The History of Life on Earth

History of Life

• Originated 3.5-4.0 billion years ago

• Fossil evidence: stromatolites

Major Episodes in the History of Life

• Prokaryotes-3.6 billion years ago

• Prokaryotes diverged into Bacteria and Archaea 2-3 billion years ago

• Photosynthetic bacteria began producing O2 2.5 billion years ago

• Eukaryotes emerged 2 billion years ago

Prebiotic Chemical Evolution

• Abiotic synthesis and accumulation of monomers

• Formation of polymers

• Formations of protobionts

• Origin of heredity during or before protobiont appearance

Protobionts

• Aggregates of abiotically produced molecules

• Maintain internal environment, different from surroundings

• Exhibit some life properties-irritability and metabolism

• Self-assemble• Microspheres and liposomes

Laboratory versions of protobionts

RNA was probably the First Genetic Material

• If DNA, a primer would be necessary

• RNA can self-replicate

• RNA is autocatalytic

• Achieves unique tertiary structure (different phenotypes)-diversity!

Abiotic Replication of RNA

Hereditary Material Enabled Darwinian Evolution

If Protobionts:

1. Incorporated genetic information

2. Selectively accumulated monomers

3. Used enzymes programmed by genes to make polymers

4. Grew and split

Then:

• Variations would lead to natural selection

• Refinements would have accumulated

• Lead to the appearance of DNA

Videos and Websites

• http://www.youtube.com/watch?v=zufaN_aetZI&feature=fvw

• http://www.youtube.com/watch?v=OandUMjhZ3g&NR=1&feature=fvwp

The Fossil Record and Geologic Time

• Role of sedimentary rocks

• Fossil dating, use of strata location vs. absolute dating

• Fossil record incomplete, favors species that existed for a long time, why?

• Role of continental drift

• Mass extinctions and adaptive radiations

Key Events in Life’s History

• Photosynthesis-oxygen revolution-2.7 billion years ago (photosynthetic bacteria)

• First eukaryotes-2.1 billion years ago, result of endosymbiosis

• Origin of multicellularity-1.5 billion years ago• Cambrian Explosion-535-525 million years ago• first predators, first hard bodied organisms, Cnidaria,

Porifera, and Mollusks, bilateral symmetry• Colonization of land-began with cyanobacteria 1 billion

years ago, necessary adaptations?

Genesis of Eukaryotes:Serial Endosymbiosis?

• Membrane-bound nucleus• Mitochondria, chloroplasts, and the

endomembrane system• Cytoskeleton• 9 + 2 flagella• Multiple chromosomes• Mitosis, meiosis, and sex

The Origin of Eukaryotes?

Secondary Endosymbiosis

Continental Drift

• Pangea: supercontinent, broke apart during Mesozoic era.

• Explains distribution of fossils and extant organisms. (lungfishes, marsupials)

• Generated by plate tectonics

• Can result in volcanoes (Krakatau, Tambora), tsunamis.

Examples of Mass Extinctions

• Approximately 12 mass extinctions• Permian extinctions-250 million years ago, 90%

species (marine) eliminated• Cretaceous extinctions-65 million years ago, 50%

marine species, dinosaurs, many plants• Asteroid impact (Alvarez or Impact Hypothesis),

crater from Cretaceous extinction- 180 km dia. Yucatan coast

• Role of the Siberian Traps (caused O2 to drop from 30% to 15% or lower during the Permian period)

Consequences of Mass Extinctions

• Widespread adaptive radiations

• Causes?

Mass extinctions

DNA, RNA and Protein Comparisons

• DNA hybridizations-compare degree of similarity between two species

• Restriction maps• DNA sequence analysis• Homologous DNA sequences-mutations

accumulate as species diverge• Molecular clocks-number of amino acid

substitutions is proportional to the elapsed time since divergence

The Origin of Evolutionary Novelty

• How do novel features that define taxonomic groups above the species level arise? (wings as an example)

• Gradual refinement of existing adaptations• Alternative functions• Exaptation: structure that evolved in one context

and later was adapted for another function. Examples: feathers, light hollow bones in birds

Genes that control development and evolutionary novelty

• Sometimes a few changes in the genome causes major changes in the morphology.

• A system of regulatory genes coordinates activities of structural genes to guide the rate and pattern of development

• Allometric growth• Heterochrony• Paedomorphosis• Homeosis

Heterochrony-alteration in the time of change in the order of one or more

events

Allometric Growth-different body parts grow at different rates. Result: adult is shaped

different form the juvenile.

Paedomorphosis: type of heterochrony that describes a condition in which the

time of sexual maturity is altered. Retention of ancestral juvenile structures

in a sexually mature adult organism

Homeosis-alteration in the placement of different body parts

Hox genes

• Homeotic genes

• Responsible for where structures develop on the embryo

• Responsible for limb formation instead of fin formation