Transcript lecture 20
Chapter 14 Origins of Life and Precambrian Evolution Origins of Life Questions What was the earth’s environment life when life arose? What is life and how did it arise from non-life? What were the first cells and what characteristics did they have? Origins of Life Earth formed between 4.5 and 4.6 Bya First few million years earth too hot to support life Sometime around 3.8 Bya life began Origins of Life Which molecule came first? Proteins or DNA Proteins do complicated biological tasks but cannot reproduce themselves DNA cannot do biological functions but replicates itself perfectly Discovery of catalytic RNA = ribozymes Origins of Life Ribozymes can do biological work and reproduce themselves It is now thought that ribozymes were the first life Ribozymes can simultaneously possess a genotype and a phenotype Has particular traits and a coding sequence Defining Life All living organisms possess a phenotype and a genotype Other characteristics defining life are controversial Growth Reproduction Evolution requires ability to record and make alteration in heritable information with some way of distinguishing valuable changes from detrimental ones Genotype and phenotype Defining Life Ribozymes have all of these characteristics and were probably the first life forms RNA World hypothesis Catalytic RNA-based life eventually evolved into system we see today DNA is a more stable molecule for holding genetic information Avoids problem of having proteins before there was DNA to encode them RNA World Experimental Evolution of Ribozymes If ribozymes evolve, they must have a way of changing and recording the change in its genome RNA evolved faster replication and a shorter genome In all the experiments, investigators had to add enzymes for ribozymes to replicate themselves Still self-replication has not been demonstrated RNA World By recognizing an RNA World, problem of self-replication is pushed back before DNA RNA World was probably not first self-replicating system Likelihood of making RNA abiotically is too minute How did the earliest self-replicating system work? First Self-Replicating System Information-containing molecules need to be made from simple inorganic compounds Chemical reactions that constructed larger molecules needed energy source Building blocks had to self-assemble Larger biomolecules had to be protected from harsh environments Panspermia Hypothesis Some argue that it is just as likely that life came from outer space as it is that life was spontaneously created on the earth Biomolecules have been found on meteroites, asteroids, and comets Other planets may have more appropriate chemical composition than earth Panspermia Hypothesis Three versions of panspermia theory Life arose on another planet in our solar system Microbes could have been dislodged by meteorite impact and traveled to earth Mars and Europa may have had water and life Trip to earth would kill most life unless it was sheltered in carbonate rock Earth bacteria survived space walk on Apollo 12 Panspermia Hypothesis Three versions of panspermia theory Life originated in another solar system and traveled to earth Would need to escape solar system’s gravity and be shielded from radiation Intelligent extraterrestrials intentionally brought life to earth to seed the galaxy with life Directed panspermia Where did Stuff of Life Come From? Meteroites from Australia contained five amino acids Contained equal proportions of both stereoisomers All living things on earth have only Lform If these formed spontaneously in space, could the same thing happen on earth? Debate about early atmosphere Reducing or oxidizing Where did Stuff of Life Come From? Up until 4 Bya earth was bombarded by impacts If life formed it was probably continually destroyed Where did Stuff of Life Come From? Oparin-Haldane Model Miller attempted to re-create the primordial soup and simulate lightning Biomolecules were spontaneously created in reducing environment It is now believed that CO2 and N2 instead of methane and ammonia were the dominant gases Oparin-Haldane model still used as a null model Cellular Life Search for the most recent common ancestor of all life = cenancestor Except viruses All extant life is made up of cells Descended from cenancestor population of interbreeding individuals Cenancestors lived at least 2 Bya Cellular Life Cells allow compartmentalization and organization Allows genotype and phenotype to be linked Genotype codes for a product and that product stays within the organism Does not diffuse to other members of population with other genotypes Lab mixtures of polyamino acids spontaneously organize themselves into cell-like structures in solution Cellular Life Examine fossil record for earliest cells 3.465 Bya first known cells from Apex Chert in Australia Cells growing in filaments Probably cyanobacteria Identification made by extant comparison Must have been cells before these because cyanobacteria are relatively advanced Cellular Life Fossil record spotty before 2.5 Bya to have continuous fossil record Apex Chert may represent extant or extinct branch of life No way to tell if those cells were really our ancestors Because fossil record is inadequate, try to reconstruct a phylogeny of all life for answers on cenancestor Phylogeny of All Living Things First attempts at a phylogeny of everything were morphological Doesn’t work for prokaryotes because they don’t have sufficient structural diversity Reconstructed a phylogeny using molecular characters Needed to find a gene that all living organisms have that has strong stabilizing selection Otherwise too much drift would accumulate Phylogeny of All Living Things Woese selected small-subunit ribosomal RNA (ssu-rRNA) Is present in all organisms Strong stabilizing selection because of important function of translating proteins Surprising results from tree that changed view of life’s phylogeny Phylogeny of All Living Things Now there are three domains Bacteria split into Bacteria and Archaea Archaea more closely related to Eukarya than Bacteria Protista should be split into many kingdoms Fungi should be split into two kingdoms Eukaryotes are minor twigs on tree of life Phylogeny of All Living Things Where is exact root on tree? There are no outgroups Root tree by genetic distance Most parsimonious solution to root is that there was one most recent common ancestor population that gave rise to all life Was relatively advanced, similar to modern bacteria Phylogeny of All Living Things Estimates of universal phylogeny would be expected to be congruent among genes This has not been found to be the case Phylogeny using HMGCoA reductase gene shows very different phylogeny Because of horizontal (or lateral) gene transfer Bacterial gene introgressed into Archaea genome Phylogeny of All Living Things We now know that horizontal gene transfer was rampant in early life and many genes may not belong to the species that have them Most recent common ancestor could have been a pool of species that exchanged genes Date for Root of Tree of Life Attempts to use molecular clock to date the root have been unsuccessful Oldest definite eukaryotic fossil is 950 My old There is not reason to believe that clock was constant over billions of years Probable eukaryotic fossil 1.5 By old Fossil cyanobacteria at least 2 Bya If Apex Chert is correct, root must be at least 3.5 By old Date for Root of Tree of Life Earliest possible date 4.4 to 3.85 Bya Youngest possible date 3.5 to 2 Bya Origin of Organelles Bacteria and Archaea are similar to fossil common ancestor Gradual refinement of form Eukarya are fundamentally different Complex cells Organelles Complex genomes with introns Some are multicellular Origin of Organelles Mitochondria and chloroplasts are derived from prokaryotes Giardia, a protist, lacks complex organelles Mitochondria and chloroplasts have double membranes and chromosomes Mitochondria are purple bacteria Chloroplasts are cyanobacteria Origin of Organelles Endosymbiont Theory of Lynn Margulis Both organelles arose by symbiotic relationship with other larger bacteria Their bacterial relatives are endoparasites Phylogeny of life including organelles shows that they are bacteria Evolution of All Life From the root of the tree of life, Bacteria and Archaea changed little Eukarya evolution involved major innovations Acquiring mitochondria and chloroplasts is part of what changed them from prokaryotes to eukaryotes