[New Scientist, November 25] Title: Life Originated from a Gigantic Organism that Covered the Entire Planet
Around 3 billion years ago, there was only one organism on Earth known as the "Last Universal Common Ancestor (LUCA)," which exhibited symptoms similar to sinusitis. It was colossal, an unprecedentedly massive life form that filled all of Earth's oceans. Subsequently, it split into three parts, eventually giving rise to the ancestors of all life forms present on Earth today.
The latest scientific research findings indicate that LUCA was the result of early life striving to survive. Over millions of years, LUCA attempted to transform the oceans into a global gene exchange factory. Cells striving to survive in a competition-free environment exchanged useful materials, effectively creating a gigantic organism that covered the globe.
Around 2.9 billion years ago, LUCA split into three distinct forms of life: single-celled bacteria, archaebacteria, and more complex eukaryotic cells that could evolve into animals and plants. It is difficult to determine what happened before the split. There are almost no fossil remains from that era, and any genes that could be traced back to that time may have undergone unrecognizable mutations.
Gustavo Caetano-Anolles from the University of Illinois at Urbana-Champaign said that it is not insurmountably difficult to depict what LUCA might have looked like. Although genetic sequences change rapidly, the three-dimensional structures of proteins encoded by genes are more resistant to the passage of time. He stated that if all existing organisms produce a protein with a roughly similar structure, then it is highly likely that this protein structure existed in LUCA. He refers to these structures as living fossils, pointing out that because protein function heavily depends on its structure, these living fossils can inform us about what LUCA could do.
To reconstruct the set of proteins that LUCA could produce, Caetano-Anolles searched a database containing proteins from 420 modern organisms, looking for structures common to all categories of proteins. According to his findings, only 5% to 11% of the structures were universal, meaning they preserved enough structures originating from LUCA.
Armen Mulkidjanian from the University of Osnabrück in Germany said, "There is ample evidence supporting the sharing of genes, enzymes, and metabolites by this colossal organism." Traces of this gene exchange system can still be seen in microbial communities that can only survive in mixed populations.
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