History of Life PDF

The History of Life 1 Geologic Time Scale http://paleo.cortland.edu/tutorial/Timescale/ Timescale3.GIF 2 1 3 4 2 5 6 3  Pre-Earth: We are made of stars 7 The Hadean...

The History of Life 1 Geologic Time Scale http://paleo.cortland.edu/tutorial/Timescale/ Timescale3.GIF 2 1 3 4 2 5 6 3  Pre-Earth: We are made of stars 7 The Hadean (ca 4600-3850 Ma) Period from the origin of the solar system to about 3850 Ma No fossil record Almost no geologic record Key Events: Differentiation of Earth into crust, mantle and core Origin of the atmosphere via volcanic outgassing (little free O2) Condensation of water vapor to form freshwater lakes, streams, etc. (likely acidic due to volcanic activity) Origin of continental crust (oldest dated rocks on earth are 3.96 billion years old) 8 4  What is “Life”? Attributes of Life include: Autonomous replication - mitosis, meiosis; higher reproductive mechanisms Critical level of complexity - simpler subunits into multiple complex combinations Ability to evolve via natural selection? 9 Requirements of life An energy source Basic chemicals (nucleic acids, proteins, minerals, etc.) An external environment that sustains life 10 5  Precambrian Time Scale ORIGIN OF LIFE http://www.stratigraphy.org/pre cambrian/prec.jpg 11 The Archean (3850-2500 Ma) Origin of life (ca 3800 Ma) Early organisms had methane, SO4 (sulfate) and H2S (hydrogen sulfide) based metabolism, producing CO2 and alcohol as by-products. Photosynthetic organisms appear (ca 3500 Ma) Respired O2 accumulates and strengthens the ozone layer, trapping free oxygen below. The atmosphere is converted to an oxygen environment (3500- 2800 Ma) 12 6 13 Earliest continents form from collision of smaller land masses (3400 Ma) Early Archean Archean-Proterozoic 14 7  Early life most likely consisted of prokaryotic bacteria-like organisms Basically a capsule of genetic material 15 Archaean Life: Various bacteria Fossil bacteria (1000 Ma) Eschericia coli Modern spirochate bacteria Modern coccus bacteria 16 8  Thermus aquaticus Taq Polymerase 17 Archaean Life: Archaea Halobacteria sp. Methanopyrus sp. Methanococcus jannaschii 18 9  Archaea are “extremophiles” 19 Archaean Life: Cyanobacteria Trichodesmium sp. - a marine cyanobacterium Spirulina sp. Nostoc sp. 20 10  Stromatolites Stromatolites are formed from fossilized cyanobacteria 21 Precambrian Time Scale EUKARYOTES http://www.stratigraphy.org/pre cambrian/prec.jpg 22 11  The Proterozoic (2500-542 Ma) Origin of eukaryotes (1500 Ma) Rapid diversification of soft-bodied multicellular animals and green algae (1500-600 Ma) 23 Eukaryotic cell Nucleus - DNA storage, RNA transcription Membrane-bound organelles - compartmentalization of functions 24 12  Serial Endosymbiosis Theory (SET) Primary endosymbiosis: Dr. Lynn Margulis Eukaryotic cells evolved when aerobic bacteria either infected or were engulfed by a larger host cell and later established a symbiotic relationship. http://www.sirinet.net/~jgjohnso/endosymbiosis.html 25 Mitochondria are thought to be derived from purple bacteria 26 13  Chloroplasts are thought to be derived from cyanobacteria 27 Evidence for chloroplasts and mitochondria as endosymbiotic organelles: Circular genomes in chloroplasts, mitochondria, and bacteria Mitochondria have cell membranes very similar to prokaryotes 28 14  Evidence for chloroplasts and mitochondria as endosymbiotic organelles: http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_24 29 Progression toward multicellularity Single-celled Eukaryotes Colonial Eukaryotes Chlamydomonas Volvox: many Chlamydomonas-like units 30 15  Progression toward multicellularity Single-celled Eukaryotes Colonial Eukaryotes Choanoflagellates Porifera: Choanoflagellate-derived units http://www.ldeo.columbia.edu/edu/dees/ees/life/slides/phyla/porifera.html 31 Progression toward multicellularity Colonial Eukaryotes Differentiated Multicellular Eukaryotes 32 16  Challenges of Multicellularity Need for support, rigidity, increases Reproduction becomes more difficult 33 Challenges of Multicellularity Surface to Volume ratio (S/V) goes down as size increases – Surface controls exchange with environment Diffusion, heat exchange, area that might contact food, all dependent on S – Metabolic demands (O2, CO2, energy) largely dependent on V Animals adjust S/V by shape Use bulk transport to supplement diffusion for long-distance movement of materials 34 17  Pretend cell is a cube: to compute..... Surface area of cube = side ´ side ´ 6 = square units (units2) Volume of cube = side ´ side ´ side = cubic units (units3) 