It's life Jim, but ..... 5/6 how we know it

Nov. 29, 2010:

NASA to Hold News Conference on Astrobiology Discovery
Science Journal Has Embargoed Details Until 11 a.m. PST On Dec. 2


It was to be three nail biting days of anxious waiting. Stress levels in scientists across the globe rose to values only previously seen the week before Apple's iPad release. At least half of Oregon packed a suitcase ready to be told alien Spock had made first contact and his buddy Tuvok had a spare room for guests. Kentucky loaded their shot guns.

Then the moment of truth dawned:

There's a bacterium 1/100th of the size of a human hair that hangs out in some pond in California and lives off arsenic.

Whereupon approximately half the audience fell asleep, half went into a state of frenzied excitement and the one person to actually understand its implications commented
that this complicated matters. To know exactly why this one person felt this way, we need to understand what makes you and the microbes in your kitchen sink blood brothers.

We have long known that life can exist in some pretty unlikely places. Organisms aptly known as 'extremophiles' have been found to thrive in temperatures exceeding 100
°C (hyperthermophiles), in solutions more acidic than lemon juice (acidophiles) and under bombardment of powerful ionising radiation (radioresistent). So while California's Mono Lake with one of the highest concentration of the deadly toxin arsenic on the planet might not make a swim resort, it is perhaps not astonishing that it should still harbour life.

However, the extremophiles, you, me and the microbes in your kitchen sink are all composed of six major elements;
carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorus, with trace other ingredients as icing to the biological cake. Until the NASA press release, these six fundamental building blocks were common to all forms of known life. In fact, while one could speculate otherwise, there was no evidence that life didn't require these six elements to exist. This meant that searches for life on other planets seemed to necessitate the detection of the presence of these seemingly essential ingredients.

The difference with the bacterium found in Mono Lake is that it is the first living entity to have been discovered that violates this cardinal rule. Instead of phosphorus, this microbe can use arsenic in its DNA. Since phosphorus is vital to cell reproduction, it might be considered a controversial move to substitute it for the second most favourite poison in Agatha Christie's famous crime novels (the first is cyanide, in case anyone was interested). It turns out that arsenic is actually chemically similar to phosphorus, sitting directly below it in the periodic table. This makes it a viable alternative.

The discovery that life can form using a different fundamental base of elements is immensely important for scientists who have been trying to imagine what life on other worlds would be like. However, there is one thing that this discovery is not which, if it were, would cause every astrobiologist in the world to pass out for at least a week.

It is not an unrelated form of life to us.

All currently known forms of life have a common origin. Nevertheless, it is possible to conceive that life might develop in multiple places on Earth independently of one another. An occurrence of a second genesis of life on Earth is known as a 'shadow biosphere' and could evolve in a completely different way to life that we know. If such a system were found, it would be evidence that life is not difficult to produce in the right conditions. That being so, it would increase the odds of life being present on other planets considerably.

Basically, it would be time to check that your best friend isn't from a small planet somewhere in the vicinity of Betelgeuse.

The case of the bacterium in Mono Lake is not a shadow biosphere. While this microbe can use arsenic, it can also use phosphorus and indeed will prefer to when given the choice. Moreover, Mono Lake has only become heavy in arsenic over the last 50 years. Prior to that, it had a source of fresh water, removing the environment in which an arsenic-dependent microbe could develop. This points to an organism that originally conformed to the established six building blocks of life, but evolved to survive in the increasingly hostile environment of Mono Lake.

While it may not be a new form of life, this tiny microbe opens the door to a huge number of questions in astrobiology. It particular, its discovery proves that we will have to be significantly more open minded when searching for life outside our planet. If there is no core pattern that nature cannot adapt when required, then what signatures can we design our detectors to search for? In fact, this microbe hasn't so much as opened a door as ripped it off its hinges.

References: the original paper for this research is science.1197258. There are also two excellent reviews at 'Not Rocket Science' and 'Bad Astronomy'.