Some of the earliest microorganisms may have been able to move around under their own power using whip-like 鈥渢ails鈥�, according to a study of fossils from 3.4 billion years ago. However, other palaeontologists say the evidence is weak 鈥� although they agree that the ability to move probably did evolve early.
The oldest confirmed fossils are 3.5 billion years old. They are all single-celled organisms like bacteria. Many researchers have claimed to have found older fossils, but none of these are widely accepted.
Researchers led by Fr茅d茅ric Delarue at Sorbonne University in Paris, France, have now described a new collection of microfossils from the Strelley Pool Formation in Western Australia. The rocks in which they were found are 3.4 billion years old.
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The cells are leaf-shaped and are 30 to 84 micrometres long, and about half that wide. The team dug them out by dissolving the surrounding rock using acid.
Such 鈥渕icrofossils鈥� sometimes turn out to be peculiar rock formations, so the team performed a number of chemical tests to confirm that they are the remains of living organisms. 鈥淲e observe nitrogen and phosphorus that are preserved in the fossils,鈥� says team member Romain Tart猫se at the University of Manchester in the UK 鈥� these are characteristic elements of life.
Abderrazak El Albani at the University of Poitiers in France says he is convinced the fossils are genuine. 鈥淚t鈥檚 very good and very well documented,鈥� he says.
But exactly what the fossils demonstrate is more controversial. A handful of the fossilised cells have a short rod sticking out at one end, which the team calls a 鈥渓ash-like appendage鈥�. 鈥淚 have about 500 specimens,鈥� says Delarue. Only four have the appendage. 鈥淢ost of them lost it during geological history,鈥� he says.
Delarue says these appendages could be the earliest evidence of an organism able to move by itself. They 鈥渨ere likely providing them with swimming capability鈥�, he says. El Albani declined to comment on this aspect of the study.
Today, microorganisms use several different methods of locomotion. Many have a flagellum: a whip-like tail made of protein that rotates at high speed and powers the cell through water. Others move using tube-shaped prosthecae, which are extensions of their outer membrane.
Delarue and his colleagues argue that the microfossils鈥� appendages are similar and must also have been used to get around. 鈥淔or me, it鈥檚 probably the finding of my life,鈥� he says.
The researchers say they cannot be sure exactly what kind of appendage the cells had. 鈥淲e probably have only a part of a locomotor organelle,鈥� says Delarue. 鈥淲e don鈥檛 know whether it was entirely preserved.鈥�
The limited number of cells with lash-like appendages is a big problem, says Tanja Bosak at the Massachusetts Institute of Technology. 鈥淚t鈥檚 really hard to make too strong a conclusion.鈥�
Bosak also points out that the appendages are much thicker than any known bacterial flagella. 鈥淭here鈥檚 nothing we know of that really looks like this today,鈥� she says. The team argues that the cells themselves are larger, meaning that the lash-like appendages are roughly the same relative size as a bacterial flagellum compared to a bacterium.
Mobility has obvious advantages, enabling microorganisms to move towards food supplies or light, so it may well have evolved early, says Bosak. 鈥淵ou just need proteins with certain properties,鈥� she says. The problem is that organelles like flagella are fragile and would probably not preserve as fossils. It may be better to use genetics to estimate when the key genes responsible for mobility first evolved, says Bosak.
Reference: bioRxiv, DOI:
