Its name is LUCA. Now it’s needing second thoughts

A new artificial lifeform is helping cast light on a profound and ancient evolutionary event, reports a team of Dutch scientists.

Unravelling the intricacies of the way in which evolution played out in deep history is made all the more difficult by a patchy fossil record, and the fact that many evolutionary phenomena are not subject to fossilisation. Evidence is therefore often sparse, and scientists are forced to rely on ingenious methods to test their theories.

New research published in the Proceedings of the National Academy of Sciences is a case in point. A team from Groningen and Wageningen universities in the Netherlands has created a novel genetically engineered organism – and in the process has refuted a long-held hypothesis about the evolutionary origin of the two microbial domains of Bacteria and Archaea.

Up until the mid-2000s, single-celled Archaea were thought to belong in the same classification as Bacteria. However, genetic comparisons revealed separate evolutionary origins, placing both groups in domains of their own.

Bacteria and Archaea split from each other very early in the history of life, about 3.5 billion years ago, and this evolutionary divide was long thought to be driven by selection pressures acting on the makeup of their cell membranes, the wall that encapsulates the cell.

This is because the organic molecules that compose the cell membranes of Archaea and Bacteria are mirror images of each other, known in the jargon as stereoisomers.

Researchers have long relied on the “discordant hypothesis” to explain this. It holds that the Last Universal Common Ancestor (LUCA) of both Bacteria and Archaea had a cell membrane made up of an unstable mixture of these mirrored lipids. The instability is thought to have led to evolutionary pressures to have cell membranes made of one or the other stereoisomer, thus eliminating the instability caused by mixing.

Researchers call this the “lipid divide”, the moment when the Bacteria and Archaea split from each other.

After a breakthrough in the production of archaeal membrane lipids at Groningen University, and another related development at Wageningen, researchers were able to engineer Escherichia coli bacteria to have cell membranes that include 30% archaeal lipids. This is exactly the kind of mixed membrane that scientists thought would be unstable in the LUCA.

What the team discovered was something of a surprise.

“This bacterium grew at normal speed and was stable,” says Groningen microbiologist Arnold Driessen. “So, this result does not support the hypothesis that a mixed membrane is inherently instable and could thus have created the lipid divide.”

While the scientists caution that E. coli has had the benefit of 3.5 billion years of evolution compared to the LUCA, the results nonetheless seem to refute the discordant hypothesis.

“The robustness of these mixed cells surprised us,” says Driessen. “We expected more problems keeping them alive. After all, what we have engineered does amount to a new life form.”