Researchers have discovered the fossilized remains of the oldest known ancestor of almost every animal in existence today. The creature, named Ikaria wariootia, is a wormlike animal about the size of a grain of rice, and it appears to be the earliest example of the bilaterian body shape that’s common to the overwhelming majority of animals ever since.
Even if you don’t recognize the term, you know the concept of a bilaterian. It’s an animal whose body has two symmetrical sides, with a mouth at one end and an anus at the other. This basic structure has proven so successful that it’s been conserved across hundreds of millions of years of evolution and countless incarnations, from ducks to dogs to dinosaurs. In fact, only a fraction of animals, like sponges and jellyfish, aren’t bilaterian.
And now, researchers have found the oldest known ancestor of this widespread group, dating back 555 million years. Ikaria looked a little like a slug, measuring between 2 and 7 mm (0.08 and 0.29 in) long and 1 and 2.5 mm (0.04 and 0.1 in) wide. It was very clearly symmetrical, and evidence points to it having a mouth, a gut and an anus – all features of bilaterians.
This timeframe neatly lines up with what evolutionary biologists have long believed about the beginning of bilaterians. Ikaria’s age places it in the Ediacaran Period, a time when life on Earth was really starting to take off.
Before the team discovered Ikaria itself, they found its home – tiny fossilized burrows in ancient South Australian rock. These were long thought to have been created by bilaterians, but the animal responsible was nowhere to be found.
This is what evolutionary biologists predicted.
Eventually though, researchers from the South Australia Museum, UC Riverside and UC San Diego realized that the nearby rock was dotted with small oval-shaped impressions. On closer inspection using a 3D laser scanner, the team found that the impressions were consistently the same shape, showed faint signs of muscle grooves, and were the right size for the burrows. They were the remains of the animals.
The burrows also showed signs of V-shaped ridges, which suggest that Ikaria moved like a worm, contracting muscles and dragging itself along. While simple by today’s standards, the researchers say that Ikaria would have been complex for its time.
“Burrows of Ikaria occur lower (in the rock record) than anything else,” says Mary Droser, co-author of the study. “It’s the oldest fossil we get with this type of complexity. Dickinsonia and other big things were probably evolutionary dead ends. This is what evolutionary biologists predicted. It’s really exciting that what we have found lines up so neatly with their prediction.”
The research was published in the journal Proceedings of the National Academy of Sciences.
Source: UC Riverside