible large evolutionary changes to developmental regulatory genes (e.g. Hox genes) n Increase in oxygen levels may have allowed larger body sizes n Larger, more complex body structures may have led to new interactions among species (predation, competition) and associated adaptive evolution n We don’t really know… n In-Class Discussion Question! Was the Cambrian explosion really an explosion? n It occurred over 20 MY in the fossil record n Molecular clock-based estimates of phyla ages are 580-1,000 MYA n n But Cambrian explosion in fossil record didn’t begin until ~540 MYA n How do we explain apparent contradiction between molecular estimates and fossil estimates? 12
