This week's science posting is in two parts. Part 1 covers a few theories about how life arose on Earth. Part 2 goes into a bit more detail on the molecules of life.
How Life came to Earth
There are two main theories on how life started on Earth: It came from Outer Space, and it started here on Earth. I will briefly describe the two approaches.
1. Panspermia
This theory claims that life came to Earth from elsewhere in the universe, and that there could be related organisms elsewhere in the galaxy. The approach was popularised by astronomers Fred Hoyle and Chandra Wickramasinghe, who believed that the process was still continuing today and that terrestrial outbreaks of disease could be caused by microbes brought to Earth by comets.
It certainly appears possible that life could be brought to Earth this way. Experiments have taken place where microbes have been taken into orbit and exposed to space where they have been exposed to cosmic rays. This took place in 2008-2010 when a piece of rock was fixed to the outside of the International Space Station. Space is a very harsh environment compared to the surface of the planet: there is no water or oxygen, and no ozone layer or atmosphere to protect against the harsh light from the sun. When the rock was brought back to Earth, laboratory tests showed that the bacteria had survived.
The surface of a comet is fairly hospitable by comparison. There might not be much of an atmosphere most of the time but there is some water and hydrocarbons which could provide a source of food. When the comet approaches the sun and warms up, clouds of gas billow out which could scatter any bacteria across the solar system where some could eventually arrive on Earth.
Evidence for Panspermia
Sadly there is no firm evidence for panspermia. No extra-terrestrial bacteria have been identified and there are no confirmed cases of bacteria of any kind being detected in comets. The closest we have is a meteorite which landed in Antarctica which has been identified as coming from Mars. This is the famous Allan Hills rock (known as ALH 84001) which had tiny features which looked like bacteria. Unfortunately these 'fossils' are much smaller than terrestrial bacteria so the identification as martian bacteria is not universally accepted.
Problems with Panspermia
While panspermia is theoretically possible, there is no evidence that it has taken place or that bacteria is present elsewhere in the solar system. The main problem is that it doesn't explain how life actually began, it only moves the problem to somewhere else, so even if panspermia is true, there is still the problem of how life began in the first place.
2. Terrestrial Abiogenesis
Abiogenesis is the process by which life arises from non-living matter. If life arose on Earth, rather than arriving from space, then the building blocks and conditions must have been present early in the Earth's history. The traditional view can be grossly over-simplified as: slowly over billions of years, different molecules reacted together to eventually create molecules capable of life.
This process might sound like a lottery - pick a molecule, see if it works, if it doesn't then try again. In reality the process is more likely to have been gradual: pick a molecule, if it doesn't work then modify it slightly and try again. This is more like the old Mastermind board game where you picked coloured pegs and tried to guess where your opponent placed them, or the Battleships game where you had to guess the locations of the boats. If a solution is 'almost correct' then it will be used as the basis for the next attempt, instead of throwing it away and starting again.
There have been attempts to replicate the conditions of early Earth to see if such molecules can spontaneously appear. The most famous of these is the 'Miller-Urey' experiment. Sparks were passed through an atmosphere of Water, Methane, Ammonia and Hydrogen to simulate lightning, which would break up the molecules to allow the atoms to re-join to form other molecules. The above gases were chosen because the elements they contain are sufficient to make 'amino acids' which are the building blocks for proteins and are vital for life (as we know it, at least).
After a week, simple sugars had formed, such as ribose (which has a chain of 5 carbon atoms, unlike the 6 carbons in glucose and fructose which I mentioned last week). The amino acid glycine (NH2CH2CO2H), had also formed.
Since then, molecules such as glycine and ethanol have been discovered in molecular clouds in space. Additionally the molecules adenine an glycine, which are known as 'Nucleotides' and are some of the building blocks of DNA, have been discovered in meteorites and comets. This proved that nature was capable of creating such molecules (and may suggest a compromise between panspermia and abiogenesis where the building blocks came from space but life arose on Earth).
The next step, from simple amino acids to actual life, is the part which causes the most problems. For life to form, we need a way of replicating molecules instead of waiting for them to spontaneously form one at a time. In cells there are two main types of molecular replication: manufacturing proteins from the template provided by the DNA (transcription/translation) and DNA replication itself (which occurs when cells divide).
Taking the first type of replication above, the first stage is Transcription. This takes a region of DNA which contains the 'recipe' for a protein and makes a copy of it, but as RNA instead of DNA (to put it simply, RNA is similar to DNA but uses slightly different building blocks). One current theory claims that early life might have been based on RNA instead of DNA so this stage can be ignored for now. I appreciate that I am skipping over a lot of detail here but some background reading can be found at here: Transcription Translation DNA Replication DNA Replication tutorial
At first glance it seems likely that the nucleotides which spontaneously formed may have joined together to form a primitive form of RNA (which stands for RiboNucleic Acid, which means it is formed from the sugar Ribose and Nucleotides, both of which have been identified in meteorites), but chemists have worked out the steps required and claimed that it isn't possible. An alternative chemical route has been suggested (Powner et.al, Nature, 14th May 2009) where each of the steps is plausible and all the of ingredients would have been available in the early environment.
Creating proteins from RNA uses a giant collection of molecules called the Ribosome, which is so complex and specialized that it cannot have spontaneously developed in its current form. There are suggestions that a much simpler 'proto-ribosome' formed, which was a much less complex enzyme which could have self-assembled (Agmon et.al, Nature Proceedings, March 2009).
In this way, life arose from simple molecules which were able to form in the 'primordial soup' which gradually led to more complex associations of molecules which ultimately led to something which we could identify as Life.
Further reading can be found here.