Early Earth

4,600–2,700 million years ago (HadeanArchean Eons)

When did the Earth form?

  • The Earth is approximately 4.6 billion years old or 4,600 million years old. As reference, the universe is thought to be around 14 million years old.

  • There are many aspects of the Earth's early environment that we know from the geology and chemistry of ancient rocks; rocks that were formed billions of years ago.

  • Evidence shows that the early Earth was bombarded by icy comets for hundreds of millions of years.

  • The atmosphere was mostly carbon dioxide with little or no oxygen, and smaller proportions of water vapor, ammonia and methane.

  • The formation of life from complex chemical beginnings probably occurred during this time.

  • This earliest time in Earth's history is referred to as the Hadean Eon (4,560–4,000 million years ago).

When did oceans and land form?

  • As the Earth cooled down, most of the water vapor condensed and formed the oceans.

  • In addition, continents began to form on Earth, along with the movement of continents (plate tectonics).

  • Oxygen levels begin to rise, and this is an indirect sign that life has evolved on Earth!

  • This time in Earth's history is referred to as the Archean Eon (4,000–2,500 million years ago).

When did life first evolve on Earth?

  • The earliest direct evidence of life is bacteria that appear at 3.5 billion or 3,465 million years ago (Schopf 1993).

  • Although, there may be traces of life from hundreds of millions years earlier (3.7 billion years ago)

  • This earliest life is microscopic, because they are single cells, such as bacteria, or things that look like bacteria, such as archaebacteria...

  • ...but these organisms are so successful that they begin to form large colonies, called stromatolites, along the ocean shorelines.

Above: Rock with fossils of archaebacteria

Above: Living stromatolites, colonies of ancient bacteria, living off the western coast of Australia

When did photosynthesis begin?