Sidebar: Evidence for a single origin of life
There is significant scientific evidence that life originated from a single source. Two major lines of evidence involve RNA and the shape of amino acids.
1. RNA Evidence
All cells use ribosomes and transfer RNA (tRNA) to synthesize proteins from chains of amino acids. This process is very similar in all living organisms. This strongly argues for a single origin.
Ribosomal RNA (rRNA) is one of the most conserved genes, meaning that it changes very little between organisms. Because rRNA is so similar among organisms, this strongly suggests a single origin.
2. Amino Acid Evidence
Amino acids are the building blocks of proteins. The three-dimensional structure of proteins is vital to their function. However, amino acids exist in two structures that can be described as left or right handed.
chirality = orientation of molecules into left or right handedness A left hand is identical to a right hand except for its chirality. Likewise, an L-amino acid (left orientation) is identical to a D-amino acid (dextral, or right orientation) except for its chirality.
Both L and D-amino acids are found on Earth and in organic materials from meteorites. Theoretically, either form should work equally well to build proteins. However, biochemical processes only use L-amino acids. This strongly suggests that life arose once, and by happenstance employed the L-amino acid configuration.
Another hypothesis is that if the earliest forms of life used both L and D-amino acids, then this binary system would have greatly complicated nutrition. Imagine that your cell needs R-amino acids, but what you eat is a combination of R and L; only half of your food can be used to make proteins--a very inefficient system. It would be much more efficient if everything you ate or recycled already had the orientation you needed. If an early organism evolved to utilize just one form (L), this would have conferred a great benefit, allowing the L species to drive its binary rivals into extinction.
[Rare bacteria employ D-amino acids in a kind of chemical warfare to defeat antibiotics. This works because antibiotics are specific for L-amino acids. However, these D-amino acids are added outside the normal protein manufacturing process by specialized enzymes.]