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Published: 2023-04-28

Study provides new insights into the evolution of complex life

NEWS How a simple prokaryotic cell, like a bacterium, first ‘swallowed’ another cell and thereby created the conditions for more complex life is a mystery. But one key to this puzzle has been uncovered, thanks to a new study by researchers from Umeå University among others.

Text: Sara-Lena Brännström

One of the great mysteries of biology is how eukaryotes arose, the group of organisms to which humans, animals and plants belong. Scientists consider this to be critical to our understanding of the evolution of life on Earth.

Eric Libby at Umeå University is the lead author of a paper recently published in the scientific journal PNAS. He has investigated the mystery together with Professor Christopher Kempes and Jordan Okie from Santa Fe Institute and Arizona State University.

“Metabolism is a fundamental challenge. If one cell swallows another, can both grow?” says Eric Libby, associate professor at the Integrated Science Lab (IceLab).

Explanation of terms

Prokaryots: Bacteria and archaea. Their cells lacks a membrane-enclosed nucleus and other organelles, and they are generally single-celled.

Eukaryots: Other organisms. Their cells have a membrane-enclosed nucleus and several other types of organelles where various life functions take place.

Endosymbiosis: A form of symbiosis between two organisms in which one, the endosymbiont, lives inside the other, the host.

Evidence suggests that eukaryotes formed when two prokaryotes — a bacterium and an archaeon — merged with bacteria taking up residence within the cell walls of archaea. This cooperative living of one cell within the other led to an entire diversity of eukaryotes, including all complex life such as us.

Traces inside modern organisms

Today, scientists see the traces of this so called endosymbiosis inside the cells of most modern eukaryotes, from mammals and birds to plants and fungi: cellular organelles like mitochondria and chloroplasts were once separate organisms. Yet, when we look around in nature, endosymbioses are rarely seen in prokaryotes.

Why? Evolutionary biologists don’t yet know. Many theories exist, but with this study the role of metabolic compatibility has been explored.

The research team used three large databases with models of the complete genomes of a variety of prokaryotes to test evolutionary stages that might limit endosymbiosis. The results show that most pairings were less fit and less evolvable than their ancestors, but not always.

Unexpected results

“In some sense, it is surprising how over half of the possible endosymbioses between prokaryotes might actually survive. It was also surprising that given two genomes in endosymbioses, they are less able to adapt than their single-genome ancestors. Both of these results went against our initial expectations,” says Eric Libby.

The findings of this study suggest that the metabolism is likely not the limiting factor in prokaryotic endosymbiosis. The next steps for the researchers will be to assess other theories, like the role the environment may play, or how one prokaryote takes in another. This is key to understanding how life may have evolved on Earth, the chances that it might exist elsewhere in the Universe, and the possibility of creating it in a lab.

About the scientific article

Eric Libby, Christopher P. Kempes, Jordan G. Okie (2023). Metabolic compatibility and the rarity of prokaryote endosymbioses. PNAS (Proceedings of the National Academy of Sciences) 120 (17). DOI: 10.1073/pnas.2206527120

For more information, please contact:

Eric Libby
Associate professor