In part 1, I discussed how early life may have arisen. In this part I will comment on one of the aspects of current terrestrial life - that of molecules symmetry or 'handedness'.
Molecular handedness, which chemists call Chirality, is a property of the 4 bonds which a carbon atom can form. If a particular carbon has 4 different groups of atoms attached to it, then there are two ways of attaching them such that one is a mirror image of the other. These are the two chiral forms, which are referred to as D- or L-isomers depending on the particular arrangement. In the 'skeleton' form of the amino acid Leucine, shown below, triangles mean the bond is pointing towards the viewer and the 'ladder' is a bond pointing away.
The centre carbon has the following 4 groups attached to it: Hydrogen NH2 'amine' group CO2H 'carboxylic acid' group CH2CH(CH3)2 branched carbon chain.
In terrestrial life, amino acids are almost exclusively the L-form and sugars are usually the D-form. There have been references to this in science fiction, such as the Arthur C. Clarke short story Technical Error where a scientist accidentally has his molecules changed to the opposite chirality and he is unable to metabolise any food.
If amino acid molecules are created by normal non-biological processes, there is usually a 50:50 mix of the D & L molecules (called a racemic mixture). Somehow, back when life was first beginning, there must have been a selective pressure on one form over the other.
Samples of amino acid have been retrieved from meteorite fragments and these turned out to have an excess of the L-isomer. Experiments have shown that polarized light can produce such a slight excess so conditions may have accidentally selected for a particular isomer and we are left with the legacy of those conditions.
Update (4/11/2011):
Earlier this week there was a discussion on the Simon Mayo show on Radio 2 which was about what would happen if the moon disappeared. This was prompted by a discussion of the film Despicable Me and the sequence where the moon was stolen.
It reminded me of a theory which stated that life wouldn't have developed on Earth if the moon didn't exist. The tides caused by the moon would have stirred up the precursor molecules and hastened any chemical reactions which would have led to life. An alternative version of this theory was presented in Star Trek: The Next Generation, where Q took Captain Picard back to the early Earth. Picard stirred up a 'soup' of organic molecules which mixed together to create life.