World Builders™                                                                    Session Four  --  Microbiology             

        The Gene Pool     

Living things must focus on two tasks, reproduction and energy capture.

Changes in Life Forms and the Rise of Oxygen

Billions of Years Ago What Happened   Oxygen in Atmosphere
4.6 Earth formed   Less than 1% pf [resent day levels
3.5 Cyanobacteria appear    
2.3 Oxygen accumulates rapidly

Eukaryotes with different types of cells appear.

2.3 - 1.8 mitochondria appear

Cell types in eukaryotes increase

Use oxygen to create energy  
1.6 Plants and animals separate 15% of present  day levels & rising
1.5 Some eukaryotes had up to ten cell types

Plastids appear, enable cells to produce oxygen through photosynthesis

1 Some eukaryotes have up to 50 cell types    
600 million -- or maybe much earlier perhaps after plastids emerged Cambrian Explosion    

Hedges SB, Jaime E Blair1, Maria L Venturi1 and Jason L Shoe: A molecular timescale of eukaryote evolution and the rise of complex multicellular life.
BMC Evolutionary Biology 2004, 4:2

S Blair Hedges1 , Jaime E Blair1 , Maria L Venturi1 and Jason L Shoe2
1NASA Astrobiology Institute and Department of Biology, The Pennsylvania State University, University Park, PA 16802, USA
2Harvard School of Dental Medicine, 188 Longwood Avenue, Boston, MA 02115, USA

BMC Evolutionary Biology 2004, 4:2

The electronic version of this article is the complete one and can be found online at:


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