What would happen to the genetic code, were it not to put together its twenty amino acids in the tidy, one-after-another order, so many textbooks suggest? An increasing body of work is addressing the earliest biology as a boundary problem of both chemistry and heredity, with “life” not being a moment, but a long-term top-down transfer of molecules able to copy and molecules able to catalyze and environments able to concentrate both again and again. In that frame, the final universal common ancestor, LUCA, ceases to be the initial line, and becomes a bottleneck: a stage at which a recognizably workable translation system and a genetic code sufficiently stabilized to count as an actual code already existed.
A recent study by genetic scientists at the University of Arizona narrows that tightening of the checkpoint by tallying the number of amino acids within protein domains identified as older than LUCA and those which became more prevalent after LUCA. The team observed that protein “domains” are reusable modules: first author Sawsan Wehbi has likened them to car parts: “It’s a part that can be used in many different cars, and wheels have been around much longer than cars.” The greater supposition is epistemological as well as historical: to suppose that “early” biology can be deduced by looking at modern biology is to silently make a supposition about what living systems will have produced as well as that which nonliving chemistry might have provided here or there and in a variety of recipes.
The most notable of them all is tryptophan the amino acid, which is commonly considered a late addition to the canonical code. The Arizona group found 1.2 percent tryptophan in their pre-LUCA set compared to 0.9 percent in their post-LUCA set- small figures with a big bearing. After a so-called “last-added” amino acid is identified more frequently in the older count, then frequency, in itself, is inadmissible as a chronicle. Another indication of the work is the geography: various parts of the early earth could have supplied varied mixtures of amino acids, not just one global “soup.”
Such a notion is in line with laboratory chemistry that is no longer about completed cells but the dynamics of linking information to action. UCL chemists showed that amino acids spontaneously and selectively reacted with RNA in water at neutral pH, in a process that used thioester activation, designed to mimic mild reactions that do not disintegrate in water to form side products. They do not mean that a ribosome abruptly shows up, but one of the most important steps, namely loading amino acids onto RNA, can be treated with chemistry that does not involve the use of modern enzymes.
In such situations, environment is the one that is left as the engineer. Simulated experiments of the alkaline hydrothermal vents have demonstrated that individual amino acids are capable of being synthesized in conditions that resemble the vent, and that the flow of electrons and pH gradient could act as controllable inputs. A single such system was a lab “vent” in which alanine was made by the transfer of electrons between iron minerals and pyruvate and the amount of it could be varied by varying the proportions of the minerals-equivalent of a dial essentially a dial of reactivity. These ventitative discussions have also been applied to early membranes, where vesicle-like structures are easiest formed in conditions similar to those of “white smoker” vents at temperatures of the order of 70o C.
None of this brings the debate about the origin of life to a single point or a pathway. Rather it changes what is a plausible path: various early “codes” might have existed, amino acids that were noncanonical might have been tested and rejected, and probably translation happened as a series of chemistries instead of the single jump. The Arizona authors wrote, as they say, that “Stepwise construction of the current code and competition among ancient codes could have occurred simultaneously,” and “[a]ncient codes might also have used noncanonical amino acids.”
The same reframing goes way out of this planet. In the same paper, the abiotic synthesis of aromatic amino acids has also been suggested to have occurred in the subsurface ocean of Enceladus, with water-rock interfaces resonant with some of the geochemical themes used to date early Earth. Were the beginnings of early genetics based upon uneven forms of supply, on modular protein parts, and upon a protracted bargaining between peptides and RNA, then the search of life in the rest of the universe may depend a little on the discovery of an Earth analogue and more on the identification of familiar handoffs in new chemistries.
