It’s a well-used riff, but it seems to be making a comeback in recent months: is there a “shadow biosphere” of lifeforms on Earth that don’t obey the known rules of biochemistry? And if so, how might we find it – let alone recognise it when if do? A nice long article; you should go read the whole thing, but here’s a few snippets:

To investigate a species of microbe fully, you first need to culture it in the laboratory and then study its biochemistry by sequencing its genome to position it on the tree of life. This technique, while undoubtedly important, has its problems.

Many microbes don’t like being plucked out of their natural habitat and cannot be cultured easily. Some resist gene sequencing.

And, because the chemical techniques used to analyse microbes are customised and targeted to life as we know it, they wouldn’t work on an alternative form of biology. Should there be a different type of microbial life out there, it is very likely to be overlooked, simply because it would be unresponsive to the biochemists’ probes used so far. In a laboratory sample it might well get thrown out with the garbage.

If you set out to study life as we know it, then what you find will inevitably be life as we know it. It’s therefore an open question whether some microbes might actually be the descendants of a different genesis.

[…]

Notwithstanding their exotic nature, to date all extremophiles that have been analysed are standard life: they belong to the same tree of life as you and me. Their existence proves that the range of conditions under which standard life can survive is much broader than previously suspected. Nevertheless there are limits.

If there is a shadow biosphere, it might be occupied by weird ‘hyper-extremophiles’ inhabiting environments beyond the reach of even the hardiest form of standard life, and have so far escaped detection because nobody thought to look for any form of life under such extreme conditions. A good example is temperature: standard hyperthermophiles seem to have an upper limit of about 130˚C – and for good reason. The intense heat disrupts vital molecules, and even with a host of repair and protection mechanisms, DNA and proteins start to unravel and disintegrate if they are subjected to temperatures much in excess of 120˚C.

Suppose we find nothing living between 130˚C and 170˚C in a deep-ocean volcanic-vent system, but then discover microbes thriving there between 170˚C and 200˚C? The discontinuity in temperature range would be a strong indicator that we were dealing with weird life as opposed to standard life that had simply pushed the temperature envelope higher.

[…]

There are plenty of other places that could be home for isolated weird extremophiles. The inner core of Chile’s Atacama Desert is one place – it is so dry and oxidising that bacteria can’t metabolise. The U.S. space agency NASA has a field station there, but so far there is no evidence for any carbon chemistry that could be attributed to weird life.

Other possible locations include the upper atmosphere, cold dry plateaus and mountain tops (where high-ultraviolet flux is a problem for standard life), ice deposits at temperatures below -40˚C, and lakes heavily contaminated with metals toxic to known life. We don’t need to confine our search to a single parameter such as temperature; it’s possible that some combination such as temperature and acidity together is more relevant.

Very speculative stuff, as science goes: it’s basically hinging on the old “white crow” aphorism, which says that the fact that you’ve never seen something doesn’t prove that the thing doesn’t exist. But we’re friends of informed speculative science around these parts, so… 🙂