While there are many theories as to how life began on Earth, no theory has been accepted as truth, according to NBC News. Biologist Diana Northup from the University of New Mexico says this is because it's hard to prove or disprove many of these theories.
Scientists say that the earliest evidence of life on Earth was found in fossilized mats of cyanobacteria in Australia. This fossilized cyanobacteria, called stromatolites, is 3.4 billion years old. This bacteria, which are still around today, are seen as being biologically complex because they have cell walls that protect their DNA. Despite this discovery, scientists believe that life started before this, possibly as far back as 3.8 billion years ago.
One of the most commonly talked about theories on how life began on Earth involves organisms that came from a distant world. Scientists who believe this theory say that these organisms became attached to comets and asteroids as they traveled through space. Some of these comets and asteroids eventually made their way to Earth, which provided the perfect atmosphere for the organisms to thrive. Biochemist David Deamer at the University of California says that these organisms probably came from multiple origins outside the Earth's solar system, so those origins are hard to trace.