How did our planet – and the life on it – get started?

What we know is that planets are a by-product of star formation. Gravity can cause massive collections of gas in space – molecular clouds – to collapse over hundreds of millions of years. As gravity pulls material within the collapsing cloud together, the centre of the cloud becomes more compressed and hotter. This dense, hot core becomes the centre of a new star.

Not all of the material makes it into the stars, however. The Hubble Space Telescope has seen newly forming stars surrounded by flat, dark rings. It is thought these ‘protoplanetary discs’ are the leftovers from star formation that gravity will continue to sculpt into planets.

Diagram of the Miller-Urey experiment

The Miller–Urey experiment.


Adapted from Yassine Mrabet/Wikipedia

It is believed that our solar system formed in a very similar way, and the ingredients for life were probably already built in. As far back as 2003, astronomers found glycine – the smallest amino acid – in molecular clouds where star formation was active, though this discovery is contentious. In 2013, cyanomethanimine was found too. It is a precursor involved in the reaction to form adenine, one of the nucleotide bases from which DNA is constructed.

It seems that the early solar system – including the early Earth – was laced with some of the basic chemicals from which life is built. But what turned an inanimate, prehistoric ‘soup’ of chemicals into the first organisms we would classify as alive?

The most famous experiment into the origin of life on our planet was performed by Stanley Miller and Harold Urey in 1953. They tried to replicate the conditions of the early Earth in the lab, mixing water vapour with the gases that were thought to have been present in the atmosphere at the time (such as methane and ammonia). Next, they subjected the mixture to electrical sparks to represent lightning.

After a day, Miller and Urey found evidence of two amino acids: glycine and alanine. Their results showed that it is possible for simple chemistry to become more complex under specific conditions. But their results were far from conclusive – the question of life’s origins is still one of the biggest mysteries in modern science.

Lead image:

Molecular cloud.

ESA/PACS and SPIRE consortia, A Rivera-Ingraham and PG Martin, University of Toronto, HOBYS Key Program (F Motte)


Further reading

Downloadable resources

About this resource

This resource was first published in ‘Space Biology’ in June 2015.

Cell biology, Ecology and environment, History
Space Biology
Education levels:
16–19, Continuing professional development