Almost all living things share the same genetic code, and the origin of this code has long been a topic of scientific debate.
In a new study, Sawsan Wehbi, a doctoral student at the University of Arizona, found strong evidence that challenges the commonly accepted theory of how the genetic code evolved. The study suggests that the order in which amino acids—the building blocks of the code—were added is different from what was previously thought to be the “consensus” on genetic code evolution.
The study found that early life favored smaller amino acid molecules over larger, more complex ones, which appeared later in evolution. Additionally, amino acids that bind to metals were incorporated into the genetic code much earlier than previously believed. The team also discovered that the genetic code we use today likely came after other codes that have since disappeared.
The researchers argue that the current understanding of how the genetic code evolved needs to be revised. They believe it relies too much on misleading lab experiments instead of evolutionary evidence. For example, a key part of the traditional view is based on the famous 1952 Urey-Miller experiment, which tried to recreate the conditions of early Earth to understand the origin of life.
Study highlights how amino acids shaped the genetic code of ancient microorganisms(Op
While the Urey-Miller experiment was significant for showing that simple chemical reactions could create life’s building blocks, including amino acids, its conclusions have been questioned. For example, the experiment didn’t produce any amino acids containing sulfur, even though sulfur was abundant on early Earth.
This led scientists to believe that sulfur-containing amino acids were later added to the genetic code. However, this result isn’t surprising because sulfur was actually left out of the experiment’s ingredients.
Dante Lauretta, a co-author and Regents Professor of Planetary Science at the University of Arizona, points out that early life’s sulfur-rich nature can offer valuable insights into astrobiology. It helps us understand the potential habitability of other planets and how we might detect signs of life, or biosignatures, in extraterrestrial environments.
“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. Such insights might refine what we look for in biosignatures, aiding the detection of lifeforms that thrive in sulfur-rich or analogous chemistries beyond Earth.”
Evidence found for the existence of the amino acid tryptophan in space
The team used a new method to analyze amino acid sequences across the entire tree of life, tracing them back to the Last Universal Common Ancestor (LUCA), which is thought to have lived about 4 billion years ago and is the shared ancestor of all life on Earth today. Unlike previous studies that focused on full-length protein sequences, Wehbi and her team concentrated on protein domains—shorter amino acid strengths.
Wehbi said, “If you think about the protein being a car, a domain is like a wheel. It’s a part that can be used in many different cars, and wheels have been around much longer than cars.”
To determine when specific amino acids were likely incorporated into the genetic code, the researchers used statistical analysis tools to compare the presence of each amino acid in protein sequences from LUCA and even earlier. Amino acids that appear more often in ancient sequences were likely added to the code early on. In contrast, LUCA’s sequences had fewer amino acids incorporated later, as these became more common in protein sequences from later life forms.
The team identified over 400 families of sequences dating back to LUCA, and more than 100 of these originated even earlier and had already evolved before LUCA. Interestingly, these ancient sequences contained more amino acids with aromatic ring structures, like tryptophan and tyrosine, later additions to the genetic code.
Masel said, “This gives hints about other genetic codes that came before ours and which have since disappeared in the abyss of geologic time. Early life seems to have liked rings.”
Journal Reference:
- Sawsan Wehbi, Andrew Wheeler, et al. Order of amino acid recruitment into the genetic code resolved by last universal common ancestor’s protein domains. PNAS. DOI: 10.1073/pnas.2410311121