鈥淚t鈥檚 like finding Moby Dick in Lake Ontario,鈥� says of the nematode worms his Princeton University team discovered living far beneath the Earth鈥檚 surface in South Africa.
The tiny worms 鈥� just 500 micrometres long 鈥� were found at depths ranging from 900 metres to 3.6 kilometres, in three gold mines in the Witwatersrand basin near Johannesburg. That鈥檚 an astonishing find given that multicellular organisms are typically only found near the surface of the Earth鈥檚 crust 鈥� Onstott鈥檚 best guess is in the top 10 metres.
The creatures seem to live in water squeezed between the mines鈥� rocks, can tolerate temperatures reaching 43聽掳C and feed off bacteria. Carbon dating of compounds dissolved in the water suggests that the worms have been living at these depths for between 3000 and 12,000 years.
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鈥淭o have complex life sustain itself for such a long period completely sealed away from everything else 鈥� from sunlight, from surface chemistry 鈥� is pretty amazing,鈥� says Caleb Scharf of the Columbia Astrobiology Center in New York City.
No place for a worm
Onstott says no one thought multicellular organisms would be found living in this so-called fracture water. He points out that microbiologists are still trying to prove and understand how even single-celled organisms can exist at these depths. 鈥淭he lack of oxygen, temperature and food is a big dissuader,鈥� he says.
鈥淲e鈥檝e had this preconception that there can only be certain types of organisms in certain environments,鈥� says Scharf. 鈥淏ut it鈥檚 not true at all. There are more complex organisms in these bizarre environments.鈥�
Animals on Mars
If complex life forms are able to survive inside cracks deep inside Earth, it raises the possibility that they might have survived undetected in similar environments on Mars.
, director of NASA鈥檚 Astrobiology Institute in Moffett Field, California, points out that Onstott has previously discovered a bacterium living 2.8聽kilometres underground, completely isolated from all other ecosystems on Earth (Science, DOI: ). The bug gets its energy from the radioactive decay of elements in the surrounding rocks. 鈥淭he significance was that you could imagine an ecosystem existing in the subsurface of a planet that didn鈥檛 have a photosynthetic biosphere, like Mars,鈥� he says.
Until now, it was thought such an ecosystem could be made of bacteria only. But Onstott鈥檚 new findings have completely changed that. 鈥淚t has extended the [earlier] work to an animal,鈥� says Pilcher.
鈥淭hese nematodes are grazing on microbes. So now you could imagine that if animal life had ever developed on a planet, and the surface of that planet became lifeless,鈥� Pilcher explains, 鈥測ou could imagine that animals [small enough to fit in tiny cracks] could coexist with microbial ecosystems all powered by radioactivity.鈥�
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