The Paleoarchean (also spelled Palaeoarchaean) is a geologic era within the Archaean. It spans the period of time 3600 Ma to 3200 Ma (million years ago)—the period being defined chronometrically and not referenced to a specific level in a rock section on Earth. The oldest ascertained life form (Well-preserved bacteria older than 3460 million years found in Western Australia) is from this era.
The oldest ancient fossil microbe-like objects are dated to be 3.5 Ga (billion years old), just a few hundred million years younger than Earth itself.
Research carried out by Dr Donald Lowe from Stanford University in California and Dr Gary Byerly from Louisiana State University suggests that the Earth continued to be bombarded by large impacts from asteroids - around 12 and 62 miles (20 and 100km) across - until 3.3 billion years ago. The researchers said at least two such impacts between 3.29 and 3.23 billion years ago were large enough to super-heat the atmosphere, boiling the surface layer of the ocean evaporating up to 330ft (100m) of seawater from the surface. For a full year the atmospheric temperature could have remained at 100°C (210°F) with the first few weeks reaching more than 500°C (930°F).