Life began on Earth 3.8 billion years ago, as BBC Nature explains. At that time Earth became home to single-celled prokaryotes. 2.1 billion years ago multicellular life began, and the first arthropods appeared 570 million years ago. Mammals have only been evolving over the last 200 million years.Continue Reading
While life on Earth began 3.8 billion years ago, the events that predated this milestone are also important to the history of life: life couldn't have begun on Earth until Earth was conducive to life.The Earth is a little over 4.5 billion years old. Before life could form, the planet's surface had to cool to become a crust that would then endure erosion, volcanic activity, and other events to create the first small continents. Those continents in turn endured many geological events that caused them to rip apart and reform repeatedly before they settled into the forms they have today.
Once life did form at least 1.3 billion years after the beginning of the planet itself, it took over a billion years for the single-celled organisms to evolve to a multicelluar state. Another 1.5 billion years went by before the arthropods appeared. After another 40 million years, there were fish; 55 million years after fish, plants began to grow on land, but it would be another 225 million years before mammals evolved. Meanwhile our species, Homo sapiens (humans), didn't come about until 200,000 years ago. Relative to the rest of life on Earth, humans are incredibly new, having only been around for 0.0004 percent of Earth's history.Learn more about Prehistory