The Precambrian Time Span is the earliest time period on the Geologic Time Scale. It stretches from the formation of the earth to around 600 million years ago and encompasses many Eons and Eras leading up to the Cambrian Period in the current Eon.
Beginning of Earth:
The earth was formed about 4.6 billion years ago in a violent explosion of energy and dust according to the rock record from Earth and other planets. For about a billion years, the earth was a barren place of volcanic action and a less than suitable atmosphere for most types of life. It wasn't until about 3.5 billion years ago that it is thought that the first signs of life formed.
Beginning of Life on Earth:
The exact way life began on Earth during the Precambrian Time is still debated in the scientific community. Some theories that have been posed over the years include the Panspermia Theory, Hydrothermal Vent Theory, and Primordial Soup. It is known, however, there was not much diversity in organism type or complexity during this extremely long period of Earth's existence.
Most of the life that existed during the Precambrian Time span were prokaryotic single celled organisms. There is actually a pretty rich history of bacteria and related unicellular organisms within the fossil record. In fact, it is now thought that the first types of unicellular organisms were extremophiles in the Archaean domain. The oldest trace of these that has been found so far is around 3.5 billion years old.
These earliest forms of life resembled cyanobacteria. They were photosynthetic blue-green algae that thrived in the extremely hot, carbon dioxide rich atmosphere. These trace fossils were found on the Western Australia coast. Other, similar fossils have been found all over the world. Their ages span about two billion years.
With so many photosynthetic organisms populating the earth, it was only a matter of time before the atmosphere started to accumulate higher levels of oxygen since oxygen is a waste product of photosynthesis. Once the atmosphere had more oxygen, many new species evolved that could use oxygen to create energy.
More Complexity Appears:
The first traces of eukaryotic cells showed up about 2.1 billion years ago according to the fossil record. These seem to be single celled eukaryotic organisms that lacked the complexity we see in most of today's eukaryotes. It took about another billion years before the more complex eukaryotes evolved, probably through endosymbiosis of prokaryotic organisms.
The more complex eukaryotic organisms began living in colonies and creating stromatolites. From these colonial structures most likely came multicellular eukaryotic organisms. The first sexually reproducing organism evolved around 1.2 billion years ago.
Evolution speeds up:
Towards the end of the Precambrian Time period, much more diversity evolved. The earth was undergoing somewhat rapid climate changes, going from completely frozen over to mild to tropical and back to freezing. The species that were able to adapt to these wild fluctuations in climate survived and flourished. The first protozoa appeared followed closely by worms. Soon after, arthropods, mollusks, and fungi showed up in the fossil record. The end of the Precambrian Time saw much more complex organisms like jellyfish, sponges, and organisms with shells come into existence.
The end of the Precambrian Time period came at the beginning of the Cambrian Period of the Phanerozoic Eon and Paleozoic Era. This time of great biological diversity and rapid increase in organism complexity is known as the Cambrian Explosion. The end of the Precambrian Time marked the start of the more quickly progressing evolution of species over Geologic Time.