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Slide 1 History of Life today's goals 1) relationship of phylogeny to classification 2) the fossil record: how it formed, and how we can use it 3) some major events in the history of life life cells photosynthesis & the oxygen revolution eukaryotes multicellular organisms Cambrian explosion of animal diversity colonization of land 4) physical context of the history of life 2 aside: pigeons! Paper from Science (2013) exploring the evolution of pigeon form, especially the evolution of the "head crest" of feathers how did the "head crest" evolve? members of all crested breeds have a single nucleotide change in a gene called EphB2 (from G T at position 600) that leads to a single amino acid change in in a protein called EphB2, from R (arginine, basic) C (cysteine, polar uncharged) aside: pigeons! EphB2 = ephrin receptor 2 -- it's a trans-membrane protein (a protein embedded in the plasma membrane – part of it sticks into the cell, part of it sticks out of the cell) -- in vertebrates, it's involved in numerous developmental processes, including of the nervous system (and mutations at that locus may play some role in Alzheimer's disease) aside: pigeons! that single amino acid change in EphB2 leads to a reversal of polarity in the way the feathers on the back of of a pigeon head grow! “normal” pigeon: feathers on back of neck point down crested pigeon: feathers on back of neck point up aside: pigeons! How many times did this particular mutation evolve? aside: pigeons! 7 micro and macroevolution "microevolution" – evolution on a small temporal scale Populations/species Speciation processes Changes in allele frequencies “macroevolution” - large-temporal scale patterns Longer time periods Larger patterns in the history of life (e.g., origin of eukaryotes, origin of photosynthesis, mass extinctions, etc.) Microevolution over long periods of time For much of the rest of the course, we'll be talking about macroevolutionary patterns length of recorded history: ~5000 years! some major transitions in the history of life multicellular organisms ~1.2 billion years ago eukaryotic cells ~1.8 billion years ago photosynthesis ~2.7 billion years ago origin of life/prokaryotic cells ~3.5 billion years ago today's goals 1) relationship of phylogeny to classification 2) the fossil record: how it formed, and how we can use it 3) some major events in the history of life life cells photosynthesis & the oxygen revolution eukaryotes multicellular organisms Cambrian explosion of animal diversity colonization of land 4) physical context of the history of life 10 the fossil record sedimentary rocks -- form at the bottom of bodies of water (e.g., oceans), when sediment in the water falls to the bottom and accumulates in layers -- pressure from the sediment above cause the bottom layers to solidify into rock -- organisms that fall into this may get compressed into the rock and become part of it – fossils -- distinct layers of sedimentary rock are called "strata" the fossil record sedimentary rocks -- if water levels goes down, or if the sedimentary rocks are lifted out of the water by various geological forces, you can see the layers! -- the oldest layers are at the bottom; the youngest at the top (this is a cliff on the Palos Verdes Peninsula, near San Pedro) the fossil record -- sequential layering of strata allows one to date fossils in a relative sense (fossils found in lower strata are older than those found in upper strata) -- in the 18th/19th centuries, geologists and biologists examined fossils worldwide, and found clear patterns: similar fossils are found in strata of the same relative ages throughout the world fossils found in younger strata are more like modern organisms than those in older strata -- the study of strata worldwide by looking at their mineral and fossil composition is called "stratigraphy" the fossil record Stratigraphy doesn't allow us to assign absolute dates to strata or fossils... the most important tool for this is radiometric dating -- this relies on the constant rate of decay of radioactive isotopes of various atoms (e.g., carbon, potassium, or uranium) -- if we know something about the initial amount of the radioactive isotope in a material (when that material was formed), then by examining its current amount of radioactive isotope, we can estimate how long ago the material was formed -- example: radioisotope dating using 14C and 12C the fossil record -- 14C is produced in the upper atmosphere, when neutrons strike nitrogen; that product decays to 14C. -- 14C decays (back to N) with a half-life of ~5,700 years). It's produced at about the same rate at which it decays. -- 12C is stable and present in a constant amount in the atmosphere -- so… there is a roughly constant ratio of 14C/12C in the atmosphere -- C gets taken up by living organisms in that ratio... -- but when organisms die, they stop taking up new carbon, and the 14C in them continues decaying… so as time passes the ratio of 14C/12C they contain declines! the fossil record -- ratio can be measured and plotted on a graph so that the age of the material can be determined -- because the half-life of 14C is so short, this