Chapter 25 The History of Life on Earth

Question u How have events in the Earth’s history contributed to life as we know it?

How did Life get started? u One Idea - Chemical Evolution: the evolution of life by abiogenesis.

Steps 1. Monomer Formation 2. Polymer Formation (macromolecules) 3. Protobiont Formation 4. Origin of self-replicating molecules (heredity)

Primitive Earth Conditions u Reducing atmosphere present. u Simple molecules u Ex: H 2 O, CH 4, H 2, NH 3

Complex Molecule Formation u Requires energy sources: u UV radiation u Radioactivity u Heat u Lightning

Oparin and Haldane 1920s u Hypothesized steps of chemical evolution from primitive earth conditions.

Miller and Urey, 1953 u Tested Oparin and Haldane’s hypothesis. u Experiment - to duplicate primitive earth conditions in the lab.

Results u Found organic monomers including Amino Acids. u Suggested that under the primitive Earth’s conditions, monomers could form.

Other Investigator's Results u All 20 Amino Acids u Glucose, Ribose, Deoxyribose etc. u Glycerol and fatty acids u Nucleotides u ATP u Monomers for all macromolecules have been found.

Polymer Synthesis u Problem: u Monomers dilute in concentration. u No enzymes for bond formation.

Possible Answer 1. Clay 2. Iron Pyrite u Both could have served as a substrate to position monomers close together. u Have metals that can act as catalysts.

Another possibility u Waves could have placed monomers on beaches. u Sun could have concentrated the solutions.

Protobionts u Aggregates of abiotically produced molecules. u Exhibit some properties of life. u Ex: Osmosis, Electrical Charge, Fission

Protobionts

Results u Protobionts have membrane- like properties and are very similar to primitive cells. u Start for natural selection that lead to cells?

Question ? u Where did the energy come from to run these early cells?

Answers u ATP. u Reduction of sulfur compounds. u Fermentation. u Rs and Ps developed much later.

Self-replicating molecules (Heredity) u DNA  RNA  Protein u Too complex for early life. u Other forms of genetic information?

RNA Hypothesis u RNA as early genetic information: u RNA polymerizes easily. u RNA can replicate itself. u RNA can catalyze reactions including protein synthesis.

Ribozymes u RNA catalysts found in modern cells. u e.g. ribosomes u Possible relic from early evolution?

Molecular Cooperation u Interaction between RNA and the proteins it made. u Proteins formed may serve as RNA replication enzymes.

Molecular Cooperation u Works best inside a membrane. u RNA benefits from the proteins it made.

Selection favored: u RNA/protein complexes inside membranes as they were the most likely to survive and reproduce.

DNA u Developed later as the genetic information. u Why? More stable than RNA

Alternate View Life developed in Volcanic Vents.

Volcanic Vents u Could easily supply the energy and chemical precursors for chemical evolution. u Most primitive life forms today are the prokaryotes found in or near these vents.

New Idea u Life started in cold environments. u Interface between liquid and solid allows concentration of materials and formation of polymeres. u Molecules last longer too.

Modern Earth u Oxidizing atmosphere. u Life present. u Prevents new abiotic formation of life. u However – some suggest that “alien” life is still present.

Hypothesis u Life as a natural outcome of chemical evolution. u Life possible on many planets in the universe (?).

Fossils u Any preserved remnant or impression of a past organism.

Types of Fossils 1. Mineralized 2. Organic Matter 3. Trace 4. Amber

Mineralized Fossils u Found in sedimentary rock. u Minerals replace cell contents. u Ex: bone, teeth, shells

Organic Matter Fossils u Retain the original organic matter. u Ex: plant leaves trapped in shale. u Comment – can sometimes extract DNA from these fossils.

Trace Fossils u Footprints and other impressions. No organic matter present.

Amber u Fossil tree resin. u Preserve whole specimen. u Usually small insects etc.

Fossils - Limitations u Rare event. u Hard to find. u Fragmentary. u Dating.

Fossil Dating Methods 1. Relative - by a fossil's position in the strata relative to index fossils. 2. Absolute - approximate age on a scale of absolute time.

