Tucson Standard

Despite the vast diversity of life on Earth, from simple bacteria to complex blue whales, nearly all organisms share the same genetic code. The origins of this code have long been debated in scientific circles.

A new study by Sawsan Wehbi, a doctoral student at the University of Arizona's Genetics Graduate Interdisciplinary Program, challenges the conventional understanding of how this universal genetic code evolved. Wehbi's research suggests that the sequence in which amino acids were incorporated into the genetic code contradicts widely accepted views.

"The genetic code is this amazing thing in which a string of DNA or RNA containing sequences of four nucleotides is translated into protein sequences using 20 different amino acids," said Joanna Masel, senior author and professor at the University of Arizona. "It's a mind-bogglingly complicated process, and our code is surprisingly good. It's nearly optimal for a whole bunch of things, and it must have evolved in stages."

The study found that early life forms preferred smaller amino acid molecules over larger ones and that metal-binding amino acids were integrated earlier than previously thought. Additionally, it posits that today's genetic code likely succeeded other codes that are now extinct.

The authors criticize current models for relying on laboratory experiments rather than evolutionary evidence. They highlight flaws in the Urey-Miller experiment of 1952, which sought to recreate early Earth's conditions but failed to produce sulfur-containing amino acids despite their abundance during that time.

Co-author Dante Lauretta from the University of Arizona notes the implications for astrobiology: "On worlds like Mars, Enceladus and Europa, where sulfur compounds are prevalent, this could inform our search for life by highlighting analogous biogeochemical cycles or microbial metabolisms."

The researchers employed a novel method to trace amino acid sequences back to LUCA (Last Universal Common Ancestor), focusing on protein domains instead of full-length proteins. This approach revealed insights into when specific amino acids were added to the genetic code.

"If you think about the protein being a car, a domain is like a wheel," Wehbi explained. "It's a part that can be used in many different cars, and wheels have been around much longer than cars."

Using statistical analysis tools, they compared ancient protein sequences to determine when each amino acid was likely recruited into the genetic code. The team identified more than 400 sequence families dating back to LUCA and discovered some originated even earlier with aromatic ring structures like tryptophan and tyrosine.

"This gives hints about other genetic codes that came before ours," Masel stated. "Early life seems to have liked rings."