Resolution: The environment of early earth wasn t anything like current conditions.

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1 1 Bio 1101 Lecture 12 Chapter Evolution of Microbial Life 2 3 Evolution of Earliest Life Different ideas about origins Spontaneous Generation Until mid-1800s Life can emerge from inanimate materials Flies on meat; mice in dirty laundry; frogs in ponds; etc. Louis Pasteur: chemist 1859: did not believe in spontaneous generation. Pasteur's Swan Neck Flask He boiled meat broth in a flask, heated the neck of the flask in a flame until it became pliable, and bent it into the shape of an S Air could enter the flask, but airborne microorganisms could not - they would settle by gravity in the neck As Pasteur had expected, no microorganisms grew. Pasteurisation of milk heat it to high temp & bottle it Pasteur s experiments demonstrated that life cannot arise from non-life Biogenesis There is no evidence that life can arise from non-life (abiogenesis or spontaneous generation) on earth today Paradox: how did the first life arise then? Resolution: The environment of early earth wasn t anything like current conditions. The atmosphere contained very little oxygen (oxygen tears apart complex molecules) There were favorable conditions for chemical reactions that generated small organic molecules (monomers) from inorganic ones Without an ozone layer (which is a product of oxygen in the atmosphere), high levels of UV radiation reached the earth. UV light and lightning could have been energy sources, catalyzing chemical reactions 1

2 Laboratory experiments (such as the famous Miller-Urey experiment see page 297) have replicated the conditions of early earth inside flasks and tubes to test hypotheses of chemical evolution: Atmosphere consisting of methane, ammonia, hydrogen, and water vapor A flask of water representing the primordial sea Electric discharges simulating lightning The result: basic organic molecules were formed and deposited in the water Small molecules joined to form polymers as water splashed onto hot rocks or little pools condensed down Eventually Complex molecules became self-replicating (DNA!) 11 First genes were probably short strands of RNA In the lab, small strands of RNA have been found to assemble without the need for special proteins (enzymes) Pre-cells may have then formed Liposomes are little hollow spheres consisting of a membrane of fats May have enclosed some nucleic acids and proteins, making a precursor to a cell Over time, these pre-cells could have become more and more cell-like Formation of first cells explained by natural chemical and biological processes The earth formed about 4.6 Billion years ago, and the first prokaryotes were living on earth 3.5 billion years ago so these little pre-cells had hundreds of millions of years to evolve Over the next 2 Billion years, only prokaryotes were on earth There are many different kinds of prokaryotes, and they can be found in just about every habitat on earth Hot or cold Saltwater or freshwater Acidic, neutral, or basic 2

3 15 2 Types of Prokaryotes Domain Bacteria Most prokaryotes are in this group The cause of many diseases Beneficial bacteria help break down food in our guts Decomposers break down dead organic material, allowing it to be recycled in the environment Domain Archaea Used to be considered the most evolutionarily primitive prokaryotic cells Extremophiles ( love extreme habitats) Adapted for harsh environments (such as would have been present in early earth) Such as Great Salt Lake, deep sea thermal vents, and anaerobic habitats Prokaryotic Structure Cocci (spherical) Bacillus (rod-shaped) Spirochetes (spiral-shaped) Lack true nucleus and membrane-bound organelles Most can reproduce at rapid rate by binary fission DNA replicated and the cell splits in two Ecological Impacts Disease Production of toxins (exotoxins or endotoxins) 1 gram of botulinum exotoxin could kill a million people Salmonella poisoning is due to an endotoxin in the cell wall of the bacteria Chemical Recycling 3

4 Cyanobacteria are important in restoring oxygen to the atmosphere; also fix nitrogen take it in from the atmosphere and convert it into a form that other plants can absorb from soil and water Some symbiotic bacteria live with roots of some plants (like legumes) and fix nitrogen, making it available for the plant Many bacteria break down organic materials (decomposition) 20 Bioremediation Used to clean up the environment Decomposers can break down sewage Some types of bacteria have also been used recently to treat oil spills (can break down the petroleum products) PROTISTS Simple, primitive eukaryotes The ancestors of all other eukaryotes plants, animals and fungi Where did they come from? Evolved about 1.7 billion years ago Many membrane-bound organelles may have formed from in-foldings of cell membranes Endosymbiont Theory: chloroplasts and mitochrondria evolved when one prokaryotic cell engulfed a smaller cell, and the two came to live together. Evidence Chloroplasts and mitochondria happen to be about the size of a small bacterium They also contain their own DNA, RNA, and ribosomes, separate from the rest of the cell The Diversity of Protists 4

