The ages of the Earth

• 4.5 billion years ago.
  Earth and sun form out of interstellar gas and dust, perhaps triggered by a supernova explosion.
  
  
• 4 billion years ago.
  Life began.
  
  
• 3 billion years ago.
  Photosynthesis (in blue-green algae).
  
  
• 570 million years ago.
  First animal with shells, eukaryote cells.
  
  
• 400 million years ago.
  First fishes.
  
  
• 300 million years ago.
  First reptiles, coal deposits.
  
  
• 250 million years ago.
  Age of reptiles.
  
  
• 200 million years ago.
  First dinosaurs, very early mammals.
  
  
• 140 million years ago.
  Age of dinosaurs, first birds.
  
  
• 65 million years ago .
  Dinosaur extinction.
  
  
• 20 million years ago.
  Probable point (+/- 5 million) at which our lineage separated from the apes.
  
  
• 4 million years ago .
  Earliest fossil of genus Homo (not our species but our genus and an ancestor).
  
  
• 200,000 years ago.
  Focus of radiation of our species in Africa. "Mitochondrial Eve," (see below).
  
  

Radioactive dating:

• Carbon-14.
• Potassium Argon.
• Other examples.

Origin of Life.

Probably some 4 billion years ago.

Original atmosphere: probably had compounds such as water, carbon dioxide, methane, ammonia, nitrogen, but no free oxygen.

Stanley Miller.

Classic experiment ~1953 in which he placed compounds believed to be in the primordial atmosphere, supplied energy (in the form of electric arcs) and produced a scum of organic molecules. Among these molecules included those found in living organism.

Biochemicals of life arose out of a soup of organic chemicals produced by different possible energy sources (lightening, solar photon, geothermal sources).

RNA may have been one of the original self replicating molecules. Natural lipids (phospholipids) in water form natural vesicles which could have been the template for the first primitive cells.

Mitochondrial Eve:

The mitochondria in living cells are thought to have evolved from primitive bacteria which were more successful at converting carbohydrates to energy. Their DNA is different from the nuclear DNA in the cells they inhabit.
In humans, mitochondria are inherited only from the mother.
Using estimates of the rate at which mutations occur in mitochondria scientists can estimate how long ago two populations had a common ancestor. The common mitochondrial ancestor of all humans, or more accurately this small group of ancestral hominids, is often called the Mitochondrial Eve. By sampling as many different human populations as possible, scientists have calculated that the common mitochondrial ancestral human lived about 200,000 years ago (this date is still in much dispute). Furthermore, African populations have the greatest differences among their mitochondrial DNA and it is assumed that this is evidence for an African origin for humankind. This later conclusion has been well supported by fossil evidence.

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