By Ivan Noble
BBC News Online science staff
There is a good reason why the DNA of living things on Earth is not like that of Steven Spielberg's ET, researchers believe.
Samples of DNA taken from humans, animals, plants, microbes and viruses have one thing in common: they form a double helix structure held together by four different chemicals called bases.
And scientists from New Zealand and Sweden have now built a computer model to show that organisms are unlikely to evolve if their "life code" is written using more or fewer bases.
The work could help our understanding of what to expect from alien lifeforms - if we ever come across them.
Reading along the double helix, the sequence of the bases can be read off as a genetic code made of four letters - A for adenine, C for cytosine, G for guanine and T for thymine.
This code carries the instructions for creating and maintaining life, but why not use two letters, or six or eight?
ET in the famous Spielberg film was said to have six-base DNA and, on the face of it, Earth-bound life could have, too.
The reason why not turns out to be in the way a prehistoric relative of DNA began to protect itself against copying errors.
HOW DNA WORKS
The double-stranded DNA molecule is held together by chemical components called bases
Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)
Groupings of these letters form the "code of life"; there are about 2.9bn base-pairs in the human genome wound into 24 distinct bundles, or chromosomes
Written in the DNA are 30,000-40,000 genes which human cells use as templates to make proteins; these sophisticated molecules build and maintain our bodies
Paul Gardner, and colleagues at Massey University, New Zealand, and Uppsala University, Sweden, used a computer model to try to explain why four turned out to be the magic number.
Evolution by supercomputer
It is not entirely clear how life first began on Earth, but many biologists believe that before our current DNA-dominated world, there was an environment known as RNA World.
RNA is a similar chemical to DNA but it is much less stable and so much less suitable for holding the blueprint information for building complex organisms.
Supporters of the RNA World theory believe that RNA evolved from simpler chemicals and only later evolved into DNA.
RNA would have existed in a constantly changing and reactive soup.
Gardner and his colleagues built a computer model to show how RNA in RNA World would have evolved into DNA.
They instructed their supercomputer to examine how RNA might have developed had it had two, six or eight bases, as well as the standard four.
They found that four- and six-base RNA molecules were the most efficient at evolving into DNA.
But four-base RNAs were the ones which were best suited to overcoming RNA's fundamental weakness: its susceptibility to making errors as it copies itself.
The two- and eight-base RNAs seem to get stuck somewhere along the evolution process, Gardner told BBC News Online.
But six-base RNAs could have survived if they had evolved a way of putting right the errors introduced by mutation.
The research poses the question of what kind of DNA extra-terrestrial life might have if a similar process of evolution had taken place on a similar planet elsewhere in the Universe.
"We found the margins between four- and six-letter alphabets to be low, so a percentage of independent lifeforms might incorporate six, or a different four.
"But it is likely that the same principles that life on Earth are based on will be used elsewhere," Gardner said.
If RNA-based lifeforms on other planets had developed the error-correcting techniques needed to repair the damage to their genetic code caused by mutation and degeneration, they may well have developed into something with six-base DNA.
"I'd love to meet an organism with a six-letter alphabet. However, they'd probably take a lot longer to sequence," he said.