The world's oldest meal reveals details about our 575-million-year-old ancestor

The last supper eaten by our earliest animal ancestors has revealed new clues about their physiology.

Scientists from The Australian National University (ANU) analyzed the forms of Ediacara biota — the world's oldest large organisms dating back 575 million years — and found that the ancient beings ate bacteria and algae that was sourced from the ocean floor, according to an institutional press release.

They examined ancient fossils that contained traces of preserved phytosterol molecules, which is a type of fat found in plants, remaining from the animals' last meal. Based on their observations, the researchers confirmed that a slug-like organism, known as Kimberalla, had a mouth and a gut and even digested food the same way as modern animals.

"Ediacara biota really are the oldest fossils large enough to be visible with your naked eyes, and they are the origin of us and all animals that exist today. These creatures are our deepest visible roots," lead author Dr. Ilya Bobrovskiy, from GFZ-Potsdam in Germany, said in the statement.

The study pronounces Kimeralla as one of the most advanced creatures of the Ediacarans. 

Kimberella left feeding marks by scraping off algae covering the sea floor

Algae, known to be rich in energy and nutrients, could have played a key role in Kimberella's growth. 

"The energy-rich food may explain why the organisms of the Ediacara biota were so large. Nearly all fossils that came before the Ediacara biota were single-celled and microscopic in size," said study co-author Professor Jochen Brocks from the ANU Research School of Earth Sciences.  

The examined molecules had all signs of what the animals ate before their death. According to Brocks, the most difficult part was distinguishing between the "signatures of the fat molecules of the creatures themselves, the algal and bacterial remains in their guts, and the decaying algal molecules from the ocean floor that were all entombed together in the fossils," said the release.

"Scientists already knew Kimberella left feeding marks by scraping off algae covering the sea floor, which suggested the animal had a gut. But it was only after analyzing the molecules of Kimberella's gut that we were able to determine what exactly it was eating and how it digested food," Brocks said. 

Preserved chemical in the fossils helps figure gut contents of animals

The team also found another animal, called Dickinsonia, which grew up to 1.4 meters in length and had a rib-like design imprinted on its body. Unlike the Kimberella, the Dickinsonia absorbed food through its body.

"Our findings suggest that the animals of the Ediacara biota, which lived on Earth prior to the 'Cambrian Explosion' of modern animal life, were a mixed bag of outright weirdos, such as Dickinsonia, and more advanced animals like Kimberella that already had some physiological properties similar to humans and other present-day animals," Bobrovskiy said. 

Even though the Dickinsonia didn't have a gut, researchers were able to figure out its feeding patterns. "Kimberella knew exactly which sterols were good for it and had an advanced fine-tuned gut to filter out all the rest. This was a Eureka moment for us; by using preserved chemicals in the fossils, we can now make gut contents of animals visible even if the gut has since long decayed," said Brocks.

The fossils were collected in 2018 by Bobrovskiy from cliffs near the White Sea in Russia's northwest.

The study paper was published in Current Biology

Study Abstract:

The oldest animals appear in the fossil record among Ediacara biota communities. They prelude animal-dominated ecosystems of the Phanerozoic and may hold clues to the appearance of modern animal phyla in the Cambrian explosion. However, little is known about the phylogeny of the Ediacaran organisms and even less about their diet and feeding behavior. An exception is mollusc-like Kimberella, for which a fossilized gut, feeding traces, and even potential coprolites have been found. By contrast, Ediacaran organic-walled tubes, such as Sabellidites and Calyptrina, are thought to belong to tube worms comparable with modern Siboglinidae that have no gut but gain their nutrition from symbiotic bacteria. Here, we examine the gut contents of Ediacaran animals using biomarker molecules. We show that 558-million-year (Ma)-old tube worm-like Calyptrina and mollusc-like Kimberella possessed a gut and shared a diet of green algae and bacteria. Despite their ancient age, sterol metabolism within the gut of both organisms was already comparable to extant invertebrates. Dickinsonia, one of the key Ediacaran animals, show no traces of dietary molecules, indicating a different feeding mode and possible external digestion analogous to modern Placozoa. Lipid biomarkers uncover a range of feeding strategies in Ediacaran communities, highlighting true eumetazoan physiology of some Ediacaran animals.