Zoologists of a certain age will remember with affection Animals Without Backbones, a classic 1938 elementary textbook by Ralph Buchsbaum that quotes a (sadly uncredited) researcher on the ubiquity of nematodes, or roundworms. Were all the matter on Earth, it says, to be made transparent apart from nematodes — the grass, the trees, the people, the animals, even the ground beneath our feet — the shapes of all these things would still be discernible, if ghostly, from their burden of worms.
It is therefore fitting, if not hugely surprising, that nematodes have now been found in the deep subsurface biosphere, a realm hitherto believed accessible only to microbes. The nematodes recovered from gold-mine workings under South Africa — reported on page 79 of this issue — are the first multicellular organisms to be discovered in such a subterranean setting.
Although underground worms may bring to mind the imaginings of science-fiction writers, these are not monsters. The cramped spaces between the grains of rocks, where these creatures presumably graze on bacteria, have straitened them. At hardly half a millimetre in length, they would barely deserve even a slither-on role in Tremors, the scifi spectacular that stars enormous worms that destabilize buildings. And our golden nematodes would be as fleas before an elephant compared with the majestic sandworms of Dune, which were the size of jumbo jets.
Yet their discovery is encouraging, even heartening, for it demonstrates, once again, that once life appears, it is hard to extinguish — and that there are few, if any, habitats in which living things might not survive, even thrive, no matter how seemingly unpromising the circumstances. Even today, when you may think that we have shaken every tree and looked under every stone on the planet, new creatures hove into view. It is rare, nowadays, to discover large vertebrates as yet unknown to science, yet such discoveries are still made.
Molecular methods have extended the reach of those formerly equipped only with traps and butterfly nets, allowing trawls of entire habitats for new forms of life — in the sea, in the upper air and even in the rich inner spaces of our guts. There is probably no slagheap too toxic, no nuclear-waste dump too radioactive, no smoking fumarole too fuming and no icy firn too frigid that it cannot be colonized by some enterprising microbe. A promise of a carbon source, a whisper of redox potential, and life will arrive. (Indeed, to encourage such endeavours, Nature promises a 500 g jar of Marmite to the research group that can identify the organism that digests this savoury spread. Tip: it is probably endemic in the intestines of Britons, to many of whom Marmite is an obsession, and absent in the more sensitive stomachs of Danes, who last week threatened to ban the comestible because of its evil added vitamins.)
Perhaps the habitats least explored and prospected for life are the most familiar: soil, sediments in ponds, and the seashore — the home of the meiofauna, creatures that make their living between particles of minerals and detritus. Meiofauna creatures at their largest are just at the verge of unaided visibility, down to about 40 micrometres in diameter — a world literally in the interstices, too small for every day, but too big to trouble micro- or molecular biologists. The meiofauna offer a community of tiny arthropods and a range of creatures otherwise encountered only in the dustier (and therefore most fascinating) pages of textbooks. This is the realm in which nematodes rule — and a kingdom to which we can now add, as Buchsbaum's unheralded scientist almost predicted, the ground deep, deep beneath our feet.
In some ways their world is hidden, inaccessible and unknown.In others it is in plain sight and all around us. To pull a nematode from the deep subsurface biosphere is an achievement. But we needn't look as far as that to find wonders that are new and strange — those who look most carefully will see a universe in a grain of sand.