Before introducing the abiogenesis hypothesis, I would like to talk about the spontaneous generation theory that states that life arises suddenly and spontaneously from lifeless material. People before the 19th century believed fervently in the spontaneous generation theory. For instance, people observed that worms would be produced in a sealed bag of garbage, not realizing that worm eggs had gotten into the garbage before the bag was sealed. It was during the 19th century that Louis Pasteur designed experiments that proved that living organisms cannot arise from non-living material. Thus, the spontaneous generation idea was refuted.
The abiogenesis theory is also called the chemical evolution theory. Like spontaneous generation, it says that life arises naturally from lifeless materials. However, the timescales over which life forms is much longer than that of the spontaneous generation theory. In other words, in abiogenesis, life arises in a series of chemical reactions that do not occur suddenly. The first life must been extremely simple forms such as small organic molecules, which combined to form larger biomolecules, and then combined to create primitive living organisms.
The panspermia theory is very different from the abiogenesis theory; it maintains that microscopic living organisms were initially carried to Earth by an asteroid or comet. In other words, primitive life originated naturally in an extraterrestrial environment and then landed on Earth. However, this hypothesis is not viewed favorably by most scientists, who argue that outer space itself is too harsh an environment to allow unprotected cells to survive over a long period of time. Thus, a theory related to panspermia called "weak panspermia" is more popular. The idea states that it was not cells, but only ingredients of life, such as organic molecules, that were delivered from space. Evidence for weak panspermia includes the Murchison meteorite, which contains over 14,000 different molecules that would be capable of helping to spawn life. The evidence not only shows the presence of organic components in outer space, but also points to the ability of these materials to reach Earth.
The theories of the origin of life on Earth remain debatable, but they are worth pondering.