Arsenic bacteria: what does it mean?

I suspect there are more than a few people feeling disappointed at the recent kerfuffle in which NASA scientists reported that they had found ‘alien life’ right here on Earth. Some people had expected or hoped that the announcement would be about Mars, or Titan, but after some reading yesterday, I concluded that neither of those options would be likely. Usually, when an announcement comes, there is a trail of scientific publications leading up to it. There wasn’t anything significant that pointed in the direction of Mars or Titan beyond what we already have. Instead, it was announced that scientists had found life in conditions where none had been thought possible, on Earth.

The significance? Well, life as we know it relies on six major elements: hydrogen, nitrogen, carbon, oxygen, sulphur and phosphorus. It was assumed that all life needs those elements, so searches for life elsewhere in the universe concentrated on the presence of these elements. What these new results prove is that it need not be so. That may not sound revolutionary. After all, there has been speculation on ammonia-based (oxygen-less) life on Titan, but speculation and providing absolute proof are two entirely different things. The major achievement of the latest results is that they provide the proof: not all organisms need to have phosphorus. Indeed, it may mean that some organisms exist that have no need for any of the other ‘essential’ elements either.

But does that make them alien, as in, non-Earthly? Well, uhm, no.

There are a few things to be noted. First, the scientists report that the bacteria in question could live as well with phosphorus as with arsenic in their DNA. In other words, arsenic was not mandatory, but rather an alternative in case phosphorus wasn’t present. Secondly, no one speculates whether this ability is a function of a highly specialised adaptation to the environment, or whether it’s a relic from some ancient life form.

Current taxonomy assumes that all life arose from a single incidence and that from there, life evolved and spread out, became more diverse. This is the tree of life concept. It assumes that life on Earth originated once. But what if this assumption is wrong and life arose more than once, in different forms? The different forms would be totally different types of life, and possibilities that some of this life still lingers is what is meant by the term shadow biosphere, which is well-explained here. But if these bacteria were truly from a separately-evolved shadow biosphere, they would be far more different from life as we know it than these arsenic-loving ones.

As is, I don’t think the research, while significant, proves anything beyond the fact that life can exist in places where we thought it couldn’t. It means we can now look for arsenic as indicator for the presence of life elsewhere. The discovery as to whether life has evolved more than once, either on Earth or elsewhere, will have to wait a bit longer.

ETA: It seems a veritable scientific storm has broken out over the veracity of the results of the papre, with a rather scathing review here

2 comments on “Arsenic bacteria: what does it mean?

  1. Still, I was quite excited. A couple of years ago, NewScientist ran a feature about the possibility of life existing on different planets and moons, and what their base biology might consist of. When I read about the arsenic bug, I thought, here we go, it wasn’t just extremely out there cogitating. Plus, there are surely stories in it?

    PS, Your link, “which is well explained here” threw up a 404.

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