Life does all kinds of things to a planet. Organisms use a world like the Earth as their personal garbage can. They dump out all manner of noxious substances in the name of metabolism. A good example is oxygen. Early photosynthetic bacteria started releasing molecular oxygen to the atmosphere perhaps as far back as 3.5 billion years ago, and possibly earlier. That oxygen was toxic to many species, but over time – as we well know – natural selection managed to come up with variants that were not only happy being bathed in oxygen but could respire it and in doing so jump onto a metabolic express train.
Today we look at oxygen in a planetary atmosphere as a possible planetary biosignature (albeit an imperfect one). Other compounds might also give away the presence of life on a distant world; atmospheric gases like methane or nitrous oxide.
But life doesn’t just mess with planetary chemistry, it also rearranges the way a planet looks. Mostly that kind of influence is subtle. Back in 2006 the scientists William Dietrich and Taylor Perron published a terrifically interesting paper in Nature called “The search for a topographic signature of life”. One thing they pointed out was that while biotic influence on phenomena like rock weathering, river dynamics, and soil erosion is quite profound in the short term, deducing that life is behind what we see (whether on Earth, Mars, or beyond) is really tricky.
All of which makes a new finding on termite-built structures in northeastern Brazil all the more intriguing. In the journal Current Biology Stephen Martin and colleagues report on “A vast, 4,000-year-old spatial pattern of termite mounds”.
These are impressive structures. Each conical mound is on average 2.5 meters high and about 9 meters across. They’re not nests, but rather the dumping of soil from an equally vast tunnel network made by the termites – tunnels currently occupied by a single species. There are at present about 200 million of these mounds – quite regularly spaced and covering an area the size of Great Britain.
As the researchers report, this enormous spread was built by the termites over thousands of years, and involved them excavating and shaping 10 cubic kilometers of soil. The regularity of spacing is likely an emergent feature due to the nature of the underlying tunnel connectivity and the availability of plant food.
This topographical feature is indeed visible, quite easily, from space – as shown here in imagery taken directly from Google Earth (also provided by other sources in the image credit below). It’s a remarkable example of the physical imprint of life on a planet – not by large animals, or even human technology, but by billions of teeny-tiny organisms going about their business century after century.
Without knowing it (or even being capable of knowing it), these busy and supremely successful termites have actually made themselves available for cosmic scrutiny. No small feat.