35 Compare cubes of different sizes (1, 4, 8, 16 units) Cube side = 1, 4, 8, 16 units 36 18  Compare cubes of different sizes (1, 4, 8, 16 units) Cube side = 1, 4, 8, 16 units Surface area (SA) = 6, 96, 384, 1056 units2 37 Compare cubes of different sizes (1, 4, 8, 16 units) Cube side = 1, 4, 8, 16 units Surface area (SA) = 6, 96, 384, 1056 units2 Volume (V) = 1, 64, 512 4096 units3 38 19  Compare cubes of different sizes (1, 4, 8, 16 units) Cube side = 1, 4, 8, 16 units Surface area (SA) = 6, 96, 384, 1056 units2 Volume (V) = 1, 64, 512 4096 units3 SA / V* = 6, 1.5, 0.75, 0.375 39 Compare cubes of different sizes (1, 4, 8, 16 units) Cube side = 1, 4, 8, 16 units Surface area (SA) = 6, 96, 384, 1056 units2 Volume (V) = 1, 64, 512 4096 units3 SA / V* = 6, 1.5, 0.75, 0.375 *(surface area to volume RATIO) 40 20  fig. 6.7 Cam 41 Opportunities of Multicellularity Cellular and tissue specialization (greater complexity) becomes possible Specialization and compartmentalization Epidermal layer to keep interior from drying, protect from microbial invasion, etc. Skeleton for support, movement Vascular system to transport gasses and nutrients Distinct reproductive cells, tissues, organs 42 21  Cambrian explosion Relatively sudden appearance of diverse animal forms in fossil record ~450 mya 43 Animal Evolution 44 22  The Cambrian Explosion Fossils of many phyla first appear in the early Cambrian (542 to ~ 530 mya) First fossil evidence of each animal phylum 45 The Cambrian Explosion (cont.) The “sudden” appearance of so many different types of animals raises a number of questions. Earlier fossils have been found, but these are of small animals or protists (cysts) without easily fossilizable (hard) parts 46 23  The Cambrian Explosion (cont.) The Cambrian is represented by several “Lagerstätten” where soft-bodied animals have been preserved E.g., the Burgess Shale (next slide) By the end of the Cambrian, all “major” phyla (perhaps all phyla) were present 47 The Burgess Shale Fauna Pikaia (a chordate) Marella (an arthropod) Anomalocaris (phylum?) Priapulid worm Hallucigenia (an onycophoran?) 48 24  The Cambrian Explosion (cont.) The explosion does not represent the origin of bilaterians (triploblasts) This occurred earlier (perhaps 1000 mya, based on molecular clock arguments) There is also evidence from trace fossils (e.g., burrows) that bilaterians were present earlier However, bilaterians diversified then, and the question is why? And why then? 49 The Cambrian Explosion (cont.) Explanations for the Cambrian explosion fall into two categories Intrinsic: something about animals changed, e.g., new developmental patterns Extrinsic: something about the environment changed, e.g., increase in available oxygen 50 25  Intrinsic Explanations Hox and Hox-like genes were duplicated in the bilaterians (bilarteral symmetry) The number of such genes correlates with complexity Figure from Garcia-Fernandez (2005) Nature Reviews Genetics, Vol. 6, pp. 881-892 51 Extrinsic Explanations So if we accept the idea (at least tentatively) that animals had the genetic mechanisms to become large well before the Cambrian, why didnʼt they? Here we may need to look at extrinsic explanations of diversification 52 26  Extrinsic Explanations (cont.) Ancient atmosphere contained insufficient O2 to allow evolution of active life styles O2 didnʼt approach current levels until sometime in the Ediacaran Without sufficient O2, large animals are possible, but not large, active animals This may be the reason there are reasonably large animals in the Ediacaran, but not, apparently, very active ones 53 Extrinsic Explanations (cont.) Possibly a mass extinction of the former biota allowed new forms to radiate at the start of the Cambrian This has happened after previous mass extinctions We know the Ediacaran animals disappeared rapidly just before the Cambrian, but whether this was due to a mass extinction is not yet certain 54 27  Extrinsic Explanations (cont.) Another ecological explanation envisions an ecosystem that reached a tipping point in complexity resulting in widespread co-evolution For instance, there may have been an “arms race” between predator and prey Each advance in predatory ability requires a countermeasure by prey, and vice versa Greater incorporation of mineralized hard parts may have started such an arms race This would also explain the sudden appearance of fossils at this time 55 The Cambrian Explosion (cont.) Questions remain: Which explanations are relevant? (All of them?) Why no new phyla (or at least “major” phyla) since the Cambrian? 56 28