[New Scientist website, November 25] Title: Life Originated from a Gigantic Organism that Covered the Globe
Around 3 billion years ago, there was only one organism on Earth known as the "Last Universal Common Ancestor (LUCA)." It was a massive entity, an unprecedentedly large life form that filled all of Earth's oceans. Subsequently, it split into three parts, eventually giving rise to the ancestors of all life on Earth today.
The latest scientific research indicates 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 trying to survive in an environment without competition exchanged useful materials with each other, effectively creating a gigantic organism that spanned the globe.
Approximately 2.9 billion years ago, LUCA split into three distinct forms of life: single-celled bacteria, archaebacteria, and more complex eukaryotic cells that could later evolve into animals and plants. It is difficult to determine what happened before this 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-Anollés from the University of Illinois at Urbana-Champaign said that reconstructing what LUCA looked like is not insurmountable. Although genetic sequences change rapidly, the three-dimensional structures of proteins encoded by genes are more resistant to the test of time. He stated that if all existing organisms produce a roughly similar protein structure, then it is highly likely that such a protein structure existed in LUCA. He referred to these structures as living fossils and pointed out that since protein functions are heavily dependent on their structures, these living fossils can tell us what LUCA could do.
To reconstruct the set of proteins LUCA could produce, Caetano-Anollés searched through a database containing proteins from 420 modern organisms, looking for structures common to all classes of proteins. According to his findings, only 5% to 11% of the structures were universal, meaning they retained enough LUCA-originated structures.
Armen Mulkidjanian from the University of Osnabrück in Germany said, "There is ample evidence supporting the sharing of genes, enzymes, and metabolites by this enormous 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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