The beginning of life on earth. That is a subject that nobody thinks about. Other than a few theories, there is not really a lot of hard evidence. Some scientists think that life emerged from chemosynthesis ,some RNA world hypothesis, and some panspermia .
Chemosynthesis is the use of energy released by inorganic chemical reactions to produce food. Most of the time, organisms that use chemosynthesis can survive in absolute darkness. Chemosynthesis is often found on the bottom of the ocean floor. On the bottom of the ocean floor lies hydrothermal vents, spewing chemical rich bubbles. These bubbles are boiling hot, saturated with toxic chemicals and heavy metals, and more acidic than vinegar. Around these vents, specialized bacteria absorb hydrogen sulfide streaming from the vents, and oxidize it to sulfur. Then, using the energy released from the oxidation, they combine carbon, hydrogen, and oxygen into sugar molecules. Bacteria that use chemosynthesis to survive are found anywhere with large amounts of petroleum deposits, methane, ammonia, and hydrogen sulfide.
Another famous theory is the RNA world hypothesis. It is the theory that RNA came before DNA. This is widely spread because RNA is capable of self replication and could have carried genetic information throughout the generations. Another reason why it is so widely spread is that RNA could by a process called reverse transcription, become DNA. With all of these qualifications, RNA is said to be the first life making molecule. In the past, RNA would have probably have done all of the functions that the modern cell would do, storing genetic information, copying itself, and performing basic metabolic functions.
The last theory is called panspermia. That is the theory that life came to earth from meteors impacting Earth. These meteors containing microorganisms spread from the rocks and gave birth to life. These organisms might have gotten onto the space rocks after collisions of planets and gotten stuck inside of it. Even if there is no sustainable environment for the bacteria to thrive in space, they would still remain dormant for a long time. Even microorganisms called tardigrades could survive without water for 30 years. That is a long time for the meteor to land on the planet. Once, tardigrades were exposed to the space environment for a long period of ten days, and over two thirds of them successfully revived after they were brought back to Earth. This is only for microorganisms though. Imagine if the microorganism was even simpler, Bacteria, with a lot less functions that complex organisms like tardigrades would probably survive for a lot longer than tardigrades.
