The blood plasma and lymph of modern animals is similar in chemical composition to seawater, strongly supporting the idea that animal life began in the oceans, but the liquid inside our cells has a quite different chemistry, suggesting that cells themselves first arose in a different environment, since the first cells are unlikely to have shared modern cells ability to maintain an interior chemistry very different to the liquid outside their membranes.
In a paper published online in the Proceedings of the National Academy of Sciences on 13 February 2012, a team of scientists led by Armen Mulkidjanian of the School of Physics at University of Osnabrück and the A. N. Belozersky Institute of Physico-Chemical Biology at Moscow State University, describe a review of our understanding of the chemistry of the earliest cells and the environment on the early Earth in these cells are thought to have inhabited, and the conclusions drawn from this.
The oldest known rocks on Earth are about 4 billion years old, but life is thought to have originated earlier than this (life does not pre-date rocks, just any rocks still around), in a period known at the Hadean. This makes the chances of finding direct evidence of the earliest life effectively nil, but does not stop us speculating, as we are still able to make inferences about the environment on the Hadean Earth.
An artist's impression of the Hadean Earth. The continents were still assembling from thousands of volcanic island arcs, the moon was closer, the sun dimmer, the planet was being constantly bombarded by meteors, the atmosphere rich in Carbon Dioxide but entirely lacking in Oxygen, consequently there was no Ozone layer and the planet was constantly bathed in ultra-violet radiation. From the Palaeos website.
The oldest, conserved, proteins found in all organisms tend to use Zinc and Manganese, but not Iron, which is used by more modern proteins that have evolved separately in different groups, implying that the first cells evolved in an environment rich in Zinc and Manganese, but poor in Iron. Water from hydrothermal vents tends to be rich in Zinc, Manganese and Iron, but the iron precipitates out of solution rapidly.
All known cells maintain an interior environment richer in Potassium than in Sodium; the reverse of the situation found in the modern ocean. We do not have any evidence that suggests the situation in the earliest oceans would have been any different. Hydrothermal vents in the seas have similar Sodium/Potassium ratios to seawater, since this is where the water in them derives from, and the contribution of Sodium and Potasium ions from the seawater outweighs the volcanic contribution. Terrestrial hydrothermal springs derive their water from precipitation (rain and snow), which lacks Sodium and Potassium ions. The Sodium/Potassium ratio in such pools is variable; those which are dominated by water that is emitted in a liquid tend to be rich in Sodium, but those where water is emitted as a gas and then condenses tend to be rich in potassium.
All cells maintain a high interior Phosphate concentration, but the oceans are not rich in Phosphates, and there is no reason to believe the early oceans were any different. Water from volcanic vents tends to be rich in Phosphates.
Previous theories have suggested that life may have originated around deep-sea hydrothermal vents, but these have steep chemical gradients, and chemistry unlike that found inside cells. Hydrothermal pools on-land tend to have stable chemistry, but are very acidic, and so have been discounted as a likely source of life by many studies. However this acidity is caused by the reaction of Sulphur-compounds with Oxygen in the atmosphere, something that could not have happened in an ancient environment lacking atmospheric Oxygen.
The other objection to terrestrial hydrothermal pools as a place of origin for life is the high level of ultra-violet radiation that would have bathed the early Earth and which is harmful to life. The modern world is protected from ultra violet radiation by the Ozone Layer, but Ozone (O₃) is a form of Oxygen, so this would not have existed prior to the evolution of an Oxygen rich atmosphere. Water also protects against ultra-violet radiation, but there needs to be enough of it; the sea would protect early cells, but shallow pools would probably not. However Sulphur from volcanic vents, if it was not reacting with oxygen from the atmosphere, would probably react with Manganese and Zinc if they were present in the same pools of water. Sulphur compounds of Manganese and Zinc are good at absorbing ultra violet radiation, offering protection to any life living in these pools.