The traditional explanation of the initial genetic code in life seems less established when ancient proteins seem to be recalling some other arrangement of amino-acids. A recent examination of protein building blocks over extended evolution time holds that a long-standing conjecture, which what came early or late in the genetic code, was biased by the selection of what remains in modern biology.
The last common ancestor of all life is LUCA: the first universal common ancestor, but the origin of all known life to have split. Comparing the patterns of amino-acids that are inferred as present in protein domains before and after LUCA, researchers at the University of Arizona co-led by Joanna Masel and first author Sawsan Wehbi argue that the traditional chronology of the incorporation of the 20 canonical amino acids into the code can be over-weighted by the relative abundance of those 20 amino acids in subsequent, complete living systems.
The argument by the team relies on protein domains which are parts of proteins that are reusable as building blocks and which are believed to be highly ancient. It is an institutional statement where Wehbi employed a mechanical analogy: “It’s a part that can be used in many different cars, and wheels have been around much longer than cars,” and wheels are much older than cars. The researchers used domain-evolution software and public sequence databases to reconstruct a broad family tree of these parts to give an estimate of the amino-acid frequencies on either side of LUCA.
One detail is hard to ignore. The last amino acid to be introduced into the genetic code, tryptophan, was found more often in the pre-LUCA set than in the post-LUCA set. The article presents 1.2% tryptophan pre LUCA and 0.9% post LUCA a 25 percent relative difference. When a so-called late amino acid exhibits more frequency in the older slice of biology, then no longer does frequency provide a working simple clock of genetic history.
The researchers do not walk at a nice step-by-step march toward the current code but instead provide a sloppier list of the earlier days when several of the coding schemes may have coexisted in rivalry. They state, “Stepwise construction of the current code and competition among ancient codes could have occurred simultaneously,” and also write that “[a]ncient codes might also have used noncanonical amino acids.” That possibility brings back an older engineering-type question of the origins of life: whether the early systems were optimized designs that survived, or prototypes that were discarded.
Hydrothermal vents are still an attractive testbed to such prototypes since they provide gradients capable of performing chemical work. Enzyme-freeEnzyme-free vent-like conditions with natural electrochemical gradients, in which CO 2 is reduced to formic acid and acetic acid using conductive iron sulfur minerals, have now been recreated using laboratory reactors. Even nanoampere-sized currents were described to have the potential to maintain essential reactions in the study, in line with the metabolism first concepts with a physically plausible energy source: small yet constant voltages generated by variations in pH and temperature.
A separate line of origins work narrows in on information transfer rather than metabolism. Experiments at the Salk Institute described an RNA enzyme that can copy functional RNAs with improved accuracy while still allowing variation an important threshold for Darwinian-style evolution at the molecular scale. Gerald Joyce summarized the motivation: “We’re chasing the dawn of evolution.” That laboratory result supports an RNA-world pathway where heredity and innovation begin before proteins and DNA dominate.
Although the earliest chemistry on Earth may still be buried, the larger collection of prebiotic elements is no longer a hypothesis. Asteroid material returned has been analyzed to contain 33 amino acids, 14 of them found in terrestrial proteins, and evidence of nucleobases as well as large amounts of nitrogen-bearing compounds; indicating that complex organics can be accumulated outside of Earth and brought by them as feedstock.
The same inquiries now take on the form of cautionary words. Enceladus, which releases ocean material into space, might also be hiding their most useful biosignatures because it tends to be positioned as an easy target. The work modeling can imply that the ocean of Enceladus can be stratified such that the vertical mixing can be resistant to the mixing of chemicals at the seafloor, meaning that chemical traces of the seafloor may be distorted or trapped before they ever reach the plumes that are sampled by spacecraft.
Collectively, these threads lead to a single direction the origin of the genetic code might have been an alternative set of chemistries, a competition between molecular and molecular standards, an environment which actively maintains some of them and selectively removes others. The more science is akin to the early Earth and ocean worlds such as Encycladus, the more the issue resembles systems engineering, which is recreating a lost architecture using some of the parts that remained.
