All life on Earth is powered by a process called chemiosmosis, where the chemical adenosine triphosphate (ATP), the rechargeable chemical ‘battery’ for life, is both broken down and re-formed during respiration to release energy used to drive the reactions of life, or metabolism. The complex enzymes required for both the creation and break down of ATP are unlikely to have existed on the Earth during the period when life first developed. This led scientists to look for a more basic chemical with similar properties to ATP, but that does not require enzymes to transfer energy.
Phosphorus is the key element in ATP, and other fundamental building blocks of life like DNA, but the form it commonly takes on Earth, phosphorus (V), is largely insoluble in water and has a low chemical reactivity. The early Earth, however, was regularly bombarded by meteorites and interstellar dust rich in exotic minerals, including the far more reactive form of phosphorus, the iron-nickel-phosphorus mineral schreibersite.
The scientists simulated the impact of such a meteorite with the hot, volcanically-active, early Earth by placing samples of the Sikhote-Alin meteorite, an iron meteorite which fell in Siberia in 1947, in acid taken from the Hveradalur geothermal area in Iceland. The rock was left to react with the acidic fluid in test tubes incubated by the surrounding hot spring for four days, followed by a further 30 days at room temperature.
In their analysis of the resulting solution the scientists found the compound pyrophosphite, a molecular ‘cousin’ of pyrophosphate – the part of ATP responsible for energy transfer. The scientists believe this compound could have acted as an earlier form of ATP in what they have dubbed ‘chemical life’.
"Chemical life would have been the intermediary step between inorganic rock and the very first living biological cell. You could think of chemical life as a machine –a robot, for example, is capable of moving and reacting to surroundings, but it is not alive. With the aid of these primitive batteries, chemicals became organised in such a way as to be capable of more complex behaviour and would have eventually developed into the living biological structures we see today," said Dr Terry Kee.
The team from NASA’s Jet Propulsion Laboratory (JPL-Caltech) working on the Curiosity rover, which landed on Mars in August last year, has recently reported the presence of phosphorus on the Red Planet.
"If Curiosity has found phosphorus in one of the forms we produced in Iceland, this may indicate that conditions on Mars were at one point suitable for the development of life in much the same way we now believe it developed on Earth," added Dr Kee.