Early Earth

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).

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).

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.