The recent discovery of an ancient bilaterian fossil

In March 2020, researchers in Australia announced the discovery of a 555 million-year-old fossil of a bilaterian animal, which they named Ikaria wariootia. The way that journalists presented this discovery made it clear that it is significant for… something. But it seemed to be difficult for them to pin down precisely what is significant about it. In this post, I’ll attempt to do that.

Here are some headlines that were used:

  • Ancestor of all animals identified in Australian fossils
  • Ancestor of All Animals – Including Humans – Identified in Australian Fossils
  • Discovery of the oldest bilaterian from the Ediacaran of South Australia
  • Earliest Ancestor of Many Modern Animals May Have Been Found in Australia
  • Fossil hunters find evidence of 555m-year-old human relative
  • Fossil worm shows us our evolutionary beginnings
  • Geologists Have Discovered Humanity’s Earliest Ancestor, And It’s A Worm
  • Meet Ikaria wariootia, the first organism with a front and back
  • Oldest ancestor of almost all animals found in Australian fossils
  • Oldest Bilaterian Fossil Found in Australia
  • Smaller than a grain of rice, this creature could be our forebear
  • This primeval worm may be the ancestor of all animals
  • This worm-like creature is the first ancestor on the human and animal family tree
  • Worm-Like Creature Was Forerunner to Most Animals
  • Wormlike Creature That Lived 555 Million Years Ago May Be the Ancestor of Almost All Living Animals

I’ll start with some background information. Roughly speaking, a bilaterian is an animal that has a front and a back, and approximate bilateral (left-right) symmetry, at least as an embryo. This body plan seems to have evolved only once on Earth, and any animals descended from the first bilaterians species are classified as bilateria.

The bulk of living bilaterians belong to one of two large groups: the protostomes or the deuterostomes. The distinction between them is based on the order in which certain body parts develop. This union of these two groups is what the news articles mean by “almost all living animals”.

The deuterostomes include mainly the vertebrates and their relatives, and (strangely enough) the starfish and their relatives. Although starfish don’t have bilateral symmetry as adults, they do as embryos, and are bilaterians.

The protostomes include most everything else: insects, crustaceans, molluscs, most worms, and much more.

There may also be a few living bilaterians that are neither protostomes nor deuterostomes. The current thinking is that this is the case for the simple worm-like animals in the group named Xenacoelomorpha.

I gather that the new Ikaria wariootia animal is suspected to be connected to the known tree of life somewhere near the protostome-deuterostome split. Many of the headlines imply that it is an ancestor of both branches. (That’s exceedingly improbable at best, but let’s assume they really mean “a reasonably close descendant of such an ancestor”.) Branch points this old are subject to a lot of uncertainty. If it is connected near the split, it could just as easily be connected to one of the branches just after the split, instead of before. We might never know.

However, the fateful split between protostomes and deuterostomes is estimated to have occurred around 600 million years ago. While there is a significant margin of error, it’s very unlikely that it could have occurred as recently as 555 million years ago.

Here’s a rough diagram of where I think Ikaria wariootia is suspected to lie on the animal tree of life:

So, any Ikaria wariootia specimen dating from the time of the fossil could not actually be an ancestor (or close descendant of an ancestor) of protostomes+deuterostomes, or of any living animal.

Ikaria wariootia was a simple creature, and it could be that the 555 million-years-ago Ikaria wariootia was not so different from its ancestor from 600+ million years ago, when it branched off from the human ancestral line. And that ancestor would also be the ancestor of humans. So, the fossil could certainly shed some light on our ancestors from that time period. I’ll acknowledge that it could be hard to express that in a catchy headline.

Another significant thing claimed for Ikaria wariootia is that it is the record oldest bilaterian fossil ever discovered. That’s a good claim to fame, but is it true?

As far as I can tell, it’s pretty much tied for the oldest known fossil that’s definitely a bilaterian. Another candidate is Kimberella, which dates to about 555 to 558 million years ago. Kimberella is more complex than Ikaria wariootia, and is believed to be a protostome. So, while the fossils are the same age, Kimberella’s distinct lineage is not as ancient as Ikaria wariootia’s might be.

I’d also like to nitpick phrases like “humanity’s earliest ancestor”. Think about it for a minute. All life on Earth is related, so the earliest ancestor of humans is also the earliest ancestor of all other life. In order for “earliest ancestor” to mean anything at all, you must qualify it in some way, such as “earliest human ancestor that was an animal”, or “earliest human ancestor that had bilateral symmetry”. I acknowledge that headline writers may not have room for that many words, but I still think they could try a little harder to be less wrong.

Another thing: There are also lots of interesting things here that are the latest, instead of the earliest. For example, the “latest common ancestor of almost all living animals”. I think there is an instinctive aversion to using the word “latest” when describing something that happened a long time ago. But if that is what you mean, then that is the word to use! You can’t just change “latest” to “earliest”, and still have your statement be correct! This reminds me of another rather silly post of mine.

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