Before bacteria and archaea became two major branches of life, their common ancestor already possessed part of the chemistry necessary for life. But it was still missing many enzymes. According to a new study, each of the two lineages would then have completed this system independently.
This system is metabolism: a network of reactions that enables cells to build their constituents and transform energy. Enzymes are the proteins that now accelerate and control most of these reactions. Archaea, for their part, are microbes distinct from bacteria, even though the two groups can look similar.
Pixabay illustration
To go back so far into the past, there are obviously no protein fossils. The authors therefore compared the enzymes found in 552 bacterial genomes and 401 archaeal genomes. They linked these proteins to 361 reactions needed to produce amino acids, DNA bases and various chemical helpers in cells.
This comparison makes it possible to search for enzymes inherited from a very ancient ancestor. The researchers attribute 166 of them to LUCA, the name given to the last common ancestor of bacteria and archaea. By contrast, 89 others would have appeared later on the branch leading to bacteria, and 38 on the branch leading to archaea. For 37 enzymes, the data remain too scattered to reach a conclusion.
One point significantly changes how this result should be interpreted: the absence of an enzyme in LUCA does not necessarily mean that the corresponding reaction was impossible. It could have been carried out in another way, before the modern protein appeared.
The authors propose that metals present in the environment performed part of this work. Iron, nickel or cobalt can accelerate chemical reactions without an enzyme. Several previous experiments have already reproduced, using such metals and under hydrothermal conditions, reactions corresponding to more than half of today’s central metabolism.
This scenario points to hydrothermal vents, where water circulates through hot rocks and triggers numerous chemical reactions. Some of these reactions produce hydrogen and expose metals capable of transforming small molecules. The chemistry of the environment could thus have supplied part of what cells now handle themselves.
The study also provides a test involving phosphite, a phosphorus compound found in some hydrothermal environments. In the presence of metals, it can participate in adding a phosphate group to AMP to form ADP. This molecule belongs to the ATP-ADP system used by cells today for their energy exchanges. The authors also link 17 modern chemical helpers to functions that metals can replace in laboratory experiments.
The proposed scenario therefore does not imply two independent origins of life. Bacteria and archaea would have inherited the same already well-developed chemical network, then replaced some mineral helpers with different enzymes. It remains to be determined how far these enzyme-free reactions can work together in the same environment and sustainably power a network comparable to that of the first cells.