We might need to rethink what we know about the oldest fossils ever found.
Some of the best evidence for early life is provided by structures called stromatolites. Many geologists assume these stromatolites were made by microbes living in shallow, sun-drenched water. This means that life, if it emerged on the deep seafloor as some scientists believe, spread to shallow regions rapidly.
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A new discovery questions that conclusion. It is a stromatolite that formed recently in the deep, dark water at the bottom of the Arabian Sea.
“I think this is unique,” saysĚý at California State University–Chico.
Layer upon layer
Stromatolites are rock-like structures made up of many thin layers. They were formed by microorganisms, some of which lived in “mats” to which sediments became attached. Similar structures still form in a few places today – including Australia’s Shark Bay.
These modern stromatolites form in shallow seas and lakes that are flooded by sunlight. As a result, researchers long assumed that stromatolites are always created by photosynthetic microbes that harness energy from sunlight to make food. However, since the 1990s a few researchers have that microbes living on the dark ocean floor – which do not use sunlight – .
We now know they can. In 2007 an expedition visited the Arabian Sea, just off the coast of Pakistan. Researchers collected what looked like a 40-centimetre-tall stromatolite from 731 metres down, in dark water containing very little oxygen. The scientists put it into a CT scanner and confirmed that it had the finely-layered internal structure of a stromatolite.
“This looks very much like a stromatolite,” agrees at the University of Connecticut in Groton, who was not involved in the study.
Life in the deep
The researchers have now worked out how the stromatolite might have formed, in the absence of light and oxygen. They suggest that microorganisms are using sulphates in the water to oxidise methane seeping out of the seafloor. This “chemosynthesis” produces sulphides. If these are then oxidised using nitrates in the water, the water will become more alkaline – triggering the precipitation of thin layers of calcium carbonate, which help build the stromatolite.
The deep-water chemosynthetic stromatolite might be a better analogue for the stromatolites from early in Earth’s history, says team member at the University of Bremen in Germany thinks. “The Shark Bay stromatolites are characterised by coarse sediment and crude layering,” he says. “The Arabian Sea stromatolites show a finely laminated fabric, typical of many [ancient] stromatolites.”
In other words, finding fossil stromatolites from 3.5 billion years ago or more is not necessarily evidence that life was thriving in light-bathed shallow seas.
“When you read textbooks, many of them will still make the point that finding stromatolites [is] evidence for shallow water environments,” says team member at the University of Hamburg in Germany. “This pattern of thought is not necessarily correct.”
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Visscher agrees that the sulphate-driven oxidation of methane could explain how the deep-water stromatolite formed. However, he says this chemosynthetic pathway probably didn’t operate on the early Earth, because nitrate probably wasn’t present in the first oceans. “It’s there now, but you can’t extend that presence much prior to about 1.8 billion years ago – which is about 2 billion years after the development of life and the first stromatolites.”
Peckmann agrees, but he says there are plenty of other forms of chemosynthesis. “We really need to delve into alternate metabolisms when viewing ancient stromatolites,” says Shapiro.
“With respect to the [early Earth], in a sense these findings open up more possibilities, rather than making things more clear,” says at the University of Minnesota, Minneapolis. “But if we want to properly understand the past, we must also appreciate and understand the complexity of the present-day microbial biosphere.”
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