In 2014, biologists digging in the black shale of Gabon's Bangombé Plateau discovered truly ancient treasure: perfectly preserved fossils of what might be the oldest multicellular organisms we've ever seen. Now, the 2.1-billion-year-old fossils of the Francevillian basin are in the spotlight again, as a closer look reveals that some of them might be the oldest organizations that ever moved on their own.
In a study published in PNAS on Monday, international scientists point out tiny, noodle-shaped structures embedded in the rock (shown in the video above), which they argument is the oldest-ever evidence of self-propelled multicellular organism. Before this study, it was thought that mobility emerged in evolutionary history about 570 million years ago. The new finds could push the origin of mobility way back in time by 1.5 billion years.
"It is plausible that the organisms behind this phenomenon moved in search of nutrients and oxygen that were produced by bacteria mats on the seafloor-water interface," said Ernest Chi Fru, Ph.D., a study co-author from Cardiff University's School of Earth and Ocean Sciences.
The paper describes the curious wavy, tubular fossils, which were at most 170 millimeters in length and about 6 millimeters across, mostly lying flat along the rock but occasionally bending up toward the sky. The "string-shaped structures," they argue through an analysis of complex 3D reconstructions of the fossils, look a lot like those known to be left behind when strings of mucus are fossilized – mucus produced by moving organizations.
They "confront traces left by motile organizations, rather than individual filaments of bacteria or sheaths / tubes," the team writes.
Furthermore, the fact that they're often oriented forward relative to the sediment bed "can be taken as evident for movement within the sediment." Close to the string-shaped fossils, the team also found evidence of fossilized "microbial mats" – carpets of single-celled organisms that were rich in nutrients and oxygen that the organisms – whatever they were – might have moved towards in search of a place to graze.
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While it's hard to find a modern-day equivalent for the organizations that might have left these behind, the team suggests they might be similar to "slugs" – not the gastropods that come out on rainy days, but the organism that forms when individual slime mold cells aggregate in hungry times. When these aggregate organisms form, they also move and leave behind trails of mucus.
We take our ability to move for granted, but for a long time the vast majority of life on Earth was bound in place, dependent on the environment to send nutrition its way or to move it to a more favorable location. Before the discovery of fossil mucus strings in Gabon, the world's "oldest trail," according to National Geographic, was a slime trail left behind at Mistaken Point in Newfoundland by crawling animals called Ediacarans about 565 million years ago. An article, published in 2010 in the journal Geology, claimed this was some of the "first evidence for locomotion."
What remains unclear is what evolutionary legacy – if any – the creatures that made these Francevillian structures passed down. After all, whatever it was that made them was buried alive in sediment. Who knows if they had relatives elsewhere that lived and led to more agile species?
"The results raise a number of fascinating questions about the history of life on Earth, and how and when organizations began to move," said Chi Fru. "Was this a primitive biological innovation, a prelude to more perfected forms of locomotion seen around us today, or was this simply an experiment that was cut short?"