How life evolved: 10 steps to the first cells
Was our oldest ancestor a proton-powered rock?
We may never be able to prove beyond any doubt how life first evolved. But of the many explanations proposed, one stands out - the idea that life evolved in hydrothermal vents deep under the sea. Not in the superhot black smokers, but more placid affairs known as alkaline hydrothermal vents.
This theory can explain life's strangest feature, and there is growing evidence to support it.
Earlier this year, for instance, lab experiments confirmed that conditions in some of the numerous pores within the vents can lead to high concentrations of large molecules. This makes the vents an ideal setting for the "RNA world" widely thought to have preceded the first cells.
If life did evolve in alkaline hydrothermal vents, it might have happened something like this:
Water percolated down into newly formed rock under the seafloor, where it reacted with minerals such as olivine, producing a warm alkaline fluid rich in hydrogen, sulphides and other chemicals - a process called serpentinisation.
This hot fluid welled up at alkaline hydrothermal vents like those at the Lost City, a vent system discovered near the Mid-Atlantic Ridge in 2000.
Unlike today's seas, the early ocean was acidic and rich in dissolved iron. When upwelling hydrothermal fluids reacted with this primordial seawater, they produced carbonate rocks riddled with tiny pores and a "foam" of iron-sulphur bubbles.
Inside the iron-sulphur bubbles, hydrogen reacted with carbon dioxide, forming simple organic molecules such as methane, formate and acetate. Some of these reactions were catalyzed by the iron-sulphur minerals. Similar iron-sulphur catalysts are still found at the heart of many proteins today.
The electrochemical gradient between the alkaline vent fluid and the acidic seawater leads to the spontaneous formation of acetyl phosphate and pyrophosphate, which act just like adenosine triphosphate or ATP, the chemical that powers living cells.
These molecules drove the...