THE ancestor of all life on Earth probably preferred moderate temperatures, not scorching heat as some biologists believe. The finding could shed light on where such early organisms lived, but only if it is confirmed.
Everything alive today can be traced back to the last universal common ancestor (LUCA), a single-celled organism that appeared early in Earth’s history. LUCA emerged at least 3.9 billion years ago, and relatively soon after split into two groups called bacteria and archaea, which today make up the majority of all living species. More complex organisms made of multiple cells, like sponges, elephants and us, only appeared billions of years later.
Ryan Catchpole and Patrick Forterre of the Pasteur Institute in Paris have re-examined the genetic evidence that LUCA was adapted to extreme heat. They think earlier work may have incorrectly traced a key gene, changing our understanding of LUCA’s habitat.
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Many biologists have argued that LUCA lived somewhere hot, like a geothermal pond, where temperatures exceed 50°C or even 100°C. They point to the many primitive archaea alive today that are adapted for heat. Organisms that live above 50°C are called thermophiles, while the hardy few that endure 80°C or more are known as hyperthermophiles.
LUCA’s genome could provide a clue as to which category it belongs in. Being so ancient, no specimens of this organism remain, but in 2016, a team led by Bill Martin at Heinrich Heine University Düsseldorf in Germany looked for universal genes found in some of the oldest branches of life, which are .
Martin’s team found 355 such genes. One, which encodes for a protein called reverse gyrase, is essential for hyperthermophiles. Although it is unclear what the gene does, it is found in all hyperthermophiles and some thermophiles, but not in “mesophiles” that live below 50°C. Its presence suggested that LUCA was at least a thermophile.
However, Catchpole and Forterre aren’t so sure. They identified 376 genes for reverse gyrase from 247 bacteria and archaea, and built a family tree to see how they had been inherited since LUCA’s time. Their tree didn’t match the known family trees for bacteria and archaea, suggesting the reverse gyrase gene had been repeatedly transferred between species.
They argue that this means it wasn’t present in LUCA, but instead arose in an early archaean. If LUCA lacked the reverse gyrase gene, it cannot have been a hyperthermophile and, the pair say, was probably a mesophile (bioRxiv, ).
“The story is not unrealistic,” says Davide Pisani at the University of Bristol, UK, but the results can be read in two ways.
One is that LUCA lacked the reverse gyrase gene, which arose in an early archaean before being transferred to the few thermophilic bacteria. But it could also be that LUCA did have the gene and it was then lost early in the evolution of bacteria. “The two scenarios cannot be distinguished,” says Pisani.
The key point about LUCA’s genes is that they imply it lived in an alkaline vent on the sea floor, says Martin. He has long argued that such vents were the birthplace of all life. Alkaline vents are never hotter than 90°C and much of the water is tens of degrees cooler, so the reverse gyrase gene might not have been essential.
“The fact we found reverse gyrase in there, that’s nice, but it’s just one gene out of 355,” says Martin.
