Fossils in rocks that are 3.8 billion years old have puzzled biologists as they look nothing like modern cells, but now it seems they may be an ancient precursor life form that was unable to control its structure
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| Unusual cells without walls can create structure (main image) that resemble those found in 3.4-billion-years-old rocks from Buck Reef, South Africa. (Credit: Dheeraj Kanaparthi et al.2024) |
The oldest rocks on Earth contain mysterious fossils of cells that appear to be unlike anything alive today, leaving biologists puzzled as to how they evolved. Now, experiments have shown that these fossils could be of primitive cells that lacked the ability to fully control their shape, making them a precursor to the modern cells we see today.
Sedimentary rocks that formed up to 3.8 billion years ago – not long after Earth itself – appear to contain fossilised cells that could be traces of very early life. These microfossils, first unearthed in 1987, have been found in multiple locations around the world, but they aren’t what biologists expected.
The first living cells are assumed to have been even simpler than the simple cells alive today, such as bacteria. Modern bacteria are tiny – just 1 or 2 micrometres wide – with no internal structures. But the fossilised cells found in ancient rocks are much larger, at around 60 to 70 micrometres across, and seem to have internal structures, says Dheeraj Kanaparthi at the Max Planck Institute of Biochemistry in Germany. “These fossils look too complex. They’re also too large,” he says. “What we find is always baffling.”
As a result, there is intense debate about what created these fossils and whether they really are the remains of living cells at all. Now, Kanaparthi has stumbled across a possible answer while studying bacteria growing around freshwater springs at the bottom of the very salty Dead Sea. Some of these bacteria had bizarre shapes, and he eventually realised that they were a type of cell without a cell wall, named L-forms after the Lister Institute in London where they were discovered in the 1930s.
Normally, the rigid cell wall of bacteria determines their shape, like putting a water balloon in a box, but stressful conditions can strip this wall away. In most situations, such “naked” bacteria swell and burst due to osmosis, as they absorb too much water. But in salty conditions, these L-forms can survive and sometimes even grow and replicate.
Kanaparthi and his colleagues have now shown that when L-forms are put in high salinities like those that might have existed in the coastal regions where the ancient fossils formed, they acquire structures that closely resemble some of the enigmatic microfossils.
Depending on which salts are present in what levels, the cells grow very large and lots of new cells form within them, creating the illusion of internal structures like those in complex cells. The new cells are released when the parent cell bursts apart. These L-forms closely resemble microfossils found in the 3.4-billion-year-old rocks of the Strelley Pool-formation in Western Australia.
In other conditions, the L-forms grow into long strings that split into separate cells, resembling microfossils found in the 3-billion-year-old Cleaverville formation, also in Australia.
“We thought these cells would just grow into large bubbles and then split apart into 1000 pieces,” says Kanaparthi. “But actually, they reproduce in a very defined manner.”
In the absence of a cell wall, the shape of these bacteria and how they reproduce are determined by the conditions to which the L-forms are exposed rather than their genes, the team found. “They’re at the mercy of the environment,” says Kanaparthi.
Their reproduction is also rather inefficient, as lots of the content of the parent cell tends to leak out during the process. But this leakiness could explain mysterious carbon deposits found outside some microfossils, says Kanaparthi. “We were able to reproduce not just the morphologies of the cells, but also all the associated organic carbon structures.”
The team also looked at how L-forms might be preserved and fossilised. They showed, for instance, that the remnants of L-forms can generate microscopic structures that resemble those found in the 3.4-billion-year Buck Reef rocks in South Africa (see above).
Based on these findings, the team proposes that primitive cells also lacked a cell wall and weren’t in full control of their shape and reproduction. They may only have gained this ability when cell walls evolved around 2.5 billion years ago.
“They have done a very comprehensive job and find examples of morphologies that bear striking resemblances to a wide range of proposed microfossils,” says Jeffery Errington at the University of Sydney, Australia, who, in 2013, first suggested that L-forms might reflect what primitive cells were like.
“Most scientists would be cautious because 3.5 billion years is a very long time for a fossil to be preserved,” says Errington. “Nevertheless, the results certainly provide support for the idea that primitive L-form-like cells inhabited the planet soon after the Earth cooled sufficiently to support carbon-based chemistry.”
“It is challenging to determine if the conditions used truly reflect those of early Earth,” says Dennis Claessen at Leiden University in the Netherlands. “Nonetheless, I agree that these cells could represent how early life forms proliferated and explain the early microfossils discovered.”
bioRxiv DOI: 10.1101/2021.08.16.456462
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