Life Develops Over Time
Radioactive dating indicates that our earth formed
about 4.5 billion years ago. How long is 4.5 billion years? One simple way
to make the numbers easier to understand is to make 4 billion years
equal to one year on our calendar. The table below*
shows the approximate dates of the earliest known fossils of various
kinds of plants and animals.
Fossils have been found for a few small mammal species that lived at the same time as dinosaurs. Selection forces allowed certain animals to survive that had genes for new variations in anatomy, functions, and brain power. Some of these changes helped certain types of mammals to occupy special niches that they were accidentally adapted for. It was as if Nature were selecting certain species to survive. Just as dinosaurs were selected against by Nature when their niches went away, new unoccupied niches selected for new species that had the right characteristics to thrive in the niches. One apparently useful adaptation was brain power.
Humans, having the best brains, appeared last. Fossils of human skulls indicate that there were pre-human species over a million years ago that had skulls much more like our skulls than those of ancient or modern apes. Modern human skulls appear in the fossil record only in relatively recent times and historical indications of abstract thinking (such as art, domestication of animals, and agriculture) apparently began about 10,000 years ago.