The vastness of geologic time proves to be mostly incomprehensible to us. One useful tool to improve our understanding involves shrinking all of Earth history into a single half-day, with our planet's birth being at midnight, and present time being noon. When reduced thusly, the major events in Earth history play out like this...
4,500 million years ago.
The Earth has formed, along with the other bodies in the solar system, from a cloud of dust and gas swirling around a protostar... our sun. The Precambrian era has begun.
4,125 million years ago.
Still in its infancy, planet Earth has cooled, has almost no free oxygen, and is a very different world from the one we know today. No life forms are yet present.
3,750 million years ago.
The oldest still-existing rocks are just forming (3,690 million years ago). The very first life is just about to appear in the form of simple, single-celled organisms (Eubacteria, Prokaryotae).
3,375 million years ago.
Simple life continues to radiate. The cyanobacteria (blue-green algae) are a dominant lifeform.
3000 million years ago.
The first photosynthetic bacteria appear. All life is restricted to the sea, which provides a fairly constant environment and protection from the sun's ultraviolet rays. With photosynthesis, the levels of atmospheric oxygen begin to rise.
2,625 million years ago.
The end of the Achean eon and the start of the Proterozoic eon of the Precambrian is in sight. The Archaebacteria (unusual bacteria) make their appearance.
2,250 million years ago.
Half of Earth history has elapsed, and the first multicellular organisms are just appearing. The first Eukaryotes arise, beginning with the protists. The first macrofossils (naked eye) will make their appearance in another 50 million years.
1,875 million years ago.
Earth is struck by two massive solar system objects, leaving 140 km craters in South Africa and Sudbury, Ontario, Canada. The first photosynthetic plants (green algae) appear.
1,500 million years ago.
World-wide radiation of photosynthetic aquatic life has significantly altered the composition of the atmosphere. Free oxygen forms ozone, blocking ultraviolet light and paving the way for life on land (although far from immediately).
1,125 million years ago.
Fungi make their appearance (Zygomycota). A mid-continent rift forms in what will become North America.
750 million years ago.
Things speed up as we near the start of the Cambrian period. Brown and red alagae have formed, and the first animals (Annelida) are immediately around the corner.
374 million years ago.
The Devonian period, the Age of Fishes, is here. In the last hour, plants and insects have begun the colonization of the land, and the first amphibians have pulled themselves out of the water. The great Carboniferous forests will shortly arise.
The first reptiles appeared early in the hour, the dinosaurs lived for about 26 minutes later in the hour, and the first hominds appeared about 39 seconds ago. Modern humans have been on Earth for the past 6 seconds or so.
Return to Earth History Resources