At what point did chemistry become biology? In what type of environment did this transformation take place? These are major questions for those seeking to understand the origins of life on Earth.
A major breakthrough was hailed as far back as 1953 in what is now the famous Miller-Urey experiment in which the chemically simple molecules of water, methane, ammonia and hydrogen were mixed together in a glass chamber into which a powerful electrical current was discharged. The idea of the experiment was to simulate lightning in the ‘reducing’ atmosphere of the early Earth. To the amazement of the scientists the watery liquid in the glass chamber began to change colour and by the end of a week was deep red – indicating the presence of complex amino acid molecules. The building blocks of proteins had been synthesised from non-living material.
Getting the geo-chemistry right
Unfortunately for Miller and Urey, the chemists had not been speaking to the geologists. We now know that the choice of simple chemicals for this experiment was not representative of the atmosphere or the oceans of the early Earth. Early Earth studies and our knowledge of our planetary neighbours show that the Earth’s earliest atmosphere was carbon-dioxide rich. It was also more ‘oxidising’ than the atmosphere used in the experiment, so in reality the early life molecules had to be constructed from a much more difficult array of starting materials. Sadly, even as recently as 2011, at a meeting on the chemical origins of life at the Royal Society in London, chemists and Earth scientists still seemed to be following parallel but non- overlapping tracks.
A deeply troubled childhood
The clues to chemical synthesis of biological molecules must start with a detailed understanding of the Earth in its infancy. And it was a very different planet from the one we know today. Watery – yes, actively volcanic – yes, but with no continental land masses and with a very different atmosphere, dominated by carbon dioxide and methane. This was a planet in the shadow of a faint sun, which should by rights have been frozen but was warmed by its blanket of insulating gasses. In its earliest stages the Earth was totally inhospitable, bombarded with meteorites and asteroids which created so much energy on Earth as to boil the upper levels of the oceans and trigger massive volcanic eruptions.
Finding the right environment
It might be that the inhospitable environments on the early Earth hold a clue to the beginnings of life on Earth. Maybe life started in a place protected from the violent activity on the planet’s surface. Maybe it began in the deep oceans. Ever since the discovery of ‘black smokers’ in the deep oceans in the late 1970’s this environment has been considered a possibility for the origin of life. Black smokers are an example of a hydrothermal vent system – that is a jet of hot water at 300-400oC, which is vented into the deep ocean. The water is hot because it has been in contact with molten rocks beneath the ocean floor, and because it is hot it has the capacity to dissolve a particular suite of chemical elements from the rocks of the ocean floor. When the hot water mixes with cold ocean-bottom water a range of chemical reactions take place which form a ‘black smoke’ of sulphide particles which rain down onto the ocean floor. This mineral-laden, high energy and today biologically diverse environment has been identified as a most probable place for the synthesis of organic molecules to become the building blocks of life.
Darwin’s warm little pond
But the very energy identified to create complex biological molecules in black smokers may also be its undoing, for it has been argued that the temperatures in hydrothermal vents are so great that the delicate molecules created would then break down. Hydrothermal vent fluids are also acidic, and deemed a chemically unsuitable medium for organic synthesis. More recently, attention has turned to what in chemical terms is a polar opposite environment – one in which highly alkaline fluids are formed. This may also have been in an oceanic environment, but a cooler one – and one in which watery fluids interact with rocks that are now buried deeper in the Earth, called the mantle.
A modern analogue of this alkaline environment can be found today where rocks which normally form deep in the ocean are now preserved on land. In these places cool alkaline vents produce methane gas, which comes into contact with modern carbon dioxide and makes mud – intriguingly similar to the ‘warm little pond’ spoken of in Charles Darwin’s letters. This may well have been the situation all over Earth in the early history of the planet.
An impossible task?
Of course studies of this type ultimately need to show how the key molecules of life – the large molecules of DNA and RNA (which today is the intermediary between DNA and proteins) – might have been synthesised on Earth and on other planets. So how far along the road are we in this process? Put simply, not very far. Not least because we know so little in detail about the earliest history of the Earth and Earth-like planets. The danger of course is to say that what has happened was impossible without some external intervention, but that leads us into the dangerous territory of a ‘God of the gaps’. A god who only ‘pops in’ to help us when we are stuck is not the God of the Bible, who Christians believe is actively involved in his world.