Here’s an announcement:
Geologists have discovered the first ancestor on the family tree that contains most animals today, including humans. The wormlike creature, Ikaria wariootia, is the earliest bilaterian, or organism with a front and back, two symmetrical sides, and openings at either end connected by a gut. It was found in Ediacaran Period deposits in Australia and was 2-7 millimeters long, with the largest the size of a grain of rice…
The earliest multicellular organisms, such as sponges and algal mats, had variable shapes. Collectively known as the Ediacaran Biota, this group contains the oldest fossils of complex, multicellular organisms. However, most of these are not directly related to animals around today, including lily pad-shaped creatures known as Dickinsonia that lack basic features of most animals, such as a mouth or gut.
The development of bilateral symmetry was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organize their bodies. A multitude of animals, from worms to insects to dinosaurs to humans, are organized around this same basic bilaterian body plan.
Evolutionary biologists studying the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs
Holly Ober/University of California–Riverside, “Ancestor of all animals identified in Australian fossils” at ScienceDaily
Very well, but how do we know that this find (excellent in itself) is the ancestor of all animals? That’s the claim in the media release. Paper. (paywall)
Here are some thoughts from a skeptical paleontologist, Günter Bechly:
Even if the attribution should turn out to be correct, the headlines — “Ancestor of all animals identified” — would be nonsense. Such hyperbole might be excused coming from ignorant popular science journalists, but that it was used in the official press release by the University of California is a shame. Nothing in this fossil discovery allows an identification as ancestor of any other group of animals or of all animals. Even being the oldest fossil record for Bilateria would not imply at all that it is ancestral to all animals. It could be an extinct side branch or only ancestral to a single phylum of worm-like animals. Evans et al. (2020) mention recent research that “suggests that the bilaterian LCA was a simple, small, mobile organism with anterior/posterior differentiation and limited sensory abilities (44–49). Remarkably, these predictions agree closely with the characters identified here for Ikaria.” However, such a simple worm-like habitus would fit with the ancestor of Bilateria, the ancestor of Xenacoelomorpha, the ancestor of Nephrozoa, the ancestors of Deuterostomia and of Protostomia, the ancestors of Lophotrochozoa and of Ecdysozoa, as well as the ancestors of several worm-like phyla like Platyhelminthes. But such scientific honesty would hardly make for a sexy headline.
Ikaria wariootia is just another problematic Ediacaran fossil that could be anything from inorganic artifact to protozoan to cnidarian and yes, maybe a bilaterian worm. Neither musculature, nor mouth, gut and anus, nor movement, nor mobility and a causal connection to the Helminthoidichnites traces is established by the actual fossil evidence. Its nothing but small oval-shaped impressions in the sediment. It is usually good paleontological practice to consider such dubious fossils as incertae sedis and not base far-reaching hypotheses on them.
Gunter Bechly, “Ancestor of All Animals in 555-Million-Year-Old Ediacaran Sediments?” at Evolution News and Science Today
So probably not.