Burrow-like structures several millimetres in diameter have been found in 2.1-billion-year-old rocks in Gabon, Africa. The structures were made by a moving lifeform of some kind, claim geologist Abderrazak El Albani at the University of Poitiers in France and his team.
The team do not know what made the trace fossils, but they speculate that it could be something similar to colonial amoeba or slime moulds – organisms made of cells that normally live separately. The trace fossils were found near bacterial mats that the mysterious lifeforms may have been feeding on. “It’s truly amazing,” says El Albani.
Previously, the earliest evidence of moving lifeforms was just a half a billion years old. There are burrows and tiny footprints in rocks of this age, probably left by small animals.
The 2.1-billion year-old burrows are very unlikely to have been produced by organisms as complex as animals, which probably appeared only between about 850 and 650 million years ago. In fact, it’s not even clear that organisms as complex as amoeba were around 2.1 billion years ago: they are eukaryotes, and the oldest eukaryotic fossils found so far are about 1.7 billion years old. So if El Albani’s interpretation is correct, these finds challenge the conventional story of life’s evolution.
On the other hand, it’s clear that multicellularity evolved on numerous different occasions. There are even multicellular organisms composed of simple – prokaryotic – cells. And lab experiments suggest it’s relatively easy for cells to evolve multicellularity.
In 2010, El Albani reporting finding what his team think are fossils of multicellular organisms in the same sedimentary rocks in the Franceville basin in Gabon, which formed in a warm, shallow ocean 2.1 billion years ago. “It’s a unique place in the world, where we have this preservation of the rocks,” he says. Most rocks are of this age have been metamorphosed by extreme heat and pressure.
Since then, his team have continued to make field trips and have collected more than 500 specimens – now including the apparent trace fossils. These organisms lived at the time when oxygen levels were relatively high, says El Albani.
Shortly after oxygen level plummeted and remained low for a billion years – the “Boring Billion”. So El Albani thinks complex lifeforms started to evolve much earlier than thought, but were then killed off. “These organisms disappeared,” he says.
But Nicholas Butterfield of the University of Cambridge is sceptical about the team’s new interpretations. “It’s such an extraordinary claim,” he says. “But they don’t give compelling data to support it.”