Scientists have made a discovery that could overturn a decades-old theory on how life got started on Earth.

For living creatures to emerge out of lifeless chemicals, Earth needed a stock of organic molecules and the right conditions to put these together.

One of the most popular theories to explain where that stockpile came from is that icy comets delivered vast amounts of concentrated chemicals directly to Earth’s surface.

However, researchers from the University of Cambridge now say this is highly unlikely.

The problem is that the vast majority of comets arrive too fast and hit the atmosphere at too steep an angle to survive the impact.

Small comets are blasted into pieces by the sudden shock, and the fragile organic molecules inside would almost certainly vaporise on impact.

That means the cometary delivery of life’s building blocks is at best a ‘one-off fluke’ rather than the steady rain of raw materials previously envisioned.

Lead author Dr Richard Anslow from Cambridge’s Institute of Astronomy says: ‘It’s kind of a romantic idea, but our results suggest it’s highly unlikely.’

To understand how life emerged, scientists need to understand where the so-called ‘feedstock’ molecules came from and how they came together.

However, figuring out how the basic building blocks could have become concentrated in high enough levels has proved challenging.

Dr Anslow told the Daily Mail: ‘Comets emerged as a promising delivery mechanism based on the observation of Solar System comets showing them to be rich in carbon- and nitrogen-bearing chemicals that we think are important for the origins of life.’

He adds that these giant space snowballs had become something of a ‘get out of jail free card’ for scientists keen to explain how Earth got the molecules needed to seed life.

In their paper, published in the journal Scientific Reports, the research focuses on hydrogen cyanide, a feedstock chemical made of hydrogen, carbon and nitrogen.

This chemical has been considered particularly promising since it is exceptionally abundant in icy comets from the outer Solar System and potentially tough enough to survive impact.

However, new computer modelling suggests that comets couldn’t have delivered nearly enough hydrogen cyanide to get life’s chemical chain reaction started.

In his previous research, Dr Anslow had theorised that comets might slow down by ‘bouncing’ through systems where planets orbit close together, delivering organic molecules as they went.

To see whether this could have happened in the Solar System, the researchers used the latest comet impact simulations and computer models of the evolution and origin of comets.

This allowed them to work out how comets would actually arrive and what would happen when they hit the atmosphere.

‘We found that the typically high speeds that comets impact the Earth at lead to the destruction of the majority of these molecules, meaning that it is not straightforward for these to reach the surface intact,’ says Dr Anslow.

Any comet smaller than 330ft (100m) wide would ‘catastrophically fragment in explosive airbursts at high altitude’, while even kilometre-sized comets would be broken apart.

After being dissolved in a planet’s worth of water, the concentrations of hydrogen cyanide delivered in this way would be orders of magnitude lower than what is needed to start life’s chemical reactions.

That suggests that comets are an unlikely source for the origin of life, meaning that the feedstock chemicals must have come from somewhere else.

Dr Anslow says: ‘There are many promising alternative hypotheses including their delivery by carbon-rich asteroids, or alternatively these molecules could have been synthesised on the young Earth, not requiring any impacts at all.’

Previous studies suggest that the heat generated by rocky asteroids’ passage through the atmosphere could provide the energy needed to forge these basic chemicals.

On the other hand, some researchers propose that Earth’s basic chemical stockpile could have been created in hydrothermal vents deep under the oceans or near the surface.

However, although scientists say they have ruled out comets as the source of life on Earth, the situation is very different elsewhere in the Solar System.

In fact, the researchers identify Mars as a promising place where comets could have given life a helping hand.

Since Mars is smaller than Earth and currently has a much thinner atmosphere, comets would be much more likely to make it to the planet’s surface during the period of the Solar System’s development when comets were far more frequent visitors.

‘Because comets typically strike Mars at lower speeds than Earth, we found that — at face value — more of these molecules survive impact.

‘However, a big caveat is that there are many uncertainties about early Martian environments, meaning that this isn’t to say successful impacts occurred onto Mars!’

In the future, the researchers say that these findings could help telescopes like the James Webb Space Telescope or the proposed Habitable Worlds Observatory look for life.

By looking for planets where ‘bouncing’ comets could have safely reached the planet’s surface, scientists can narrow down the number of places they need to search.