Underwater camera looking at a hydrothermal vent in the Caribbean

Origins of life

It’s the biggest question of all: how did life get going?

There has been life on Earth for about 3.5 billion years. Once it began, evolutionary processes could shape its future – but they need something to work on. Although there are several possible theories, the origin of life remains uncertain.

One of the most famous contenders is the ‘primordial soup’ model. This is well illustrated by Stanley Miller’s famous experiment in the 1950s, where he mixed some simple chemicals, applied a strong charge, and stood well back. Lo and behold, he managed to create complex ‘biological’ compounds such as amino acids.

Unfortunately, there are big problems with this model. For a start, the conditions Miller mimicked in the lab were unlike those now thought to exist at the dawn of life on Earth. And ‘life’ is more than just making a few complex molecules.

There are at least two key aspects to life:

  • the ability to carry out controlled chemical reactions (metabolism)
  • the ability to store and pass on information (eg in DNA).

With these powers, self-replication is possible, so a chemical entity can continue to make copies of itself.

But there is a chicken-and-egg issue here. Metabolism usually depends on proteins, yet the information to make proteins is stored in DNA. So which came first, proteins or DNA?

A possible solution could be DNA’s cousin, RNA, which can both store information (eg some virus genomes are made of RNA) and catalyse chemical reactions. Many believe that there was once an ‘RNA world’, populated by self-replicating RNA molecules.

But where did these molecules come from? Some people argue for a variant of the primordial soup model, in which pools of chemical broth became highly concentrated, allowing unusual chemical reactions to take place. British geologist Graham Cairns-Smith thinks that clays may have been important, because of their self-organising properties.

In the 1990s, it was suggested that life might have emerged at hydrothermal vents – extraordinary geological and biological structures deep on the ocean floor where energy from inside the planet is released into the seas. These are home to highly unusual organisms, carrying out odd chemical reactions.

Finally, some people believe that the idea of life arising spontaneously on Earth is so fantastic that it couldn’t have happened. Perhaps life was ‘seeded’ on Earth, brought in on meteorites. But then, if that’s the case, where did those life forms come from?

Lead image:

Underwater explorer shines its lights on a dense aggregation of shrimp at the Von Damm hydrothermal vent field in the Caribbean.

Image courtesy of NOAA Okeanos Explorer Program, MCR Expedition 2011/Flickr CC BY NC ND


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About this resource

This resource was first published in ‘Evolution’ in January 2007 and reviewed and updated in December 2014.

Cell biology, Genetics and genomics, Microbiology, Ecology and environment, History
Education levels:
16–19, Continuing professional development