The Origin and Early History of Life

Chapter 4

 

Origin of Life Hypotheses

•Special Creation

–Supernatural force created life

–Not repeatable, not testable, not science

•Extraterrestrial Origin

–Panspermia - Asteroids carried complex organic molecules to earth.

–Nearly impossible to test, marginally science (if at all)

–Doesn’t answer the question, only changes the geography of it

•Spontaneous Origin

–Life evolved from inanimate matter

–Replication of conditions of “origin” should produce life or at least complex organic molecules

 

Where Life Started

•Early atmosphere composed of CO2, N2, H2O

–little O2 (from photosynthesis much later in Earth’s history)

•Degrades amino acids

–Debate about “reducing” nature of atmosphere (ie. availability of H+)

 

Where Life Started

•Potential Sites of Origin

–Ocean’s Edge – would require H+ in atmosphere

–Under Frozen Seas – would require cold waters

–Deep in Earth’s Crust - ?

–Clay - ?

–Deep Sea Vents - H+ readily available, abundance of archaebacteria

 

Miller-Urey Experiment

•Reproduced conditions at oceans edge in reducing atmosphere

–May have resembled early Earth

–Formed amino acids with electricity

–Similar experiments produced many different amino acids and nucleic acids

 

Chemical Evolution

•Did RNA or Proteins evolve first?

–RNA - complex molecules couldn’t form without a heredity molecule.

–Protein – enzymes responsible for nearly all cells activity (some functions done solely by RNA)

–Peptide-Nucleic Acid - peptide nucleic acid precursor to RNA.

 

Cell Origin Hypotheses

•Most well supported hypotheses involve ocean

•Phospholipid bubbles spontaneously form in sea water

–Bubble Hypothesis- shielded hydrophobic regions of molecules from water

•Primary abiogenesis- Chemical-concentrating bubbles allowed cells to develop chemical complexity

 

Has life ever been generated in a lab?

•No

•Scientific hypotheses of origin of life are testable and work with repeatable phenomena

•If life arose from inanimate matter 3.8 bya, then macromolecules that make up cells should arise during conditions resembling the Earth at that time.

 

Earliest Cells

•Microfossils - fossilized forms of microscopic organisms

–Resemble Archaebacteria.

•Prokaryotes - Lack nucleus.

•Eukaryotes - Contain nucleus.

 

Extant Organisms Similar to Earliest Cells

•Archaebacteria - live in extremely hostile conditions.

–Lack peptidoglycan cell walls.

–Unusual lipids in cell membranes.

•Methanogens

–Anaerobic

•Halophiles

•Thermophiles

 

Other Bacteria

•Eubacteria – other major bacterial group.

–Strong cell walls and simpler gene architecture.

–Cyanobacteria - Photosynthetic (3 bya)

 

First Eukaryotic Cells

•First appeared ~ 1.5 bya. (maybe earlier)

–Membrane bound nucleus

–Endoplasmic Reticulum - network of internal membranes.

–likely evolved from infolding of outer membranes.

 

Mitochondria and Chloroplasts

•Endosymbiosis

–Aerobic bacteria within larger eukaryote = mitochondria.

–Photosynthetic bacteria within larger eukaryote = chloroplasts

 

Chloroplast Structure

Sexual Reproduction and Multicellularity

•Many Eukaryotic organisms sexually reproduce.

–Genetic diversity.

•Multicellularity allows for specialization

–May facilitate increases diversity

 

Kingdoms of Life

•Archaebacteria

•Eubacteria

•Protista (not all closely related to each other)

–Category that just won’t go away!

•Fungi

•Plantae

•Animalia