type of dating only works for materials ~100-50,000 years old the fossil record -- for older materials, you can date igneous rocks (rocks formed from cooling magma or lava) using other radioactive isotopes with longer half-lives: e.g., potassium (which decays to argon) and uranium (which decays to lead) used to date many events in the past few billion years the fossil record Geologists have used stratigraphy to put together a relative timeline for major events in history of life (estimate) Radiometric dating has been used to assign actual dates to these eons, eras, and periods (refine estimates) the fossil record Geologists have used stratigraphy to put together a relative timeline for major events in history of life (estimate) Radiometric dating has been used to assign actual dates to these eons, eras, and periods (refine estimates) we will go into more detail on some of these events today's goals 1) relationship of phylogeny to classification 2) the fossil record: how it formed, and how we can use it 3) some major events in the history of life life cells photosynthesis & the oxygen revolution eukaryotes multicellular organisms Cambrian explosion of animal diversity colonization of land 4) physical context of the history of life 20 major events: life (3.5-4 bya) -- the earth formed 4.6 billion years ago (bya). Until about 4 bya, the earth was too hot to support life... -- but we have fossil evidence of prokaryotic cells from ~3.5 bya -- so life must have originated 3.5-4 bya. How? Four things likely happened... 1) abiotic synthesis of organic compounds 2) abiotic synthesis of macromolecules 3) formation of "protocells" 4) formation of self-replicating, information- containing, catalytic molecules (e.g., RNA) 1) abiotic synthesis of organic compounds (e.g., amino acids, nitrogenous bases, monosaccharides) many scientists have tried to mimic the early earth's atmosphere and add energy (which could have come from UV radiation, lightning, or hydrothermal vents) to see if simple organic compounds can be formed e.g., Miller & Urey (1954) simulated early earth conditions, and found that >20 different AA, purines, and pyrimidines could be formed major events: life (3.5-4 bya) 2) abiotic synthesis of macromolecules (polymers) Many ideas on how this could have happened Experiments showing that formation of RNA from nucleotides is possible, as well as formation of polymers of amino acids 3) formation of "protocells" if polymers are to form, they need to be protected. Something needs to form a boundary. when you add some kinds of lipids to water, they self-assemble into spherical vesicles whose walls are a lipid bilayer. In the center is a droplet of water, which could contain information-containing polymers, for example... major events: life (3.5-4 bya) 4) formation of information-containing, self-replicating, catalytic molecules -- cells need both information-containing molecules (e.g., DNA), and catalytic molecules (e.g., proteins) -- one type of molecule can do both: RNA! RNA contains information. A single strand can serve as a template for making an identical strand, so it can replicate that information (with occasional mutations... which permit selection to occur!) because RNA can fold on itself and acquire tertiary structure, it can serve as a catalyst for other reactions. Catalytic RNA molecules are called ribozymes -- so early life may have been RNA-based (the "RNA world” hypothesis)... with a later transition to DNA to contain information, as it is more stable than RNA, and to protein for catalysis major events: life (3.5-4 bya) major events: cells (3.5 bya) -- the earliest direct evidence of cells in the fossil record is provided by fossil stromatolites: these are structures formed by films of prokaryotes (especially a kind of prokaryote called "cyanobacteria"). A film grows, then another grows on top, etc... and eventually a large, layered structure forms. -- these prokaryotic cells appear in the fossil record 3.5 bya! Modern stromatolites are still being formed, mostly in high-salinity lagoons (e.g., in western Australia) fossil stromatolites modern stromatolites major events: photosynthesis & oxygen (~2.7 bya) -- most existing atmospheric oxygen was produced by photosynthesis (where a water molecule is split into hydrogen and oxygen) -- before 2.7 bya, there was no photosynthesis, and very little free oxygen in the atmosphere... but then some prokaryotes gained the ability to photosynthesize -- atmospheric oxygen increased slowly for a few hundred million years, then dramatically ~2.4 bya -- the rapid increase in oxygen levels at 2.4 bya is known as the oxygen revolution, and it had many important effects... first photosynthetic prokaryotes major events: photosynthesis & oxygen (~2.7 bya) -- some forms of oxygen can damage cells. Increased O2 levels probably made many prokaryotic species go extinct; some others survived, but in habitats where O2 was scarce ("anaerobic habitats") -- increased O2 in the atmosphere allowed aerobic (using O2) metabolism to evolve; aerobic metabolism is used by many prokaryotes and almost all eukaryotes -- so: O2 levels on earth have been controlled by living organisms, and also strongly affected