5 All are eukaryotic Most are unicellular Must have very complex cells Complete organisms Some are colonial or multicellular Although most are small, there are very large Protists as well, such as the giant kelps (~200 ft long) 4 Major Groups: Protozoans Slime Molds Algae Seaweeds Where Do Protists Live? Most are aquatic (planktonic or sessile) Planktonic protists -- important part of the aquatic food web; plankton are the tiny organisms that float freely in bodies of water, like lakes and oceans. Photosynthetic plankton and bacteria in our oceans account for about 40% of all photosynthesis on earth! Think of this in terms of our atmosphere... Some protists are free-living, some live in symbiotic relationships with other organisms, and some are parasitic Terrestrial protists must live in damp places. Protists must have water for sexual reproduction, to carry their gametes. 5-minute break 1. Protozoans Animal-like protists Heterotrophs Eat bacteria or other protists Live in a variety of aquatic habitats Will cover 4 groups: Flagellates Amoebas Apicomplexans Ciliates Flagellates Move by use of flagella (one or more) Most free-living 5

6 Some parasitic Giardia, which infects humans and causes severe diarrhea Trypanosoma, which causes African sleeping sickness (tsetse fly intermediate host) Amoebas No flagella or cilia Move by pseudopodia Temporary extensions of the cell Brain Eater! What is the brain eating amoeba? Brain eating amoeba in the news: Apicomplexans All parasitic Includes the Plasmodium which cause malaria Anophales mosquito is the intermediate host Flu-like symptoms; may progress to kidney failure, seizures, and death 1-3 million people die each year from malaria Ciliates Use cilia to move and feed Most familiar example: Paramecium Live in freshwater 2. Slime Molds Fungus-like protists (heterotrophs) Not evolutionarily closely related to true fungi Plasmodial slime molds: found on leaf litter and logs, in moist areas (forests) Single-cell with many nuclei 38 6

7 3. Algae Plant-like protists Book discusses single-celled, but there are a variety of multicellular algae as well Must live in wet or moist habitats; lack a cuticle Groups we will discuss: Dinoflagellates Diatoms Green Algae Seaweed Dinoflagellates Mostly unicellular Cells covered with cellulose plates Each has two flagella; move like a spinning top Most are photosynthetic, although some are not and ingest other microorganisms Many are endosymbionts with marine invertebrates; these dinoflagellates are called zooxanthellae, and provide carbohydrates for their invertebrate hosts; the hosts (for example, corals) provide shelter and CO2 for the zooxanthellae. Responsible for almost all primary production in coral reefs Also the organism responsible for Red Tides Produce neurotoxins that kill fish, and also humans Diatoms Most are unicellular Usually reproduce asexually by cell division Two-part shell containing silica (glass-like) Classified according to pattern of shell Planktonic and sessile forms Freshwater and marine habitats Important in primary production (photosynthesis) Massive accumulations of shells form diatomaceous earth, which is mined and used for a variety of purposes (insulation, filtering, abrasives, etc.) White Cliffs of Dover a massive accumulation of diatoms Green Algae Photosynthetic; aquatic and terrestrial (moist) habitats 7

8 Variety of growth forms (unicellular, colonial, coenocytic, & multicellular) Most are motile, at least during some point in life A green algae was probably the ancestor of modern plants Characteristics in common: pigments (chlorophyll a & b and carotenoids) storage products (mainly starch) cell walls with cellulose Important in many food webs; oxygenate water Reproduction: both sexual (by forming gametes in unicellular gametangia) and asexual (by either cell division, fragmentation, or spores) Volvox, a colonial green algae Seaweeds Large, multicellular marine algae 3 main groups classified based on their pigments: green, red, and brown algae Can be harvested for food: A brown algae called kombu used in soups in Japan and Korea A red algae called nori used to wrap sushi Gel-like substance in seaweed used as thickener in puddings, ice cream, etc. A green algae ancestor, called a Charophycean, gave rise to true land plants Next time, plant evolution & diversity All for today 8