Today’s Feature: Is Earth’s DNA Unique or a Universal Constant?



"Life has been using a standard set of 20 amino acids to build proteins for more than 3 billion years," said Stephen J. Freeland of the NASA Astrobiology Institute at the University of Hawaii. "It's becoming increasingly clear that many other amino acids were plausible candidates, and although there's been speculation and even assumptions about what life was doing, there's been very little in the way of testable hypotheses."

"To the best of our knowledge, the original chemicals chosen by known life do not constitute a unique set; other choices could have been made, and maybe were made if life started elsewhere many times," said Paul Davies -leading authority in astrobiology, director of BEYOND: Center for Fundamental Concepts in Science and co-director of the ASU Cosmology Initiative.

Freeland and his University of Hawaii colleague Gayle K. Philip devised a test to try to learn if the 20 amino acids Earth's life uses were randomly chosen, or if they were the only possible ones that could have done the job.

"Technically there is an infinite variety of amino acids," Freeland told Astrobiology Magazine. "Within that infinity there are lots more than the 20 that were available [when life originated on Earth] as far as we can tell."

The researchers defined a likely pool of candidate amino acids from which life drew its 20, starting with the amino acids that have been discovered within the Murchison meteorite, a space rock that fell in Murchison, Victoria in Australia in September 1969, thought to date from the early solar system, and to represent a sample of which compounds existed in the solar system and on Earth before life began.

The scientists then used computers to estimate the fundamental properties of the 20 amino acids life uses, such as size, charge and hydrophilicity, or the extent to which the molecules are attracted to water.
"We know that these three are important to the ways they build proteins," Freeland said.
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Freeland and Philip analyzed whether these properties could have been achieved with as much coverage and efficiency with other combinations of 20 amino acids. The researchers discovered that life seemingly did not choose its 20 building blocks randomly.

"We found that chance alone would be extremely unlikely to pick a set of amino acids that outperforms life's choice," Freeland said.

The researchers think early life on Earth probably used a version of natural selection to choose these amino acids. Some combinations of other amino acids were likely tried, but none proved quite as fit, so no other combinations ended up producing the numbers of successful offspring that the existing set achieved.

"Here we found a very simple test that begins to show us that life knew exactly what it was doing," Freeland said. "This is consistent with the idea that there was natural selection going on."
Getting at the question of why nature chose the 20 amino acids it did is experimentally difficult, said Aaron Burton, a NASA Postdoctoral Program Fellow who works as an astrochemist at NASA's Goddard Space Flight Center in Greenbelt, Md.

"Although a number of experiments have shown that unnatural amino acids can be incorporated into the genetic alphabet of organisms, it may never be possible to experimentally simulate sufficient evolutionary time periods to truly compare alternate amino acid alphabets," said Burton, who was not involved in the new study. "As a result, studies such as those presented by Philip and Freeland offer interesting insights and provide a framework for formulating hypotheses that can actually be tested in the lab."

Right now the race is on to directly find amino acids elsewhere in the solar system. Some hints that they abound have been found on meteorites that have landed on Earth from outer space, as well as from missions such as NASA's Stardust probe, which sampled the coma of comet Wild 2 in 2004.

"All signs are that amino acids are going to be found throughout the galaxy," Freeland said. "They are apparently obvious building blocks with which to construct life. What we're finding hints at a certain level of predictability in the way things turned out."

Elsewhere, a recent mathematical analysis says that life as we know it is written into the laws of reality.  DNA contains four deoxyribonucleotides, with the nitrogenous bases adenine (A), guanine (G), cytosine (C), and thymine (T). The code contained within the base sequence is ultimately translated into proteins, which are constructed from the 20 amino acids.

An energy analysis by Ralph Pudritz, a theoretical astrophysicist and director of the Origins Institute at McMaster University shows that the first ten amino acids are likely to form at relatively low temperatures and pressures, and the calculated odds of formation match the concentrations of these life-chemicals found in meteorite samples.

They also match those in simulations of early Earth, and most critically, those simulations were performed by other people.  The implications are staggering: good news for anyone worried about how we're alone, and bad news for anyone who demands some kind of "Designer" to put life together – it seems that physics can assemble the organic jigsaw all by itself, thank you very much, and has probably done so throughout space since the beginning of everything.

The study indicates that you don't need a miracle to arrive at the chemical cocktail for early life, just a decently large asteroid with the right components.  That's all.  The entire universe could be stuffed with life, from the earliest prebiotic protein-a-likes to fully DNAed descendants. The path from one to the other is long, but we've had thirteen and a half billion years so far and it's happened at least once.

The other ten amino acids aren't as easy to form, but they'll still turn up – and the process of "stepwise evolution" means that once the simpler systems work, they can grab the rarer "epic drops" of more sophisticated chemicals as they occur – kind of a World of Lifecraft except you literally get a life when you play.  And once even the most sophisticated structure is part of a replicating organism, there's plenty to go round.

Early Earth was covered with carbonaceous material from meteorites and comets that provided the raw materials from which first life emerged. In his new book, The Eerie Silence, astrophysicist Paul Davies of Arizona State University suggests that the original cells would have been able to pick and choose from the early Earth's organic cocktail. To the best of our knowledge, he writes, "the twenty-one chosen by known life do not constitute a unique set; other choices could have been made, and maybe were made if life started elsewhere many times."

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