Natural Selection Image Map

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.
Data taken from The Extraordinary Story of Life on Earth, 1996. Prometheus Books.

Date Event
Jan. 1
(4 B yrs)
Earth forms from asteroids, star debris
Jan. 5
(3.8 B)
Earth cools. Continuous rain
Feb. 14
(3.5 B)
Bacteria appear
May 16
(2.5 B)
Continents move. Huge varieties of bacteria.
Aug 24
(1.4 B)
Photosynthetic bacteria make oxygen (1% of air). Ozone layer protects earth from UV
Sept. 10
(1.2 B)
Oceans form
Sept. 28
(1 B)
Single cell animals appear in oceans
Oct. 16
(800 M)
Colonial animals appear (coral)
Oct. 27
(700 M)
Sponges appear
Nov. 5
(600 M)
7% oxygen in air. Jellyfish.
Sexual reproduction. Origin of nervous systems.
Nov. 8
(570 M)
Continents broke off, began drifting around the globe
Nov. 22
(430 M)
Plants arrive on land
Date Event
Nov. 27
(380 M)
Insects appear
(370 M) Vertebrates and fish appear
Nov. 29
(355 M)
Amphibians appear
Dec. 3
(310 M)
Great forests, swamps
Birds appear
Dec. 5
(280 M)
Reptiles appear
Dec. 9
(280 M)
Dinosaurs appear
Dec. 23
(85 M)
Flowers (sex in plants)
Dec. 25
(65 M)
Dinosaurs become extinct
Dec. 26
(63 M)
Small mammals proliferate
Dec. 28
(35 M)
Huge mammals appear
Dec. 31
7:30 PM
(2 M)
Primitive humans appear
Dec. 31
11:50 PM
(0.8 M)
Modern humans appear

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.

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