Pre-Cambrian

Pre-Cambrian

Hadean: the Earth we live on is approximately \(\text{4.6}\) billion years old. The conditions that allowed for the emergence of life lasted approximately \(\text{500}\) million years. It was an environment in which the Earth's crust cooled and the oceans and atmosphere began to form. In this environment, a variety of complex chemical reactions occurred, resulting in the production of the earliest molecules capable of making copies of themselves.

Archean: when life in the form of uni-cellular organisms first developed, the Earth's early atmosphere consisted entirely of volcanic gases, and there was no free oxygen. Prokaryotes evolved, with photosynthesising bacteria (known as cyanobacteria) emerging approximately 3 billion years ago. The presence of photosynthesising cyanobacteria resulted in the release of oxygen into the atmosphere.

The early single-celled organisms (bacteria and cyanobacteria) lived together in aquatic colonies. These colonies were formed by trapping sediments and minerals floating in water and by producing a mucus which bound everything together. As the colony of bacteria, minerals and sediments grew, so did the structure that they made, and layer upon layer built up. In order to capture as much sunlight as possible, the shape of the top of this sedimented algal mat was curved. These curved and multi-layered structures, called stromatolites, were preserved and exist as fossils. Some stromatolites survive to this day, in scattered locations around the globe (see figure below).

Stromatolites have been found at Wondergat, Northwest Province, South Africa.

Stromatolites in Shark Bay, Australia.

Proterozoic: over the next 800 million years, the earliest forms of sexual reproduction developed, thus greatly increasing the diversity of organisms. The first multicellular organisms consisting of cells specialised to perform specific functions began to evolve. The soft-bodied organisms, known as Swartpuntia, first appeared approximately \(\text{600}\) million years ago and went extinct by the start of the Cambrian, \(\text{543}\) million years ago. Swartpuntia fossils were discovered in Namibia and are some of the oldest fossils known to exist from before the Cambrian period.

Earliest soft-bodied animals found in Namibia. A simplified reconstruction of Swartpuntia.