You have probably heard this story by now:
But now researchers have discovered a bacterium that appears to have replaced that life-enabling phosphorus with its toxic cousin arsenic, raising new and provocative questions about the origins and nature of life.
News of the discovery caused a scientific commotion this week, including calls to NASA from the White House asking whether a second line of earthly life has been found.
Whether or not the bacteria actually replace phosphorus with arsenic is something that will eventually be sorted out. But for now, we can be confident that no “second line of earthly life has been found.”
“This is different from anything we’ve seen before,” said Mary Voytek, senior scientist for NASA’s program in astrobiology, the arm of the agency involved specifically in the search for life beyond Earth and for how life began here.
“These bugs haven’t just replaced one useful element with another; they have the arsenic in the basic building blocks of their makeup,” she said. “We don’t know if the arsenic replaced phosphorus or if it was there from the very beginning – in which case it would strongly suggest the existence of a shadow biosphere.”
Yes, we do know it wasn’t there from the beginning. How?
16S rRNA sequencing identified these bacteria as a species of Halomonas. These bacteria belong to the group Gammaproteobacteria, specifically the Oceanospirillales. Below is a phylogenetic tree.
We would find this “shadow life form” among the small branch labeled proteobacteria. That branch would split (not shown) into smaller branches labeled alpha, beta, gamma, etc. The tiny gamma branch would split (not shown) into twigs, one labeled as Oceanospirillales. And this twig would split into tiny twigs, where one would be this new species.
Look, if these bacteria were examples of “shadow life,” they would not fit anywhere on that entire tree! They would represent an entirely new domain. No, they would represent something even more radically different, such that the entire tree of life would be separated from the single lineage of bacteria that tolerate arsenic. They are not going to nest among a particular lineage of gammaproteobacteria.
Think of it this way. Say that someone discovered a new species of field mouse that was able to incorporate arsenic into its DNA. Would anyone seriously entertain the notion that these mice represented some shadow chordates? Because, that is basically what the NASA scientists are selling.