Of all the scientific mysteries, this is probably the greatest one of all – how did life on Earth begin? We are not talking about how it evolved into the diversity of lifeforms we see today. We are talking about how it originated in the first place.
For all his immense insight into evolution, Charles Darwin himself was stumped. He suggested that whatever the mechanism was that had led to the first replicating lifeforms, it most probably arose in some "warm little pond", a primordial soup of pre-biotic ingredients where the seed of life first germinated on the early Earth.
Now scientists have developed an experiment demonstrating how the very first self-replicating molecules may have formed about 4 billion years ago when the Earth was like any other lifeless planet that had yet to experience the radical transformation of living, breathing creatures.
John Sutherland and colleagues at Manchester University have broken new ground by being able to synthesise almost from scratch two of the four building blocks of RNA, the self-replicating molecule that many scientist believe to be the most likely contender for the original molecule of life. Dr Sutherland believes that he has shown how it was possible to make all the building blocks of RNA from the simple chemicals that would have existed on Earth 4 billion years ago.
"We've made the building blocks of RNA from what was around on the early Earth and is still around in interstellar space and in the atmosphere of Saturn's moon Titan," Dr Sutherland said.
"We haven't yet made the RNA molecule itself but we've made two of the four sub-units or building blocks. It suggests that making the molecule is possible. The building blocks are strung together and doing that is actually easier than making the building blocks themselves," he said.
RNA is the less familiar cousin of DNA, the genetic blueprint of life. Like DNA, the RNA molecule can carry and transmit information from one generation to the next. But unlike DNA, RNA is a relatively simple molecule that many scientists believed could have been quite easy to synthesise in the harsh environment of the early Earth. The trouble with this idea – which is more than 40 years old – is that no one has been able to join up the three components, the sugars, bases and phosphates that make up each of the four building blocks of RNA, under the sort of conditions that existed 4 billion years ago. Dr Sutherland, however, has shown in a study published in the journal Nature that this is indeed possible.
"The trouble is, the human eye sees the three components of RNA and so the human brain assumes that to make the molecule you should combine those three components. People have found that they can make the sugars and the bases but the key thing they can't do is to join them together," Dr Sutherland said.
"And so for 40 or so years they have worked on the problem and have become so frustrated that they have decided that RNA, although very desirable, is just too complicated and so there must have been a simpler molecule that spawned RNA. We've just changed the order of assembly of the pieces, but it's overcome the dogma that it cannot be done," he said.
In trying to explain how life began on Earth, scientists have attempted to formulate theories to account for how the first self-replicating molecule came into existence. One of the earliest theories was the "primordial soup", where simple molecules mixed together in a broth that was regularly energised by ultraviolet light and electric storms.
Over time, these simple molecules would have combined to form more complex substances containing the all-important atomic ingredients of life – oxygen, carbon, hydrogen and nitrogen. Although scientists were able to make the building blocks of proteins in this way, they failed to do the same with DNA or RNA.
Scientists first proposed that RNA preceded proteins in the 1960s, but it was not until the 1980s that they received strong support for the idea. Thomas Cech at the University of Colorado and Sidney Altman at Yale, found that RNA could act as a catalyst by speeding up a chemical reaction and yet being unchanged in the process – a feat normally reserved for enzymes.
This was the first hard evidence that RNA, a molecule that can replicate and store genetic information, could also have triggered the first synthesis of life's proteins. Most scientists now believe that there was an "RNA world" early in the Earth's history from which all present-day life is ultimately descended.