(Apologies for the title, but I had to work a pun in somehow…)
NASA’s cryptic announcement of news that would impact the “search for extraterrestrial life” led to a lot of speculation – mostly among those hoping for the announcement of the definitive discovery of life elsewhere.
After the press conference yesterday, it turns out the story was instead about newly-discovered life on Earth. Scientists funded by NASA’s astrobiology program have discovered bacteria living deep in the mud of Mono Lake in California which have the ability to use the element arsenic, which is toxic to nearly all other life. NASA reports:
“Researchers conducting tests in the harsh environment of Mono Lake in California have discovered the first known microorganism on Earth able to thrive and reproduce using the toxic chemical arsenic. The microorganism substitutes arsenic for phosphorus in its cell components.”
What’s remarkable is that these bacteria do not simply tolerate arsenic (as some other organisms are known to do), they actually appear to incorporate it into their fundamental biochemistry.
Life is chemically based on certain fundamental types of organic (carbon) compounds that are found in all organisms. In some of these compounds, the element phosphorus is an indispensable ingredient. A good example is DNA: life’s ubiquitous genetic material is composed of molecules called nucleotides, which consist of a nitrogenous base, a sugar, and a phosphate group which serves as the backbone that holds DNA together.
It seems that these newly discovered bacteria are able to use arsenic instead of phosphorus in these compounds. Phosphorus and arsenic are in the same chemical family, so the replacement is understandable in terms of chemical similarities between the two elements, but arsenic is nevertheless different enough that the fact that these bacteria can use it in place of phosphorus is astonishing, and just how they can do so is unclear. In fact, it is arsenic’s tendency to replace phosphorus and gum up the biochemical works of cells that makes arsenic so toxic – yet these bacteria seem to use that very ability to their advantage.
Despite the excitement, a couple of things should be noted about this discovery:
First, the bacteria do not use arsenic preferentially or as a matter-of-course. Under normal conditions, they use phosphorus just like all other life as we know it. It is only under extreme laboratory conditions in which the bacteria were grown in an arsenic-rich and phosphorus-poor environment that they seemed to replace their phosphorus with arsenic.
Secondly, like all major scientific announcements, this one will need to go through rigorous confirmation. NASA invited astrobiologist Steven Benner to act as devil’s advocate:
“I’m the guy they bring in to throw the wet blanket over all the enthusiasm,” [Benner] joked.
He was impressed by the finding that bacteria could get by with so little phosphorus and so much arsenic, but he questioned the conclusion that the arsenic was truly taking the place of phosphorus. Benner explained that chemists have long been familiar with the properties of arsenate compounds. “We know, for example, that they fall apart in water quickly,” he said. “Those structures are not going to survive in water.””
Biologist are undoubtedly already planning to repeat and refine the initial observations to see if these findings will stand up to scrutiny.
Assuming the discovery does hold, commentary on its significance has been varied. NASA’s own emphasis has been on the implications of the discovery of life on earth with a radically different biochemistry on the search for extraterrestrial life. Other commentators have discussed the discovery in terms of evolution, some claiming it supports and others that it challenges standard evolutionary science.
What I find most interesting about the discovery, however, is its confirmation of the view that life must be understood from the top-down, not from the bottom-up as the scientific instinct would have it. The scientific tendency is to understand things solely in terms of the parts that make them up; thus life is just a product of certain kinds of atoms and molecules. In this view, if we discover another set of molecules participating in the set of activities we call “life”, we have fundamentally changed our idea of what life is. This is the thinking behind the oft-repeated statement of NASA official Ed Weiler that this discovery “expands the definition of life.” Life was previously the activity of a certain set of molecules – and now it includes a wider set of molecules. This reductionist view is rightly a centerpiece of scientific investigation – but it does have its limits.
For instance we can think more deeply of life in terms of living beings as a whole or a unity, not a collection of parts. A living thing is not just a set of biomolecules, it is an entity that is fundamentally one and carries out immanent activity. In this view, adding new elements to the molecules of living beings is an expanded description of life, but not really a new definition.
Indeed, the most remarkable thing about this discovery is not that the bacteria simply used a different fundamental biochemistry, but that they were able to change their own fundamental biochemistry. If we think of organisms as being built-up from their basic parts, this is puzzling. If we think of them as wholes that can act on their own basic parts, however, it makes more sense. This is life that is not merely the result of basic parts working from the bottom-up; this is life that is working from the top-down to adjust its own basic parts. Even among bacteria, commonly considered the simplest form of life, we cannot reduce living things to simple sums of biochemistry.
