One of the key moments in the evolution of the animal world was the appearance of the bilateral symmetry, which involves the speculative correspondence of extremities and organs, as well as a body structured in different layers, with mouth, digestive tract and anus. The vast majority of modern animals, including humans, present this bodily plan.
When did animals with bilateral symmetry arise? It is believed that the former would have appeared before the Cambrian explosion 542 million years ago, a stage characterised by the sudden emergence and diversification of many types of bilateral animals that succeeded in occupying very diverse habitats.
The study of evolutionary genetics predicts that the oldest ancestor of bilateral animals would have been simple and small, and with very rudimentary sensory organs. The problem was to find this type of fossil (small and with many soft parts that are conserved worse): it was almost like looking for a needle in a haystack.
The solution came at the site of Nilpena, in southern Australia, in the time of Ediacara (a period before the Cambrian explosion and to which belong the oldest fossils of complex and multi-cellular animals). A series of fossilised burrows had been located there that appeared to have been excavated by bilateral animals, but so far no trace of these creatures had been found.
A team of researchers from the University of California, Riverside, detected a series of tiny, oval prints near some of these burrows. To analyse them in detail, they used a 3D laser scanner that revealed that they were before a cylindrical body with head and tail and somewhat flattened musculature. These animals had bodies between two and seven millimetres long and between one and 2.5 wide (just a grain of rice): just the right size to have dug the burrows.
Scientists have named the creature Ikaria wariootia, and it could be the first organism with bilateral symmetry, the common ancestor of a large family that includes animals as diverse and complex as insects, dinosaurs and the human being himself. Its name is a tribute to the original custodians of the land on which it was found: in the Adnyamathanha language, the word Ikaria means "meeting place".
All that can be deduced from such a small body
Despite its relatively simple form, Ikaria was a very complex animal compared to other fossils belonging to the fauna of Ediacara, many of them with other kinds of bodily plans or that led to evolutionary dead ends. It was able to bury itself in thin layers of well-oxygenated sand at the bottom of the ocean in search of organic matter, indicating that it possessed some sort of rudimentary sensory capacity. Its shape also indicates that its front end was different from the rear, a factor that supports the directed movement perceived in the excavation of the burrows.
On the other hand, transverse ridges in the form of ‘V’ are observed in the burrows, and this suggests that Ikaria moved by contraction of the muscles of its body, like modern worms. This type of locomotion is called peristaltic movement. In addition, evidence of sediment removal has been found inside the burrows: Ikaria would feed on organic matter and would surely have an anus, mouth and intestine.
"This is what evolutionary biologists had predicted," explains Mary Droser, one of the authors of the paper. "It is really exciting to have found something that fits perfectly with your prediction".
Reference: Evans et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. PNAS doi.org/10.1073/pnas.2001045117