Nobody knows WHAT conditions are needed for life to begin. Some scientists have suggested that living cells may have been brought to Earth by a comet. When the Giotto probe investigated Halley’s Comet in 1986, it found molecules that were similar to living cells. If a comet like this collided with Earth at the right time, then life may have taken hold. Another theory is that powerful lightning bolts flashing through Earth’s early atmosphere may have caused chemical reactions, which created living cells.
One of the first ideas, popularised by biochemist Sidney Fox in the wake of the Miller-Urey experiment, was that amino acids assembled into simple proteins. In modern organisms, proteins perform a huge range of functions, including acting as enzymes that speed up essential chemical reactions. However, this proteins-first hypothesis has largely fallen out of favour.
A much more popular notion is that life began with RNA, a close cousin of DNA, in an “RNA World”. RNA can carry genes and copy itself just like DNA, but it can also fold up and act as an enzyme, just like a protein. The idea was that organisms based solely on RNA arose first, and only later developed DNA and protein.
The RNA World has amassed a lot of supporting evidence, but it is not clear that RNA alone was enough. In recent years, some researchers have suggested that RNA only really reaches its potential when it is paired with proteins – and that both must have existed for life to get started.
A third school of thought is that the first organisms were simple blobs or bubbles. These “protocells” would have resembled modern cells in one key attribute: they acted as containers for all the other components of life. More advanced protocells developed by the Nobel Prize winning biologist Jack Szostak also contain self-replicating RNA.
The final hypothesis is that life began with a series of chemical reactions that extracted energy from the environment and used that energy to build the molecules of life. This “metabolism-first” idea was championed in the late 1980s by Günter Wachtershauser, a German chemist turned patent lawyer. Wachtershauser envisioned a series of chemical reactions taking place on crystals of iron pyrite (“fool’s gold”), a scheme he dubbed the “Iron-Sulphur World”. However, nowadays this idea has been supplanted by Michael Russell’s suggestion that the first life was powered by currents of electrically-charged protons within alkaline vents on the sea bed.
While we cannot know for sure which of these scenarios played out on our planet, successfully creating life from chemicals in the lab would at least tell us which of the proposed mechanisms actually works.