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Mysterious micro-burrows in desert marble and limestone were probably made by microbes that lived millions of years ago.
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Exactly what kind of microbes bored into the rock remains unknown, as does whether they still exist or have long since gone extinct.
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Whatever formed the burrows had to be alive, since researchers were able to rule out weathering and abiotic processes.
For most organisms on Earth, rocks are objects, not food. But for one oddball microbe, desert limestone seems to have been on the menu. Whether or not this mysterious form of life still exists or went extinct eons ago is, however, yet unknown.
Recently, geologist Cees Passchier from Johannes Gutenberg University Mainz came across what looked like tiny burrows in marble and limestone in the deserts of Namibia, Oman, and Saudi Arabia. Erosion had exposed the fossil burrows, and while nothing was crawling in there anymore, Passchier and his research team investigated further and found biological material inside.
“A biotic origin of the observed structures supposes the presence of liquid water, without which biological growth would be impossible,” the team said in a study recently published in the . “The investigated areas are currently arid, but experience occasional rain showers and regular dense coastal fog, while wet periods occurred in the past.”
So, what type of microogranisms could have made the burrows? Bacteria, fungi and lichens have shown that they can survive extreme conditions, and some of them are , meaning they live inside rocks. Passchier wanted to see if the organisms that created the burrows could possibly belong to any of these groups. Fungi can bore through rocks and leave behind tubes, and some types of cyanobacteria also thrive off limestone or marble.
It is unlikely that the mystery organisms were cyanobacteria—they need sunlight for photosynthesis, so they don’t bore nearly as deep into rock as the burrows that the team found. Fungi secrete digestive agents that were not present in the rock, and they also create a complex network of hyphae, or filaments, known as a mycelium. Mycelial networks tend to have order to them. The burrows were parallel and evenly spaced, which would be unusual for fungi, and there were no other patterns observed. So, they probably aren’t the culprits, either.
Because the burrows were found to be too wide to have only been made by one organism at a time, and they showed growth rings, its was more likely that they were formed by colonies of microbes. Calcium carbonate dust found in the tunnels is also a common excretion from microbes that live in these types of rocks. However, no fossilized organisms have yet been found—just evidence of their existence.
These setbacks, however, did not rule out the idea of life. On the contrary—while weathering or abiotic chemical processes can create structures mistaken for signs of life, a thorough microscopic examination showed this was not the case. The chemical composition of rock samples from inside the burrows showed that whatever made them had to have been alive.
“As no known chemical or physical weathering mechanism can explain this phenomenon with the microstructural and geochemical observations presented here, and the micro-burrows form inside the host rock,” Passchier and his colleagues said in the same study, “we suggest that they are of biological origin.”
Whatever microbes carved out the tunnels have been long dead, though there are questions around whether the mystery species still exists. Maybe it’s still creeping around somewhere, digging new tunnel systems for us to one day uncover.
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