Lecture 28 11 09 2012 WATCH POCKET MOUSE VIDEO ON EXAM Monophyletic group clade lineage a group that includes an ancestral taxon and all of its descendants Will be characterized by shared derived traits synapomorphies Phylogenies should reflect evolutionary history thus they should reflect monophyletic groupings Taxon any named group of organisms Character trait any feature that a taxon possess Derived apomorhpic trait an evolutionary novelty unique to a group that evolved in the common ancestor We can also build phylogenetic trees from homologous gene sequences in the same way identifying molecular synapomorphies Step 1 align the sequences Step 2 identify variable positions Step 3 assign synapomorphies to ingroup variable positions Parsimony principle the simpler the explanation the more likely to be Using the parsimony principle the first cladogram is preferred because it provides the simplest explanation for the observed pattern How might we construct an inferred phylogenetic tree for all life on the true planet Identify a homologous character set found in all life forms but that also shows variation that can be separated into plseiomorphic and apomorphic changes to yield synapomorphies throughout the tree 1 DNA 2 DNA is found in the nucleus 3 Protein synthesis takes place in the cytoplasm Ribosomes are the universal ribonucleoprotein particles that translate the genetic code into proteins Typical bacterial ribosomes consist of 57 different molecules 3 rRNAs and 54 proteins and can dissociate into a small and a large subunit Each subunit has a structural RNA backbone that is homologous among all life forms that can be sequenced aligned and analyzed for plesiomorphic and synapomorphic states Tree of life based on ribosomal RNA sequences 3 domains 1 bacteria 2 archaea 3 eucarya forms HOW OLD IS THE EARTH Root is the hypothetical position of the common ancestor of all life Meteorites formed 4 58 Gya billions of years ago Moon formed 4 51 Gya Earth must be about the same age but no direct radiometric dating is possible because Earth was initially molten First evidence of life 3 8 billion years ago Samples of the earth s oldest rocks are extremely rare because rock is constantly recycled or sinks to the hot mantle of the earth At some plate boundaries such as oceanic ridges the plates separate and molten rock wells up in the gap The rock solidifies and adds crust symmetrically to both plates causing seafloor spreading At subduction zones where plates move toward each other the more dense plate dives below the less dense one forming a trench Using the Fossil Record Fossil record the only source of direct evidence about what prehistoric organisms looked like where they lived and when they existed Paleontologists scientists who study fossils 1 They recognize that they are limited to asking questions about tiny and scattered segments on the tree of life Yet analyzing fossils is the only way scientists have of examining the physical appearance of extinct forms and inferring how Most fossils form when an organism is buried in sediment before they lived How do fossils form decomposition occurs PROCESS 1 Seeds pollen and leaves fall in swamp 2 Tree falls 3 Flood buries remains in sand and mud 4 Over millions of years remains are buried further After burial the fossilization process may take different routes Taphonomy the branch of paleontology that studies these different processes Types of fossils o Intact fossil pollen o Compression fossil leaf o Cast fossil bark o Permineralized fossil trunk There are several features and limitations of the fossil record that must be recognized o Habitat bias o Taxonomic bias bigger you are more likely a part of you will fossilize o Temporal bias depends on age of actual rocks on the surface o Abundance bias if a fossil is common it s going to bias your view compared to ones that are rare Mite fossil examples o Fossilization in amber tree sap o Permineralized fossil fossilized around a geyser o Compression fossil Earth s history is separated into four eons The first 3 eons Hadean Archaean and Proterozoic make up Precambrian time lasted about 4 Ga 87 of earth s history The Precambrian includes many important events o Origin of life o Origin of photosynthesis o Origin of oxygen in atmosphere Life was exclusively unicellular during most of the Precambrian and O2 was virtually absent from oceans and atmosphere for 2 billion years Discriminating true microbial fossils from microscopic pseudofossil look alikes can be difficult However analyses of chemical composition of individual microscopic fossils substantiate the biological origin of the earliest known cellular fossils The most common shapes of prokaryotes Spherical Rod shaped Spiral Atmosphere 3 billion years ago before photosynthesis Nitrogen CO2 methan ammonia hydrogen very little oxygen Life forms 3 billion years ago diverse anaerobic prokaryotes However a lineage of bacteria evolved photosynthesis and this changed everything Bacterial Photoautotrophs Cyanobacterial stromatolites are still formed today They are photosynthetic bacteria and produce oxygen as a waste Their activity ultimately changed atmospheric and oceanic chemistry in a profound manner as O2 and a byproduct of photosynthesis slowly accumulated over hundreds of millions of product years Life on the planet has been primarily microbial through evolutionary time Animals plants and fungi have evolved relatively recently and make up the tip of the eukaryote lineage Approximate origin of eukaryotes was 1 8 billion years ago Oldest animal fossil postdate this by over 1 billion years why Not clear but one possibility is that prior to late in the Proterozoic era the concentration of oxygen in the atmosphere though high enough to permit the existence of protists single celled eukaryotes was too low to support multicellular animals Multicellular animals cannot exist at ambient oxygen levels below 10 of the modern atmosphere however protists can survive lower Biologist recognize about 33 basic body plans in living animals These levels constitue the animal phyla Cambrian explosion the appearance of all the principle animal lineages in the fossil record over a period of a few 10s of millions of years starting around 542 million years ago 11 09 2012 11 09 2012