The Origin and Diversity of Life

Biodiversity ~1.5 Million species identified Many more remain to be identified

Possible origin(s) of life on earth? Extraterrestrial aliens brought it Came with meteors from other planets Divine creator Chemicals from primordial soup combined None of the above

Review of what have we learned Cell theory –All living organisms are made of cells, and all living cells come from other living cells. Molecular basis of inheritance –DNA encodes genes which make-up and control living organisms. Evolutionary change –Life-forms have evolved varying characteristics to adapt to varied environments. Evolutionary conservation –Some characteristics of earlier organisms are preserved and passed on to future generations.

Fundamental Properties of Life Cellular organization Sensitivity Growth Development Reproduction Regulation Homeostasis Heredity Evolution

Where did the first cell come from? Or….Can life arise from non-life? Can we test this scientifically? –Conditions on early earth –Formation of organic molecules –Chemical evolution –Primitive cell –Prokaryotes –Eukaryotes –Multicellular organisms

Origins of Life The Earth formed as a hot mass of molten rock about 4.5 billion years ago (BYA) –As it cooled, chemically-rich oceans were formed from water condensation Life arose spontaneously –Ocean’s edge, hydrothermal deep-sea vents, or elsewhere

Conditions on Early Earth Seems likely that Earth’s first organisms emerged and lived at very high temperatures First organisms emerged between 3.8 and 2.5 BYA Early atmosphere composition not agreed on –May have been a reducing atmosphere –Would have made it easier to form carbon-rich molecules

Reproduced early atmosphere –Assembled reducing atmosphere rich in hydrogen with no oxygen gas –Atmosphere placed over liquid water –Temperature below 100ºC –Simulate lightning with sparks Miller and Urey Experiment

Found within a week that methane gas (CH 4 ) converted into other simple carbon compounds –Compounds combined to form simple molecules and then more complex molecules Later experiments produced more than 30 carbon compounds including amino acids and nucleotides What Was Found?

Chemical Evolution If life originally arose from non-life, how might this have happened? Consider the following scenario Synthesis and accumulation of small organic molecules Joining of these monomers into polymers Aggregation of these molecules into droplets to form localized microenvironments Origin of heredity

Sidney Fox (University of Miami) demonstrated the abiotic polymerization of organic monomers Polymers were formed when dilute solutions of organic molecules were dripped onto hot sand, clay, or rock “Proteinoids” Clay can serve to concentrate these molecules Monomers bind to charged sites on clay particles Metal ions in clay have catalytic function Polymer Formation

A.DNA B.RNA C.Protein DNA  RNA  PROTEIN (Information) (Information and Catalytic) (Catalytic and Structural) Which came first? Most Likely

RNA may have been first genetic material –Ribozyme activity Amino acids polymerized into proteins Metabolic pathways emerged –Primitive organisms may have been autotrophic – built what they needed Lipid bubbles could increase the probability of metabolic reactions –Leads to cell membranes Other innovations contributed to diversity of life Probable Scenario

Classification of Organisms More than 2000 years ago, Aristotle divided living things into animals and plants Later, basic units were called genera –Felis (cats) and Equus (horses) In the 1750s, Carolus Linnaeus instituted the use of two-part names, or binomials –Apis mellifera the European honeybee –Genus name capitalized, all in italics

Taxonomy is the science of classifying living things –A classification level is called a taxon Scientific names avoid the confusion caused by common names Taxonomy

The Linnaean Hierarchy Taxa are based on shared characteristics –Domain → → → Species Early taxonomists not aware of distinction between derived and ancestral traits –Many hierarchies now being re-examined Categories at the different levels may include many, a few, or only one taxon Limitations –Many higher ranks are not monophyletic –Linnaean ranks not equivalent in any meaningful way

3 domain system –Domain Archaea –Domain Bacteria –Domain Eukarya Domain Bacteria (Bacteria) Domain Archaea (Archaebacteria) Domain Eukarya (Eukaryotes) Common Ancestor

Organisms Grouping Organisms Carl Woese proposed a 6-kingdom system 4 eukaryotic kingdoms Plantae Fungi Animalia Protista – did not fit into 3 other kingdoms –2 prokaryotic kingdoms Archaea Bacteria Tree based on rRNA analysis Archaea and Eukarya are more closely related to each other than to bacteria Each fundamentally different

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Prokaryotes Eukaryotes

Bacteria Most abundant organisms on Earth Key roles in biosphere –Extract nitrogen from the air, and recycle carbon and sulfur –Perform much of the world’s photosynthesis Responsible for many forms of disease Highly diverse Most taxonomists recognize 12–15 different groups

Archaea Shared characteristics –Cell walls lack peptidoglycan (found in bacteria) –Membrane lipids are different from all other organisms –Distinct rRNA sequences Divided into three general categories –Methanogens –Extremophiles –Nonextreme archaea

Methanogens –Use H 2 to reduce CO 2 to CH 4 –Strict anaerobes that live in swamps and guts Extremophiles –Thermophiles – High temperatures (60–80ºC) –Halophiles – High salt –Acidophiles – Low pH (pH = 0.7) Archaea

Eukarya Prokaryotes ruled the earth for at least one billion years Eukaryotes appeared about 2.5 BYA Their structure and function allowed multicellular life to evolve Eukaryotes have a complex cell organization –Extensive endomembrane system divides the cell into functional compartments

Mitochondria and Chloroplasts Mitochondria and chloroplasts most likely gained entry by endosymbiosis Mitochondria were derived from purple nonsulfur bacteria Chloroplasts from cyanobacteria

ArchaebacteriaAnimaliaFungiProtistaPlantaeBacteria Brown algae Photosynthetic protists Thermophiles Halophiles Methanogens Ancestral eukaryotic cell Purple bacteria Photosynthetic bacteria Other bacteria Chloroplasts Mitochondria Red algae Green algae Nonphotosynthetic protists 34

Viruses Are literally “parasitic” chemicals –DNA or RNA wrapped in protein Cannot reproduce on their own Not considered alive – cannot be placed in a kingdom Viewed as detached fragments of a genome Tobacco mosaic virus (TMV) first discovered in 1933