Opinion (Gustavo Caetano-Anolles, from NewScientist): “Once upon a time, 3 billion years ago, there lived a single organism called LUCA. It was enormous: a mega-organism like none seen since, it filled the planet’s oceans before splitting into three and giving birth to the ancestors of all living things on Earth today.
“This strange picture is emerging from efforts to pin down the last universal common ancestor – not the first life that emerged on Earth but the life form that gave rise to all others.
“The latest results suggest LUCA was the result of early life’s fight to survive, attempts at which turned the ocean into a global genetic swap shop for hundreds of millions of years. Cells struggling to survive on their own exchanged useful parts with each other without competition – effectively creating a global mega-organism.
“It was around 2.9 billion years ago that LUCA split into the three domains of life: the single-celled bacteria and archaea, and the more complex eukaryotes that gave rise to animals and plants. It’s hard to know what happened before the split. Hardly any fossil evidence remains from this time, and any genes that date that far back are likely to have mutated beyond recognition.
“That isn’t an insuperable obstacle to painting LUCA’s portrait, says Gustavo Caetano-Anollés of the University of Illinois at Urbana-Champaign. While the sequence of genes changes quickly, the three-dimensional structure of the proteins they code for is more resistant to the test of time. So if all organisms today make a protein with the same overall structure, he says, it’s a good bet that the structure was present in LUCA. He calls such structures living fossils, and points out that since the function of a protein is highly dependent on its structure, they could tell us what LUCA could do.
“To reconstruct the set of proteins LUCA could make, Caetano-Anollés searched a database of proteins from 420 modern organisms, looking for structures that were common to all. Of the structures he found, just 5 to 11 per cent were universal, meaning they were conserved enough to have originated in LUCA.
“By looking at their function, he concludes that LUCA had enzymes to break down and extract energy from nutrients, and some protein-making equipment, but it lacked the enzymes for making and reading DNA molecules.”