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The question of how life began is one of the most profound in science, and although many theories exist, scientists still cannot agree on an answer. It continues to be a topic for debate, as understanding life’s origin would help us grasp our place in the universe, as well as guiding our search for extraterrestrial life.

The first idea to capture scientists’ attention was the “primordial soup”: the notion that when Earth was young, the oceans were filled with simple chemicals important for life. These would eventually self-assemble into simple living cells. This idea was proposed in the 1920s by two researchers working independently: Alexander Oparin in the USSR and British geneticist J. B. S. Haldane.

The primordial soup hypothesis received dramatic support in 1953 when a young American graduate student named Stanley Miller, supervised by Nobel Prize winner Harold Urey, conducted a famous experiment. Miller mixed four simple chemicals in glass tubes, which were heated and shocked with electrical sparks to mimic lightning. The experiment made several amino acids, the building blocks of proteins. The Miller-Urey experiment showed that the chemicals of life could form naturally.

However, making life from scratch has proved to be a lot more complicated than Miller’s experiment suggested. Several competing hypotheses have been proposed over the decades and today the field is highly polarised. Scientists disagree about which chemical components of life came first, which of life’s processes came first, and where on Earth life first arose.

Even the timing of life’s origin is in question. All we know for sure is that it happened after Earth formed 4.5 billion years ago, and before 3.4 billion years ago – the time of the oldest confirmed fossils. Many palaeontologists have tried to narrow the window by identifying older traces of life but these findings are disputed.

As to the location, many still favour the sea but not necessarily the open sea: a vocal minority of researchers think life began in alkaline vents on the sea floor. Others think life began in ponds on land, perhaps geothermal pools like those in Yellowstone. Many other locations have been proposed, such as ice. A minority of scientists argue that life must have begun elsewhere in the universe and been carried to Earth, an idea known as “panspermia”. However, most researchers think this falls foul of Occam’s razor, especially as no extraterrestrial life has been found.

The process that created life

The thorniest question is the mechanism by which life began. Which of the many processes that take place in living organisms emerged first?

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 Wächtershäuser, a German chemist turned patent lawyer. Wächtershäuser 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. Michael Marshall