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All life on Earth is said to have come from one common ancestor, a single-celled organism known as life's Last Universal Common Ancestor (Luca).

But what Luca looked like, how it lived, and how humans evolved from it is still a four-billion-year-old mystery.

Researchers believe they have now discovered clues that may help tackle this mystery - and it's all down to the organism's 'leaky' membrane.

Using mathematical modelling, researchers claim the Last Universal Common Ancestor (LUCA) passed energy through a 'leaky' membrane. This caused it to grow and evolve. As it moved to new surroundings, membranes evolved independently causing bactera and archaea organisms to evolve separately (shown in this illustration)

The study, by University College London, claims this membrane may explain why all cells use the same complex mechanism to harvest energy.

It may also explain why two types of fundamental single-celled organisms - bacteria and archaea - have different cell membranes.

The team of researchers, led by Dr Nick Lane from the university's Biosciences department, made the discovery using mathematical modelling.

They claim that the leaky membrane allowed Luca to harness the energy from its surroundings, which were most likely vents on the ocean floor.

The cells were able to allow protons to enter and exit at the same time, causing them to grow at an accelerated rate.

Researchers believe Luca lived in an area where ancient seawater, packed with positively charged particles called protons, mixed with warm alkaline vent fluid, which contained fewer protons.

The difference in the concentration of protons across these two environments enabled protons to flow into the cell.

This drives the production of a molecule called adenosine triphosphate (ATP), which makes modern-day cells grow, for example.

However, scientists believe this was only possible if the membrane was 'leaky' as it allows protons to leave and enter the cell spontaneously to power such growth.

Dr Lane said: 'I find this work just beautiful - it constrains a sequence of steps going from the strange cell that seems to have been the ancestor of all life today, right through to the deep division between modern cells.

This tree diagram reveals how LUCA eventually split into bacteria, archaea and eucaryota, or eukaryote. Eucaryota includes humans and other animals, plants, and fungi

'From a single basic idea, the model can explain the fundamental differences between bacteria and archaea. Is it right? I'd like to think so, but more importantly, it makes some clear predictions that we plan to test in the future.'

Dr Lane added: 'In these deep sea vents, there is a continuous flow of alkaline fluids, which mix with the ocean waters.

'When they mix, the fluids neutralise each other, and that stops any build-up of charge which would otherwise prevent protons flowing into the cell.

'If the first cells had leaky membranes, then protons could enter and then be neutralised, or leave again, almost as if there was no barrier at all.

The researchers claim that this evolution process explains why membrane bioenergetics are universal in archaea (pictured) and bacteria - yet ion pumps and membranes are different, which has been the big unknown

'What we've shown is that the rate at which protons enter and leave is high enough to power the growth of cells via proteins embedded in the membrane.

'So Luca could have been powered by natural proton gradients in vents, but only if it had a really leaky membrane, completely unlike today's cells.'

Lead author Victor Sojo, a researcher in UCL's Biosciences department, added: 'Exploiting gradients is universal across all life, but understanding how Luca used one to drive growth gave us a bit of a chicken-and-egg problem: Luca wouldn't make a gradient if it didn't know how to exploit it, but how could it learn how to exploit a gradient if it didn't make one in the first place?

'We propose that natural proton gradients provide a solution because Luca didn't have to make the gradient; it was already there for free. We show that modern 'non-leaky' membranes had to evolve later, and they did so independently in archaea and bacteria.

'This explains why membrane bioenergetics are universal in archaea and bacteria yet ion pumps and membranes are different, which has been the big unknown.'

The findings are published in the journal PLOS Biology.