Absolute - Methods 1. Radioactive 2. Isomer Ratios

Radioactive u Estimated from half-life products in the fossil. u Ex: Carbon - 14 Potassium – 40 u Different isotopes have different half-lives.

Isomer Ratios u Ratio of L- and D- amino acid isomers. u L- used by living things. u D- not used by living things.

Death u L- form  D- form u Age can be calculated from the ratio of L-/D- as long as the temperature of the area is taken into account.

What do fossils tell us? u That the geographical distribution of organisms has changed over time. u Reason? – The Earth has changed over its history.

Key Events u Origin of Life and single-celled organisms u Ps and Oxygen revolution u First Eukaryotes u Origin of Multicellularity u The Cambrian Explosion u Colonization of Land u Mass extinctions and radiations

Origin of Life u Dates to about 3.5 billion. u Stomatolites are a fossil evidence. u Prokaryotic cells – asexual reproduction only.

Ps and Oxygen Revolution u Early atmosphere was a reducing atmosphere. u Oxygen produced by Ps. u Oxygen caused Fe precipitation (rusting). u Oxygen levels up to 30+%.

Result u Favored aerobic Rs. u Loss of many anaerobic life forms.

First Eukaryotes u Appeared about 2.1 billion years ago. u Serial endosymbiosis model suggests how.

Serial Endosymbiosis u Folding of cell membrane to create nuclear and endomembrane system. u Engulfing of aerobic heterotrophic prokaryote. u Engulfing of photosynthetic prokaryote.

Evidences u Circular DNA without nucleosomes in mitochondria and chloroplast. u Small ribosomes in mitochondria & chloroplasts.

Origin of Multicellularity u Started about 1.2 billion years ago. u Time of “snowball” Earth. u Diversification started about 565 million years ago when Earth thawed.

Cambrian Explosion u million years ago. u Explosion of life forms especially large predators. u Many prey capturing adaptations and defense mechanisms seen in fossils.

Colonization of Land u Plants – 420 million years ago. u Animals – 365 million years ago. u Problems – desiccation, gravity, light, temperature

Mass Extinctions & Adaptive Radiations u Fossils indicate that life on earth has shifted regularly. u Most life has gone extinct. u Severe climate changes have happened. u Why?

Continental Drift u The movement of the earth's crustal plates over time. u Drift is correlated with events of mass extinctions and adaptive radiations of life.

Result of plate movement u Geographical Isolation. u New environments formed. u Old environments lost. u As the environments changed, so did Life.

Example u Australian fauna and flora are unique. u Separated early and remained isolated for 50 million years.

Mass Extinctions u The sudden loss of many species in geologic time. u May be caused by asteroid hits or other disasters.

Examples u 5 major extinction events u Permian Extinction u Cretaceous Extinction

Permian Extinction u 250 million years ago. u 90% of species lost.

Cretaceous Extinction u 65 million years ago. u Loss of the dinosaurs. u Good evidence that this event was caused by an asteroid that hit in the Yucatan, causing a “nuclear winter”.

New Ideas u See article on BlackBoard.

Result of Mass Extinctions u Climate changes. u Areas are open for the surviving species to exploit. u Rapid period of speciation (adaptive radiation). u Many new species are formed in a very short period of time.

Evolution Novelty u Where can new body forms come from? u Evo-Devo – slight genetic divergences can produce major body form changes.

Examples u Heterochrony – change in the timing of developmental events or growth rates. u Homeotic genes – changes in spatial patterns.

Specific examples u Expression of Hox Gene 7 and Ubx for the insect body plan. u Pitx1 in stickleback fish

Homework u Read Chapter 26 u Chapter 25 – today u Practice Exam – analysis sheets – Tue. 3/4 u Lab - TBA

Species Selection u Speciation = birth u Extinction = death u Species that leave the most new species are the most successful. u Encourages “branches” in the Tree of Life.

Summary u Chemical Evolution steps. u Recognize the use and limits of fossils. u Key Evens in Earth’s History. u What happens to evolution in mass extinctions. u Examples of Evo-Devo.