Geological Succession

The time at which species appear in the geological record perfectly matches up with what we would expect to see if evolution were true. Invertebrates appear before vertebrates, fish before frogs, and reptiles before mammals. We never see human fossils mixed with Trilobite fossils or squirrel fossils mixed with dinosaur fossils.


Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution. Early geologists, in the 1700s and 1800s, noticed how fossils seemed to occur in sequences: certain assemblages of fossils were always found below other assemblages. The first work was done in England and France.

• Around 1800, William Smith in England, who was a canal surveyor, noticed that he could map out great tracts of rocks on the basis of their contained fossils. The sequences he saw in one part of the country could be correlated (matched) precisely with the sequences in another. He, and others at the time, had discovered the first principles of stratigraphy - that older rocks lie below younger rocks and that fossils occur in a particular, predictable order.
  
  
• Then, geologists began to build up the stratigraphic column, the familiar listing of divisions of geological time Jurassic, Cretaceous, Tertiary, and so on. Each time unit was characterized by particular fossils. The scheme worked all round the world, without fail.
  
  
• From the 1830s onwards, geologists noted how fossils became more complex through time. The oldest rocks contained no fossils, then came simple sea creatures, then more complex ones like fishes, then came life on land, then reptiles, then mammals, and finally humans. Clearly, there was some kind of 'progress' going on.
  
  
• All became clear, of course, in 1859 when Charles Darwin published his "On the origin of species". The 'progress' shown by the fossils was a documentation of the grand pattern of evolution through long spans of time.