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IT BEGAN when something fell apart. Somewhere on Earth, over 3.5 billion years ago, a bubble of fat may have spontaneously broken into smaller ones, giving rise to one of life鈥檚 most fundamental properties 鈥� the ability to make copies of itself.

That鈥檚 according to of the Howard Hughes Medical Institute in Boston, who has demonstrated this simple division in the lab. Versions of it are found in many bacteria today, lending some credence to the theory. The simplified cell division also doesn鈥檛 require any genes or the complex machinery modern cells use to divide, which suggests it could have been under way before living organisms appeared.

We can鈥檛 be sure that鈥檚 what happened, says of Newcastle University in the UK. 鈥淏ut it鈥檚 a very plausible explanation. It actually works.鈥�

Barring viruses, all modern organisms are built of cells that divide in a process that underlies their ability to grow and reproduce.

To understand how early life worked, Szostak has spent years trying to create a protocell similar to what may have preceded the first cells on Earth. He focuses on fatty acids. When dissolved in water, these organic molecules can group together into bubbles, or vesicles, similar to what the first protocells may have looked like. A few years ago, his team found that if they kept adding fatty acids, the vesicles grew into long, thin tubes. The tubes were fragile and easily broke up into daughter vesicles 鈥� a crude form of cell division (see diagram).

The finding was an implausible model for early cell division, though: the vesicles have no way to make extra fatty acids and natural supplies would have been scarce.

Szostak has now found a way to trigger the division without feeding them. By evaporating the solution his vesicles float in, he was able to make them grow and divide ().

Instead of requiring an external source of fatty acids, says Szostak, early protocells just needed a 鈥� similar to the one Darwin speculated life began in. When the sun shone water would evaporate, the fatty acids would become more concentrated, and the protocells would divide.

Could this really be how cell division began on Earth? 厂锄辞蝉迟补办鈥檚 vesicles are all made from the same simple fatty acids, whereas early Earth was probably covered in a messy mixture of related molecules, cautions of the University of Osnabr眉ck in Germany. It would be best to check whether vesicles made from such mixtures also divide when concentrations are raised. But he thinks they should, and that 厂锄辞蝉迟补办鈥檚 results will be generally applicable.

A bigger issue is whether the vesicles could have existed at all, says of the Scripps Institute of Oceanography in La Jolla, California. Even on land, water may have been too salty or acidic for them to be stable.

Support for 厂锄辞蝉迟补办鈥檚 idea comes from the behaviour of antibiotic-resistant bacteria. Almost all bacteria have rigid outer walls, which are the target of many antibiotics. In response to the drugs, some bacteria shed their outer walls, turning them into amorphous blobs. Errington has found that they then divide differently: some bud off smaller blobs, others create vesicles within themselves that eventually escape as daughter cells, and some extend a long tube that breaks up into small daughter cells 鈥� much like 厂锄辞蝉迟补办鈥檚 vesicles ().

鈥淭he similarity between Jeff Errington鈥檚 work and our observations of division of filamentous vesicles is striking,鈥� says Szostak. Mulkidjanian says that the simple cell division described by Errington could be a relic of how cells first reproduced billions of years ago.

Errington has also found that bacteria without walls can still divide even if all genes involved in this process have been deleted. As with 厂锄辞蝉迟补办鈥檚 vesicles, their division seems to rely solely on the physical properties of the membrane, suggesting that early protocells may not have required a complex genome to reproduce ().

鈥淚f you take out a gene and the cell dies, that makes it pretty clear what鈥檚 essential,鈥� says geneticist Craig Venter, who is trying to create a minimal cell by removing inessential genes. He calls 厂锄辞蝉迟补办鈥檚 findings 鈥渇ascinating鈥�.

If dividing bubbles really were at the root of life on Earth, where did they form? vesicles only form in fresh water. He and Mulkidjanian suspect that the first protocells appeared in shallow ponds on Earth鈥檚 first continents, not in the sea as origin-of-life researchers often assume.

鈥淭he first protocells may have appeared in shallow ponds on Earth鈥檚 first continents, not in the sea鈥�

Last year Mulkidjanian published evidence that cool water basins at geothermal fields are good candidates. They contain plenty of essential chemicals like phosphorus and metal ions, giving them the same chemical make-up as the interiors of modern cells (New Scientist, 18 February 2012, p 6).

Such ponds would have gone through cycles of evaporation by sunlight 鈥� making dissolved substances more concentrated 鈥� and dilution by rainfall, he says, good conditions for 厂锄辞蝉迟补办鈥檚 vesicles to divide. That wouldn鈥檛 happen in the ocean, which is too big for evaporation to have a significant effect.

Although many researchers remain partial to a marine origin for life, Szostak agrees with Mulkidjanian: 鈥淚f you need evaporation together with fresh water and temperature fluctuations, then I think you have a stronger argument for a geothermally active terrestrial environment.鈥