One of the key moments in the evolution of the animal world was the appearance of bilateral symmetry, which implies the mirror correspondence of limbs and organs, as well as a body structured in various layers, with a mouth, digestive tract and anus. The vast majority of modern animals, including humans, have this body plan.
When did animals with bilateral symmetry arise? It is believed that the first would have appeared before the Cambrian explosion, 542 million years ago, a stage characterized by the sudden appearance and diversification of many types of bilateral animals that managed to occupy 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 sense organs. The problem was to find this type of fossil (small and with many soft parts that are worse preserved): it was practically like looking for a needle in a haystack.
The solution came at the Nilpena site, in southern Australia, belonging to the Ediacaran era (a period before the Cambrian explosion and to which the oldest fossils of complex and multicellular animals belong). A series of fossilized burrows had been located there that appeared to have been dug 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 impressions near some of these burrows. To analyze them in detail, they resorted to a 3D laser scanner that revealed that they were in front of a cylindrical body with a head and tail and somewhat flattened muscles. These animals had bodies between 2 and 7 millimeters long and between 1 and 2.5 wide (just a grain of rice): just the right size to have excavated 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 different and complex as insects, dinosaurs and the human being. Its name is a tribute to the original custodians of the land in which it was found: in the Adnyamathanha language, the word Ikari a means “meeting place”.
Everything 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 Ediacaran fauna, many of them with other types of body plans or that led to evolutionary dead ends. It was capable of burying itself in thin layers of well-oxygenated sand at the bottom of the ocean in search of organic matter, indicating that it possessed some kind of rudimentary sensory ability. Its shape also indicates that its front end was different from its rear end, a factor that supports the directed movement seen in burrowing.
On the other hand, transverse ridges in the shape of a ‘V’ are observed in the burrows, and this suggests that Ikari moved by contracting the muscles of his body, just like modern worms. This type of locomotion is called peristaltic movement. In addition, signs of sediment removal have 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 work. “It is really exciting to have found something that fits perfectly with his prediction.”
Reference: Evans et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. PNAS doi.org/10.1073/pnas.2001045117
