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'A Planetary Mega-Organism'

About three billion years ago, life on Earth split into three domains of life—bacteria, archaea, and eukaryotes. Before that life on Earth consisted of a "planetary mega-organism" called LUCA, or the last universal common ancestor, says New Scientist's Michael Marshall. Now, researchers are busy trying to paint a picture of LUCA and determine exactly how it evolved and split into the three domains of life.

"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," Marshall says. "Cells struggling to survive on their own exchanged useful parts with each other without competition — effectively creating a global mega-organism."

Some researchers are studying the three-dimensional structures of proteins, which change less quickly than genes over time, to determine what LUCA's proteins might have looked like. Others are piecing together the evolution of certain crucial metabolic enzymes to see how LUCA used natural resources for energy, Marshall says. It's also possible, he adds, that LUCA's cell membranes were so porous and that the organism has such poor control over the proteins it produced, that genes, enzymes, and proteins all mixed together and made it easier for cells to share what they needed.

"Only when some of the cells evolved ways of producing everything they needed could the mega-organism have broken apart," Marshall says. "We don't know why this happened, but it appears to have coincided with the appearance of oxygen in the atmosphere, around 2.9 billion years ago. Regardless of the cause, life on Earth was never the same again."

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