Systematics

• Geologic evidence shows Earth formed 4.6 BYA

  
• The planets probably formed from debris cast off from our Sun following its condensation from interstellar gas and dust of a nebula about 5-7 BYA
• Earth was semi-molten → gradually cooled enough to form a solid rock crust
• Decay of radioactive elements of Earth caused partial melting of the planet's interior, with denser materials making up the core
• Differential temperatures of the core and crust set convection currents in motion in the mantle
• Gases released into the primitive atmosphere

  • ammonia = NH3
  • carbon monoxide = CO
  • hydrogen sulfide = H2S
  • methane = CH4
  • nitrogen = N2
  • water vapor = H2O
• Water vapor condensed and fell through the atmosphere → dissolving gases; "poisonous" rain
• Run-off of rainwater from the land carried dissolved minerals into a shallow proto-ocean
• The collective materials in the proto-ocean formed a "hot, thin primordial soup"
• Ribozyme


  An RNA molecule capable of enzymatic actions, e.g., the self-splicing introns of some RNAs, which can excise themselves from the molecule without the help of protein enzymes
• The earliest organism may have been no more than a ribozyme
• Early Atmosphere probably contained CO2, CO, Nitrogen, Ammonia, Methane. No oxygen to support life
• The Earth cooled down → enough for liquid water to form

  3.8 BYA
• Microscopic organisms → earliest life form
• Preserved in the hard structures called "stromatolites"
• Prokaryotes and anaerobic heterotrophs that fed on organic molecules in the primordial soup
• Cyanobacteria (2.4 BYA) → Earth's first photo-synthesizers, making food using water and the Sun's energy and releasing oxygen as a result
• Cyanobacteria catalyzed a sudden, dramatic rise in oxygen → killed microbes that could not tolerate oxygen (Great Oxidation Event)
• The Great Oxidation Event was recorded on seafloor rocks → oxidation reaction caused bands of iron
• Oxygen levels were lowered eventually → reduce in cyanobacteria number
• Anaerobic respiration


  C6H12O6 (glucose) → energy + C2H3OH (ethanol) + CO2
• In the atmosphere, UV radiation

  3CO2 + UV → O3 (ozone) + 3CO (carbon monoxide)
• Animal bodies have various cells each doing a distinct job
• Microbes are just single cells with no organelles and no nuclei to package their DNA
• Eukaryotic organisms → true nucleus, DNA, and membrane-bound organelles evolved between 1.4 and 1.6 BYA
• DNA became packaged in nuclei
• The new complex cells ("eukaryotic cells") boasted specialized parts playing specialized roles that supported the whole cell
• Cells also began living together → benefits (feed efficiently, grow bigger, protection)
• Living collectively, cells began to support the needs of the group by each cell doing a specific job (specialization), similar to stem cells
• Clusters of specialized, cooperating cells eventually became the first animals (800 MYA)
• Poriferans (Sponges) were among the earliest animals
• While chemical compounds from sponges are preserved in rocks as old as 700 million years, molecular evidence points to sponges developing even earlier
• Oxygen levels in the ocean were still low compared to today, but sponges are able to tolerate conditions of low oxygen
• Ediacaran Period aka Vendian Period → more organisms aside from sponges have proliferated


  580 MYA
• Fossil evidence of Ediacaran organisms can be found in sedimentary rocks around the world
• Ediacaran Period → end of global Marinoan glaciation
• Distinctive carbonate aka "cap carbonate" from glacial deposits
• By the end of the Ediacaran, oxygen levels rose, approaching levels sufficient to sustain oxygen-based life
• The early sponges may actually have helped boost oxygen by eating bacteria
• Tracks of an organism named Dickinsonia costata suggest that it may have been moved along the sea bottom, presumably feasting on mats of microbes