Oxygen - the key factor
The Proterozoic, the period in the earth’s history between 2500 and 570 million years ago, began where the Archaic period ended. The development of a continental crust eventually led to the formation of mountains. First, sand and clay sediments were formed. Later, iron sediments were formed on a vast scale due to the rise of living organisms. More sea plants led to extra oxygen being produced, increasing the percentage of oxygen in the atmosphere.
Erosion changes the surface
The atmosphere’s changes had their consequences on weathering processes, such as erosion and sedimentation. Weathering is the breaking down of rock due to atmospheric phenomena and vegetation. Erosion is the process by which loose rocks are transported by water, ice or wind and deposited somewhere new. The composition of the atmosphere influenced the development of the planet. From 3.8 billion years ago, only unicellular organisms were alive. 2.3 billion years ago, organisms with a cell nucleus (karyon) that could not survive without oxygen joined them.
Variation, selection and transmission
The next step was the rise of multicellular organisms and later of organisms that could breed. The multicellular organisms paved the way for evolution. Evolution is made up of three elements: (1) variation of organisms, (2) selection of the fittest, and (3) transmission of genetic material to the next generation. Nowadays, it is generally accepted that species have developed gradually through Darwinian evolution.