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Deep Time

One of the most challenging concepts in thinking about evolution is the idea of "deep time," the profoundly different timescale involved in geological and evolutionary processes than those we deal with in our daily lives. It is rare for those of us in North America to see buildings that are more than 100 years old. If the geological history of the world were represented by the height of the Empire State Building, the time since mastodons walked across North America 14,000 years ago would be represented by the thickness of a single dime.

James Hutton: Scottish geologistJames Hutton: Scottish geologist James Hutton (1726-1797) was one of the first scientists to realize the scale of time on Earth.

Prior to Hutton, many people assumed that not only was the Earth very young, but that the rocks of the Earth all formed at approximately the same time. Hutton was able to demonstrate by the way that rocks cut across and intruded each other that there had been a sequence of formation. Moreover, he was able to recognize unconformities—areas where layers of rock were eroded away, sometimes tilted, and then overlain by subsequent deposits. Hutton recognized in these unconformities an ancient Earth, with “no vestige of a beginning, no prospect of an end.” This is “deep time.”

Hutton’s conception of time was greatly refined in the mid-twentieth century when it became possible, for the first time, to date rocks accurately using radioisotope decay. Shortly before this, astronomers began to use red-shifted light to understand the dimensions of the universe.

Eagle NebulaEagle Nebula

Einstein further complicated the scientific understanding of time by demonstrating that time itself is malleable, that there is no steady and universal clock ticking in the cosmos. Near very massive objects, such as black holes, time almost stops relative to the rest of the universe.

Most cosmologists now believe the universe began expanding about 13.7 Ga (billions of years ago). Surprisingly, there may not have been a 14 Ga. As strange and counter-intuitive as it sounds, time and matter began at the same moment.

The major events of deep time, from an anthropocentric point of view, are listed below.

Timeline (Ga = billions of years, Ma = millions, Ka = thousands):

  • 13.7 Ga: universe/time begin
  • 4.56 Ga: solar system forms
  • 4.54 Ga: Earth forms
  • 4.408 Ga: oldest mineral yet discovered on Earth
  • 4.031 Ga: oldest rock yet identified on Earth
  • 3.85 Ga: isotopically “light” carbon, the earliest chemical signature of life
  • ~3.5 Ga: stromatolites fossils
  • 630-542 Ma: soft-bodied Ediacarans
  • 542 Ma: Cambrian boundary
  • 251 Ma: largest mass extinction on Earth
  • ~200 Ma: Pangaea, all the continents on Earth assembled
  • 65 Ma: second largest mass extinction on Earth
  • 3.9 Ma: Australopithecines, such as “Lucy”
  • ~350 Ka: Neanderthals
  • ~130 Ka: anatomically-modern humans