What are Some Milestones in Evolution?
The evolution of life on Earth begins an extremely long time ago: approximately 4,000 million years in the past. Life began shortly after the crust of the Earth cooled and well-defined continents and oceans formed, when the planet was only about 600 million years old. This ancient period is called the Hadean eon for the unpleasant circumstances which prevailed on the planet at that time. Despite heavy meteor bombardment and surface temperatures much higher than today, primitive self-reproducing RNA and DNA strands somehow emerged, and eventually evolved into proto-cells that are the ancestors of all earthly life. There are a still a lot of things we don’t know about abiogenesis, the scientific name for the origin of life and the starting point of evolution. It may have originated deep underground, or at hot spots in the ocean with the right chemical reactions. Because the atmosphere at that time completely lacked oxygen, the first simple cells used carbon dioxide as a carbon source and
