Cell Evolution and Extraterrestrials

bubbles - possible precursors to cells  

• each cell's interior differs from the exterior
• molecules w/ hydrophobic regions spontaneoulsy form bubbles in water
• edges of early oceans exposed to methane, simple organic molecules, and radiation
• primary abiogenesis - theory developed by Alexander Oparin
  • early cells evolved in conditions very different from current conditions
  • protobionts - early bubblelike structures that separated their contents from the environment
  • idea became popular after the Urey-Miller experiment
• Lerman's bubble hypothesis - shows how organic molecules became more complex
  • underwater volcanoes release gases in bubbles
  • gases in bubbles react to form simple organic molecules
  • bubbles pop and release contents into the air once they reach the surface
  • UV rays and energy sources make the simple organic molecules form more complex molecules
  • complex molecules fall back into the water and become in enclosed in bubbles
• other names for bubbles - microspheres, protocells, protobionts, micelles, liposomes, coacervates (depending on what the bubbles contain)
• coacervates - lipid bubbles that form an outer 2-layer boundary; can grow by adding more lipid molecules from the environment; can divide by pinching in 2 like bacteria
• microspheres carrying out metabolic reactions survive longer than those w/o protein or lipids inside
• bubbles better able to use the molecules/energy from the early oceans and produce offspring w/ similar characteristics would live longer
• protein microspheres - could possibly have a genetic system, do not form in water (able to form on dry land though)
• discovery of RNA enzymes >> support for idea that RNA molecules (not lipid/protein bubbles) were 1st lifeforms

microfossils - fossilized form of microscopic life  

• about 1-2 micrometers in diameter
• single-celled, lacked outer appendages, similar to present-day bacteria
• prokaryotes - lack nucleus (found in eukaryotes); very simple organic body plan
• earliest records go back 2.5 billion years
• 1st eukaryotes appeared 1.5 billion years ago

archaebacteria - "ancient ones"; live in environments similar to that of early earth  

• simplest organisms
• methanogens - produce methane, can't live in presence of oxygen (grows anaerobically); have DNA, lipid cell membrane, cell wall, metabolism based on ATP
• lack of peptidoglycan in their cell walls (found in other prokaryotes)
• uses strange lipid not found in any other organisms
• extreme halophiles - "salt lovers"; live in very salty environments, like the Dead Sea
• extreme thermophiles - "heat lovers"; live near volcanic vents; could be successors of earliest organisms due to ability to live w/ high heat
• DNA shows that it split from other life 2 billion years ago

bacteria - 2nd major prokaryote group; larger group than archaebacteria  

• have very strong cell walls
• account for the majority of prokaryotes living today
• some can use light as energy (photosynthetic)
• cyanobacteria - aka blue-green algae; played important role in increasing the amount of oxygen/ozone in the atmosphere

eukaryotes - 1st microfossils different from prokaryotes  

• all organisms other than prokarotes
• may go back 2.7 billions years, but fossil evidence only goes back 1.5 billion years
• have internal membranes and thicker cell walls
• early forms were as large as 60 micrometers in diameter
• possess internal structure called the nucleus (possibly evolved from the endoplasmic reticulum that isolated the nucleus)
• endosymbiotic bacteria - bacteria that live in other cells and perform functions for it
• theory of endosymbiosis - claims that bacteria living inside larger bacteria eventually evolved into mitochondria, chloroplasts, and other cellular parts
• developed sexual reproduction, able to frequently recombine genes

multicellularity - promoted diversity  

• started when eukaryotic cells started living in colonies
• colonies began working as a single unit
• allows for specialization, giving specific tasks to certain cells

6 kingoms  

• Bacteria - prokaryotic organisms w/ peptidoglycan cell wall
• Archaebacteria - prokaryotes w/o peptidoglycan in cell wall
• Protista - eukaryotic, unicellular (except for certain types of algae); can be photosynthetic/heterotrophic
• Fungi - eukaryotic, multicellular (except for yeast), heterotrophic; have chitin cell walls
• Plantae - eukaryotic, multicellular, photosynthetic
• Animalia - eukaryotic, multicellular, motile, heterotrophic

extraterrestrial life?  

• at least 10% of all stars can have planetary systems
• highly unlikely that earth is the only planet w/ life
• Mars meteorite - oldest rock known to science (4.5 billion years old); contained small patches similar to microfossils and bacteria (but many times smaller)
• Europa - Jupiter's moon; most likely known place for extraterrestrial life due to liquid ocean under icy surface