In 1946, a geologist rummaging in the Ediacara Hills in Australia found curious pancake-shaped, finely ribbed imprints on the underside of rocks. Reginald Sprigg named the fossils medusoids because they resembled jellyfish.
These fossils, and similar specimens found nearly all over the world, have puzzled palaeontologists ever since. They look vaguely animal-like but predate the Cambrian Explosion, an abruptevent 541m years ago that heralded the rise of the major animal groups. Taxonomic guesses for so-called Ediacaran fossils, which feature palmlike fronds and spindles as well as ridged discs, have varied wildly: were they fungi, giant single-celled organisms, marine worms or maybe even category-defying life forms lying somewhere between plant and animal?
Now, a chemical analysis has offered clues. Scientists in Australia have found ancient traces of cholesterol in Dickinsonia, one such species. Cholesteroid, which is left behind as cholesterol decays, is a biomarker thought to be unique to animals.
This makes Dickinsonia, which lived between about 570m and 540m years ago, officially the oldest macroscopic animal in the rock record — in other words, the oldest animal that can be seen with the naked eye. The research, led by Ilya Bobrovskiy at the Australian National University, was published in the journal Science.
Not that Dickinsonia, which lived in predator-free seas and could reach four feet across, looks all that much. It has been imagined as a “flat, inflated bag” with the consistency of a thick jellyfish; it is likely to have clung to the sea floor, grazing on microbes and absorbing food through its quilted skin. It was, basically, a living blob. It has no recognisable surviving descendants.
The latest finding adds to previous research suggesting Dickinsonia, with its ribbed pattern, was an animal. A team of researchers, led by Renee Hoekzema at Oxford university, looked at specimens of different sizes and wondered if Dickinsonia grew like a worm, which adds segments as it develops.
Larger specimens, presumed to represent older organisms, seemed to have more “ribs” than smaller examples, with the ribs appearing fatter. In other words, Dickinsonia seemed to grow just like animals do, unfurling to a developmental template still common today.
The facts feed into an intriguing picture: creatures, even if only bloblike, existed close to 570m years ago and then seemingly vanished.
It was the Cambrian explosion 30m years later that marked the beginning of animal life as we know it. Along came skeletons, shells, legs and other mobility-boosting appendages, compound eyes and teeth. Humans belong among the chordates, which include vertebrates (animals with backbones). Arthropods include insects and crustaceans.
The trigger for such frenetic evolutionary activity in the Cambrian period has long been debated, but one possibility is a sudden abundance of oxygen. Such an environmental change could have fuelled the development of energy-intensive adaptations such as muscles and nervous systems.
This fresh revelation of the world’s oldest macroscopic animal provokes new thinking on the evolution of life. There could be more archaic beasts to come: animals may have been languishing in the seas 700m years ago.
The late Adolf Seilacher, a German palaeontologist, once described Ediacaran fossils as “strange as life on another planet, but easier to reach”. The rocky imprints of Dickinsonia suggest an eyeless, earless, mouthless alien but they also display a beguiling bilateral symmetry. Many species, including homo sapiens, demonstrate the same. In studying the evolution of such ancient species, we hold a mirror up to our own.
The writer is a science commentator
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