3.8 Billion B.C.T. - The Archean Eon Begins

Around 3.8 Billion B.C.T., the Archean Eon began.

The Archean (also spelled Archaean) is a geologic eon before the Proterozoic Eon, before 2.5 billion years ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.

If you were able to travel back to visit the Earth during the Archean Eon, you would likely not recognize it as the same planet we inhabit today. The atmosphere was very different from what we breathe today.  At that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also, during this time, the Earth's crust cooled enough that rocks and continental plates began to form.

It was early in the Archean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archean was bacterial. The Archean coast was home to mounded colonies of photosynthetic bacteria called stromatolites. Stromatolites have been found as fossils in early Archean rocks of South Africa and western Australia. Stromatolites increased in abundance throughout the Archean, but began to decline during the Proterozoic. They are not common today, but they can still be found in Shark Bay, Australia.