Are these the oldest crawl tracks on earth?

2.1 billion years old traces in the sediment give Pal ontologen R tsel on

2.1 billion years ago, amoeba-like creatures may have left these traces in the sediment. © Abderrazak El Albani
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Enigmatic discovery: In Gabon, paleontologists have discovered 2.1 billion year old thread-like fossils that do not fit into any scheme. Because they are too big for bacteria, but too old for multicellular animals. Those who left these millimeter-thick traces, the researchers can therefore only suspect. According to this, temporary associations of amoebae-like unicellular organisms could have left these fossilized traces of mucus. If confirmed, it would be the earliest evidence of motile organisms.

When did the first organisms on our planet come into existence? So far, this question is controversial, because fossils of such animals are difficult to distinguish from purely geochemical deposits. It seems that about 3.5 billion years ago the first bacteria were already present. By contrast, 3.7 billion years ago bacterial fossils proved to be nonbiological. The first multicellular animal lived according to previous findings after "only" 558 million years ago.

Thread-shaped traces in the shale

Another early life testimony may have been discovered by Abderrazak El Albani of the University of Poitiers and his team in Gabon. In the southeast of this African country was about 2.1 billion years ago a shallow coastal sea, whose mud is preserved today as fine, dark slate deposits. In this formation, the researchers came across unusual thread-like structures.

The fugitive tracks are made of pyrite, are up to six millimeters thick and can be 17 centimeters long. Abderrazak El Albani / CNRS, Università de Poitiers

"The filamentary structures are up to six millimeters thick and extend up to 170 millimeters through the sediment, " the palaeontologists report. Most of these creeks are straight or slightly twisted and unbranched, there are also some creeks that unite or divide into one. "The texture and mineralogical composition inside these thread structures and their environment are clearly different, " the researchers said.

Petrified traces of mucus?

But what is it? According to El Albani and his team, the shape and composition of these sites do not match abiotic structures such as bare rock crevices or petrified traces of liquid. Instead, they must be fossils, according to the convincing of the Pal ontologen. They suspect that these fossil relics are fossilized slime traces of early organisms that once moved through the ocean floor. display

The only problem is that the tracks, with a thickness of several millimeters, are far too large to be derived from bacteria. "Known microbial structures are much smaller than ours, " say the scientists. But larger, multicellular living things arose after less than 1.5 billion years later. "Given its simple structure and its great age, there is no simple explanation, " El Albani and his team say.

Similar behavior as slime molds

According to the researchers, the producers of these tracks may be eukaryotic protozoa, possibly similar to early amos. "These amoeba-like organisms usually live on a perennial basis and feed on microbial mats, " said El Albani and his team. "But in times of hunger, they accumulate into aggregates, which then crawl through the sediment to reach a new microbial mat."

Palontologists compare the behavior of these early creatures with that of today's slime mold Dictyostelium discoideum. These unicellular organisms form groups of up to 100, 000 individual cells that form a nudibranch-like structure in the event of a lack of food. This now behaves like a multicellular organism and creeps forward on a slime trail. However, today's slime molds occur only in the soil and not in the marine sediment, in addition, their tracks are significantly smaller than the fossil thread structures.

Micro-computed tomography of the fossil fossils. Abderrazak El Albani

Am benartige beings with a tendency to aggregation

Nevertheless, the primeval producers of these tracks may have possessed at least one similar, occasionally aggregating lifestyle: "We do not say that these structures were produced by slime molds, " the researchers emphasize. "But we propose an analogous situation in which amoeba-like organisms with a similar aggregation ability leave these traces."

What sentient beings were allerging to and what happened to them in the further course of evolution, remains puzzling for the time being. "Whether these filamentous structures represent a failed experiment of evolution or were a prelude to evolutionary innovation remains unclear, " conclude El Albani and his team. The fossils are but another indication of the relatively complex life forms that already existed a good billion before the first animals. (Proceedings of the National Academy of Sciences, 2019: doi: 10.1073 / pnas.1815721116)

Source: PNAS, CNRS

- Nadja Podbregar