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鈥檚 history. LUCA emerged at least 3.9 billion years ago, and relatively soon after聽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聽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聽LUCA鈥檚 habitat.
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Many biologists have argued that LUCA lived somewhere hot, like a geothermal pond, where temperatures exceed 50掳C or even 100掳C. They point to the many primitive archaea alive today that are adapted for heat. Organisms that live above 50掳C are called thermophiles, while the hardy few聽that endure 80掳C or more are known as hyperthermophiles.
LUCA鈥檚 genome could provide a聽clue as to which category it belongs in. Being so ancient, no聽specimens of this organism remain, but in 2016, a team led by聽Bill Martin at Heinrich Heine University D眉sseldorf in Germany looked for universal genes found in some of the oldest branches of life, which are .
Martin鈥檚 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 鈥渕esophiles鈥� that live below 50掳C. Its presence suggested that LUCA was at least a thermophile.
However, Catchpole and Forterre aren鈥檛 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鈥檚 time. Their tree didn鈥檛 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聽wasn鈥檛 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, ).
鈥淭he story is not unrealistic,鈥� 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聽transferred to the few thermophilic bacteria. But it could also be that LUCA did have聽the gene and it was then lost early in the evolution of bacteria. 鈥淭he two scenarios cannot be distinguished,鈥� says Pisani.
The key point about LUCA鈥檚 genes is that they imply it lived in聽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掳C聽and much of the water is tens of degrees cooler, so the reverse gyrase gene might not have been essential.
鈥淭he fact we found reverse gyrase in there, that鈥檚 nice, but it鈥檚聽just one gene out of 355,鈥� says聽Martin.
