Life on Earth began 4 billion years ago during the Hadean Aeon. The origin of life, called abiogenesis, is still not fully understood. The earliest known life forms were prokaryotic cells, followed by photosynthetic cyanobacteria that caused the oxygen catastrophe. Eukaryotic cells evolved 2.1 billion years ago, and complex multicellular organisms emerged 600 million years ago. The Cambrian Explosion, 542-530 million years ago, led to the development of basic body plans and complex organisms, eventually leading to intelligence.
The evolution of life on Earth began a long time ago: about 4,000 million years in the past. Life began shortly after the Earth’s crust 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 Aeon due to the unfortunate circumstances that prevailed on the planet at that time. Despite the heavy bombardment of meteorites and the much hotter surface temperatures of today, primitive strands of self-reproducing RNA and DNA somehow emerged, eventually evolving into proto-cells that are the ancestors of all earthly life.
There’s still a lot we don’t know about abiogenesis, the scientific name for the origin of life and the starting point of evolution. It could have originated deep underground or in ocean hotspots with the right chemical reactions. Since the atmosphere at that time was completely devoid of oxygen, the first simple cells used carbon dioxide as a carbon source and oxidized inorganic materials to extract energy. In any case, this early life left no fossils, so we don’t know much about it.
Some of the earliest known life forms were prokaryotic cells (without a nucleus) that may have started leaving basic microfossils about 3.5 billion years ago. These cells used glycolysis, essentially the conversion of sugars into ATP, as the basis of their livelihoods. About 3 billion years ago, perhaps the most significant evolutionary milestone occurred before the emergence of our species. It was the advent of photosynthesis, introduced by cyanobacteria which now cover the earth’s surface in large numbers. Using water as a reducing agent, they produced oxygen as a byproduct, causing atmospheric oxygen levels to rise and continue to rise. This milestone in evolution has been called the oxygen catastrophe because it slaughtered most organisms worldwide, which could not survive in an oxygenated environment. We can tell when this happened because the period layers begin to contain iron oxide or rust.
2.100 million years ago, more advanced cells called eukaryotes evolved, with specialized nuclei and organelles. Humans are large masses of eukaryotic cells. It took another billion years for complex multicellular colonies to be spawned by evolution, followed by the descendants of all animals, the choanoflagellates, just 600 million years ago. These were the first multicellular organisms with specialized components that allowed them to navigate for food and evolve crude sensors. Evolution had created our forerunners.
The evolutionary advance that led to all basic body plans and complex organisms in general was the Cambrian Explosion, the leap in evolution that occurred approximately 542-530 million years ago. This set the stage for everything else. It was only a matter of time before life began colonizing the earth, developing complex nervous systems and eventually intelligence.
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