What specific evidence could provide insight into the original genetic code before the incorporation of additional amino acids? How did the earliest cells manage to manufacture the more complex amino acids like histidine and tyrosine? What mechanisms might have driven the transition from simple to complex protein structures in early cellular life?
Investigators examine traits in different species, such as a specific protein. They assume that the different versions of the trait evolved from a common ancestor and attempt to envision the ancestral version, such as an ancestral protein’s amino acid sequence. Imagine what steps could have transformed the ancestral version into the modern versions, such as the series of amino acid changes in an evolving protein. Researchers often rely on circular reasoning, a pattern demonstrated in a recent article in PNAS, “Order of amino acid recruitment into the genetic code resolved by the last universal common ancestor’s protein domains.” The article, which claims to elucidate the origin and evolution of the genetic code, is entirely based on this flawed reasoning. It rarely attempts to demonstrate the plausibility of the proposed steps, instead focusing on crafting an engaging narrative. Enigma of the Genetic CodeThe origin of the genetic code is one of the most challenging and intractable problems in the origin of life research field. Even the most primitive biological information storage and retrieval system presents a formidable set of requirements:The sequence of nucleotides in DNA must encode the sequences of amino acids corresponding to all essential cellular proteins.
A complex protein must separate the two DNA strands to allow genetic information to be accessed. The protein in modern cells is called helicase.
Another suite of proteins must translate the information in RNA into the amino acid sequences corresponding to proteins.
Origins researchers assume that the original system was more straightforward than today. The modern genetic code encodes 20+ amino acids (e.g., valine) into sets of three nucleotides known as codons (e.g., GTA). The first autonomous cell is believed to have used only around half of the amino acids today. Trifonov (2000) proposes that the original set includes the following nine:
The other amino acids are believed to have been added to the code sequentially. The last ones to be added could not have formed through natural processes in non-trace quantities, so cells are believed to have evolved the chemical pathways to manufacture them before they were incorporated into the genetic code. The amino acids that must have initially been manufactured in cells include those with the more complicated atomic structures, such as histidine (His, H) and tyrosine.
SOURCE: WICKAPEDIA
Previous studies postulated the order of amino acid incorporation based on such factors as the ease with which the amino acids could have formed on the early Earth through simple chemistry, the complexity of the amino acids’ structure, and their biosynthetic pathways in modern cells. In contrast, the PNAS study compared the sequences of closely related proteins in modern organisms to reconstruct the sequences of ancestral proteins believed to reside in the last universal common ancestor (LUCA) of life today. The investigators also reconstructed sequences of proteins thought to reside in even earlier cells. They compared the sequences in ancient proteins to those today to postulate the amino acids’ order of incorporation.
Circular Reasoning and Causal Circularity
Neither the PNAS study nor earlier studies present substantive details about how the genetic code originated or how amino acids were later added. They assume that everything transpired through undirected natural processes and then, based on circumstantial evidence, construct the order in which they were incorporated. When serious questions are raised about the details of what would have been required to engineer the genetic systems or significantly modify them, the entire narrative
