A Brief Diversion: The Geological Time Scale 
                                                                      Now that we’re getting into time units of higher 
                             Earth’s Oldest Life II                   resolution, we should look at how the geological time 
                                                                      scale works.
                          The Rise of the Eukaryotes
                                                                      This will allow us to talk about time divisions by name 
                                                                      instead of having to talk about time in longhand (by 
                                                                      number of years) !
                        Structure of Geologic Time Scale                          Geologic time scale
                       •Eon – the greatest expanse of time 
                       •Era – subdivision of Eon                        • Eons
                       •Period – subdivision of Era                       –Phanerozoic
                       •Epoch – subdivision of Period                      (“visible life”) –
                                                                           the most recent             Oldest undisputed 
                  Eons       Eras       Periods    Epochs                  eon, began about            eukaryotes
                                                                           545 million years           Well-developed
                                                                                                       redbeds
                                                                           ago                         BIF peaks here
                                                                          –Proterozoic
                                                                          –Archean                     Oldest stromatolites
                                                                          –Hadean–the                  Oldest rocks
                                                                           oldest eon
                                                                                                       Origin of earth
                            Smaller divisions of time
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                                       Geologic time scale                                     Period – subdivision of an 
                                                                                                   era 
                                                                                               Names derived from:
                                                  Era – subdivision of an eon                  1.  “Type” localities (e.g. 
                                                                                                   Jurassic, named after 
                                                  Eras of the Phanerozoic eon                      Jura Mountains)
                                                                                               2.  Rock characteristics 
                                                                                                   (e.g. Carboniferous, 
                                                  Cenozoic (“recent life”)                         coal-rich rocks in the 
                                                                                                   UK)
                                                  Mesozoic (“middle life”)                     3.  From various whims 
                                                  Paleozoic (“ancient life”)                       (e.g. Silurian, named 
                                                                                                   after Celtic tribe of 
                                                                                                   Wales)
                                                                                               -in other words, a big 
                                                                                                   mess !  But it works.
                                                                                                  A major innovation that accompanied the rise of oxygen 
                                                                                                     in the Earth’s hydrosphere and atmosphere was the 
                                                                                                     rise of the eukaryotes.
                                                                                                  The origin of the first eukaryotic cell is obscure, but it is 
                                     And Onward to Eukaryotes…                                       clear that a number of features had to be gained by 
                                                                                                     ancestor prokaryotes. 
                                                                                                  1. Larger size
                                                                                                  2. Membrane-bounded nucleus
                                                                                                  3. Organelles (especially mitochondria and 
                                                                                                     chloroplasts).
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                                                   Increase in cell size                                                                        Acquisition of Nucleus
                          As a general rule, eukaryotic cells are about ten times bigger than                         Making a nucleus is not a huge deal.  It has been 
                          prokaryotic cells.                                                                          speculated that the nucleus was produced by the 
                          Prokaryotic cells: 1 -10 microns Eukaryotic cells: 50-500 microns                           invagination of the cell membrane, and engulfing of genetic 
                                                                                                                      material within the cell.
                          Thus the transition from prokaryote to eukaryote must have involved an                      In this model, the membrane of the nucleus was derived 
                          increase in size.
                                                                                                                      from the cell membrane.
                                                                                                                      Genetic material in 
                                                                                                                      chromosome-like 
                                                                                                                      clump
                                                                                                                       Infolded 
                           Largest known bacterium: 200 microns in dia                                                 membrane 
                           Thiomargarita namibiensis                                                                   surrounds genetic 
                                                                                                                       material
                           A prokaryote of this size may have given rise to the first eukaryote
                                              Acquisition of Organelles                                                                   Acquisition of Organelles
                           The origin of mitochondria and chloroplasts demands a                                            Serial Endosymbiosis Hypothesis (Lynn Margulis)
                           more complex explanation for that of the nucleus.
                           The current belief is that both mitochondria and                                                                                                        cyanobacteria
                           chloroplasts  originated as either invaders or slaves within 
                           a large prokaryotic host cell.                                                                                                    S          plant cell
                           Mitochondria could represent oxidizing bacteria that later                                                     S                             animal cell
                           became integrated with a host cell.
                           Likewise, chloroplasts may well represent cyanobacteria                                                                                                purple bacteria
                           that later became integrated with a host cell. 
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                                 Giardia: Protist with no mitochondrion
                                 -Likes anaerobic environment
                                 -eats anaerobic bacteria
                        Evidence for Origin of Mitochondia and Chloroplasts                               Why the Association in the First Place ?
                                        As “Slave Bacteria” 
                    Mitochondria and Chloroplasts are of similar size as bacteria             One can imagine that mitochondria could process the 
                    (about 1-10 microns)                                                      waste of the host cell, and produce energy that would 
                    Mitochondria and Chloroplasts have complex membrane                       benefit the host cell (so the proto-mitochondrion would be 
                    systems, similar to bacteria.                                             handy to hold on to).
                    Mitochondria and Chloroplasts are somewhat self-contained,                Similarly, a chloroplast would use the waste gas from 
                    as if they derived from functional cells.                                 aerobic respiration (carbon dioxide) and make food for the 
                    Mitochondria and Chloroplasts divide by binary fission,                   host cell (so again, why not freeload on the guest).
                    similar to bacteria. 
                    Plus…the mitochondrial and chloroplast DNA are different 
                    than the host cell !
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