Lecture 3 – Origin of Life

Lecture 3 – Origin of Life Fossils similar to spherical and filamentous prokaryotes have been recovered from stromatolites 3.5 billion years old in western Australia and southern Africa. Stromatolites: Banded domes of sediment similar to the layered mats constructed by colonies of bacteria and cyanobacteria currently living in salty marshes. The origin of life was possible in Earth’s ancient environment, which was different from today: - There was little atmospheric oxygen. - Lightning, volcanic activity, meteorite bombardment, and ultraviolet radiation were more intense. One hypothesis about the first living organisms is that they were the products of a chemical evolution that occurred in four stages: 1. Abiotic synthesis and accumulation of monomers, small organic molecules that are the building blocks for more complex molecules. - In the 1920’s,A.I. Oparin and J.B.S. Haldane independently postulated that the non-oxidizing atmosphere and greater UV radiation on primitive Earth favored reactions that built complex organic molecules from simple monomers as building blocks. This is not possible today because: A. Oxygen in current Earth’s oxidizing environment attacks chemical bonds, removing electrons. Early Earth had little oxygen. B. The modern atmosphere has a layer of ozone now that screens UV radiation. - Stanley Miller and Harold Urey tested the Oparin/Haldane hypothesis. They simulated conditions on early Earth by constructing an apparatus containing , 2O, H2H CH4, and NH3. - Their simulated environment produced some amino acids and other organic molecules. - Now we know that the atmosphere of early Earth probably contained CO, CO 2, and N2. - Additional experiments have produced all 20 amino acids, ATP, some sugars, lipids, and purine and pyrimidine bases of RNA and DNA. 2. Joining of monomers into polymers (e.g. proteins and nucleic acids). - Polymers are synthesized by dehydration reactions. For example: H + OH ® + H2O Abiotic polymerization reactions in early-earth conditions must have occurred: - Without the help of enzymes. - With dilute solutions of monomers. Abiotic polymerization does occur with dilute solutions of monomers under certain laboratory conditions: - Dilute solutions of organic monomers are dripped onto hot sand, clay, or rock. Water vaporizes and concentrates the monomers on the substrate. 3. Formation of protobionts. Protobionts: Aggregates of abiotically produced molecules able to maintain an internal environment different from their surroundings and exhibiting some life properties such as metabolism and excitability. There is experimental evidence for the spontaneous formation of protobionts: - When mixed with water,proteinoids (abiotically synthesized polypeptides) self assemble into microspheres surrounded by a selectively permeable protein membrane. These microspheres: - Undergo osmotic swelling and shrinking. - Have potential energy in the form of membrane potential. 4. Origin of heredity before protobiont appearance. Today’s cells transcribe RNA from DNA, which is then translated into proteins. This chain of command must have evolved from a simpler mechanism of heritable control. - One hypothesis is that before DNA, RNAs were the first genes. Evidence to support this hypothesis includes: - Short RNA polymers have been abiotically synthesized in test tubes without enzymes to assist. Origin of Heredity Perhaps this hypothetical membrane-bound protobiont: - Incorporated genetic information. - Selectively accumulated monomers from its surroundings. - Used enzymes programmed by genes to make polymers and carry out other chemical reactions. - Grew and split, distributing copies of its genes to offspring. Mutations could also effect this protobiont and therefore it too could be subject to natural selection, bringing about the formation of the ancient prokaryotes. Photosynthesis and the Oxygen Revolution • Most atmospheric oxygen (O 2) is of biological origin • O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations • By about 2.7 billion years ago,2 O began accumulating in the atmosphere and rusting iron-rich terrestrial rocks • This “oxygen revolution” from 2.7 to 2.3 billion years ago caused the extinction of many prokaryotic groups • Some groups survived and adapted using cellular respiration to harvest energy • The early rise in O 2 was likely caused by ancient cyanobacteria • A later increase in the rise of2 O might have been caused by the evolution of eukaryotic cells containing chloroplasts The First Eukaryotes • The oldest fossils of eukaryotic cells date back 2.1 billion years • Eukaryotic cells have a nuclear envelope, mitochondria, endoplasmic reticulum, and a cytoskeleton • The endosymbiont theory proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells • An endosymbiont is a cell that lives within a host cell • The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites • In the process of becoming more interdependent, the host and endosymbionts would have become a single organism • Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events • Key evidence supporting an endosymbiotic origin of mitochondria and plastids: • Inner membranes are similar to plasma membranes of prokaryotes • Division is similar in these organelles and some prokaryotes • These organelles transcribe and translate their own DNA • Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes The Origin of Multicellularity • The evolution of eukaryotic cells allowed for a greater range of unicellular forms • A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals • Comparisons of DNA sequences date the common ancestor of multicellular eukaryotes to 1.5 billion years ago • The oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago