See also:

Earliest Fossils

The RNA World

External Links:

The Beginnings of Life on Earth (Christian de Duve)

The Origin of Life on Earth (Leslie Orgel)

Let There be Life (New Scientist)

The Origin of Life

When?

The fossil record of life on Earth goes back to at least 3.5 billion years and perhaps 3.8 billion years (see Earliest Fossils). The origin of life must have occurred earlier than this but probably not much earlier, as for the first few hundred million years of Earth's history the planet was subject to an intense bombardment of debris left over from the formation of the solar system. The largest impacts would have probably sterilized the planet destroying any nascent life. The most likely time for life's origin thus appears to be between about 4 billion and 3.8 billion years ago.

Where?

A current popular choice for the location of the origin of life is at deep sea hydrothermal vents. This suggestion is prompted by the discovery of extensive ecosystems around these vents, the finding that hyperthermophilic (high temperature) organisms seem to be among the most ancient branches of the tree of life, and the fact that such locations would provide interesting chemistry and a ready energy source. A deep sea location would also be more protected from the heavy bombarment than one nearer the surface.But others argue that such high temperatures would destroy important chemicals such as RNA, and that the origin must have occurred at more moderate temperatures nearer the surface. (see The RNA World)

Did life originate on Earth at all? Another possibility is that life originated on another planet - Mars is the most likely - and was carried to Earth by meteorites. We have many examples of Martian meteorites, and in the heavy bombardment phase the transfer of material between planets would have been much more common. It doesn't seem too implausible that a bacteria like organism could have survived the journey.

How?

We know that all life today shares a common chemistry based on the use of DNA to carry genetic information, protein enzymes to act as catalysts and drive the cells complex chemical processes, a universal genetic code which enables a DNA sequence to specify a protein, and a protein synthesis machinery based on RNA. This basic structure must have been inherited from the common ancestor of all life. This common ancestor also known as LUCA (Last Universal Common Ancestor) lived billions of years ago, but hints as to its nature can be revelead through molecular phylogeny - the comparison of gene sequences to determine the tree of life - with LUCA at its root.

The RNA World

To understand how the DNA/protein structre of LUCA came about we have to face the 'chicken and egg' problem. DNA can only replicate with the help of protein enzymes, but these proteins require DNA to specify their structure. A solution is provided by the concept of the RNA World. RNA molecules can replicate and carry information (like DNA) and act as catalysts (ribozymes) like proteins. This ability, together with the current role of RNA in what appear to be primitive features of the cell chemistry suggest that originally RNA could have provided both the genome and the catalysts and these roles could subsequently have been taken over by DNA and proteins. This stage is known as the RNA World

The Pre-RNA World

Could such an RNA World have sprung into being directly from the chance operation of prebiotic chemistry. This seems unlikely - making the components of RNA and assembling them into a viable RNA World capable of further evolution doesn't seem easy. Thus many Origins of Life researchers are now investigating possible pre-RNA Worlds - simpler systems using some other genetic material, systems in which evolution by natural selection could operate, but which are more plausible as products of prebiotic chemistry. There are many suggestions, ranging from simpler variants of RNA/DNA called peptide nucelic acids to Graham Cairns-Smith's proposal of inorganic clay crystals. Others argure that rather than looking for replicating molecules we should instead focus on proto-metabolism - networks of reactions that might be forerunners of the complex metabolic pathways of modern cells.