The origin of life on Earth is still somewhat of a mystery. Many different theories have been proposed, and there is no known consensus on which one is correct. Although the Primordial Soup Theory was proven to be most likely incorrect, other theories are still considered, such as hydrothermal vents and the Panspermia Theory.
Panspermia: Seeds Everywhere
The word "Panspermia" comes from the Greek language and means "seeds everywhere". The seeds, in this case, would not only be the building blocks of life, such as amino acids and monosaccharides, but also small extremophile organisms. The theory states that these "seeds" were dispersed "everywhere" from outer space and most likely came from meteor impacts. It has been proven through meteor remnants and craters on Earth that early Earth endured innumerable meteor strikes due to a lack of an atmosphere that could burn the up upon entry.
Greek Philosopher Anaxagoras
This theory was actually first mentioned by Greek Philosopher Anaxagoras around 500 BC. The next mention of the idea that life came from outer space wasn't until the late 1700s when Benoit de Maillet described the "seeds" being rained down to the oceans from the heavens.
It wasn't until later in the 1800s when the theory really started to pick up steam. Several scientists, including Lord Kelvin, implied that life came to Earth on "stones" from another world which began life on Earth. In 1973, Leslie Orgel and Nobel prize winner Francis Crick published the idea of "directed panspermia", meaning an advanced life form sent life to Earth to fulfill a purpose.
The Theory is Still Supported Today
The Panspermia Theory is still supported today by several influential scientists, such as Stephen Hawking. This theory of early life is one of the reasons Hawking urges more space exploration. It is also a point of interest for many organizations trying to contact intelligent life on other planets.
While it may be hard to imagine these "hitchhikers" of life riding along at top speed through outer space, it is actually something that happens quite often. Most proponents of the Panspermia hypothesis actually believe the precursors to life were what was actually brought to the surface of the earth on the high-speed meteors that were constantly striking the infant planet. These precursors, or building blocks, of life, are organic molecules that could be used to make the first very primitive cells. Certain types of carbohydrates and lipids would have been necessary to form life. Amino acids and parts of nucleic acids would also be necessary for life to form.
Meteors that fall to the earth today are always analyzed for these sorts of organic molecules as a clue to how the Panspermia hypothesis may have worked. Amino acids are common on these meteors that make it through today's atmosphere. Since amino acids are the building blocks of proteins, if they originally came to Earth on meteors, they could then congregate in the oceans to make simple proteins and enzymes that would be instrumental in putting together the first, very primitive, prokaryotic cells.