By Brandon Keim
"This may implicate a universal structure of the first genetic codes anywhere," said astrophysicist Ralph Pudritz of McMaster University in Hamilton, Ontario.
There are exactly 20 standard amino acids — complex molecules that combine to form proteins, which carry out instructions specified by RNA and DNA, its double-stranded and self-replicating descendant.
Ten were synthesized in the famous 1953 Miller-Urey experiments, which modeled conditions believed to exist in Earth's early atmosphere and volcano-heated pools. Those 10 amino acids have also been found in meteorites, prompting debate over their role in sparking life on Earth and, perhaps, elsewhere.
Pudritz's analysis, co-authored with McMaster University biophysicist Paul Higgs and published Monday on arXiv, doesn't settle the former debate, but it does suggest that basic amino acids are even more common than thought, requiring little more than a relatively warm meteorite of sufficient size to form. And that's just the start.
If the observed patterns of amino acid formation — simple acids require low levels of energy to coalesce, and complex acids need more energy — indeed follow thermodynamic laws, then the basic narrative of life's emergence could be universal.
"Thermodynamics is fundamental," said Pudritz. "It must hold through all points of the universe. If you can show there are certain frequencies that fall in a natural way like this, there is an implied universality. It has to be tested, but it seems to make a lot of sense."
Pudritz and Higgs tabulated the types and frequencies of amino acids found in primordial Earth experiments, then correlated the results on a graph of temperature versus atmospheric pressure at which the acids likely formed.
The 10 amino acids synthesized in primordial Earth experiments tended to arise at relatively low temperatures and pressures, and are chemically simple. Other, more complex acids formed less frequently, and require more temperature and pressure. Their distribution follows a clear, possibly thermodynamic, curve.
"The most frequent amino acid that forms is the one that's least-demanding, energetically. There's less and less amino acids that require more energy to form. That's very sensible, from a thermodynamic point of view," said Pudritz.
Internal conditions of meteorites are unknown, but some scientists believe that certain large meteorites are both warm and hydrated, making them roughly analogous to the relatively temperate environment of Earth's youth.
"There's a theory," said Pudritz, "that they could be made in the warm interiors of large-enough meteorites."
This is necessarily speculative, but it would explain why the 10 amino acids most common in primordial Earth experiments are also the most common acids found in meteorites.
Pudritz and Higgs speculate that these 10 common amino acids met the needs of the earliest replicating molecules, with other, rarer acids used by the nascent genetic code as they formed or arrived — a process called "stepwise evolution," culminating in the genes that gathered 3.6 billion years ago in a common ancestor of all complex life.
If simulations of interactions between these 10 acids indeed support molecules that can copy themselves, said Pudritz, then it's possible that they could support an ur-genetic code on Earth and elsewhere.
"There's a possible universality," he said, "for any code that would use amino acids."
Harvard University systems biologist Irene Chen, who specializes in the evolution of molecules, called the work "interesting," but noted that "in the absence of some experimental backup, it's generally difficult to know if this kind of analysis is a Panglossian argument."
The ultimate experimental backup, of course, is finding aliens. In the meantime, the ending of Battlestar Galactica seems a bit less implausible.
By Paul G. Higgs, Ralph E. Pudritz.
arXiv, April 6, 2009.
- A Theory of Evolution for Evolution
- Self-Replicating Chemicals Evolve Into Lifelike Ecosystem
- Proof That Meteors Could Have Sparked Life on Earth
- Primordial Earth Echoed in Arsenic-Eating Bacteria
- Forgotten Experiment May Explain Origins of Life
- Organism Sets Mutation Speed Record, May Explain Life's Origins
- Evolution as Biological Thermodynamics
Images: 1. Case University, Valadkhan Lab/Chesley Bonestell. 2,3. arXiv