Animal ancestor primera

The ancient stone at Nilpena fossil site, about 600 kilometres north of Adelaide, South Australia, has yielded some of the world’s most important fossil finds.

In 1946, geologist Reg Sprigg was exploring for minerals at the old Ediacara minefield in the western Flinders Ranges when he discovered fossils in rocks.

The soft-bodied creatures he found were of such significance that some were named after him, and the Ediacaran Period was named after the site, which is now known as Nilpena.

Now, a team led by geologists from the University of California Riverside, US, has discovered at Nilpena the first ancestor on the family tree that includes most familiar animals today, including humans.

The creature they’ve named 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 is described in a paper in the journal Proceedings of the National Academy of Sciences.

The earliest multicellular organisms, such as sponges and algal mats, had variable shapes. The group – known as the Ediacaran Biota – contains the oldest fossils of complex, multicellular organisms. 

But most of these are not directly related to animals around today, including the 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. It gives organisms the ability to move purposefully and a common, yet successful way to organise their bodies. 

A multitude of animals through history, from worms to insects to dinosaurs to humans, are organised 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. It was thought to be highly unlikely that the fossilised remains of such an animal could be found or identified.

For 15 years, scientists agreed that fossilised burrows found in 555 million-year-old Ediacaran Period deposits in Nilpena were made by bilaterians. But there was no sign of the creature that made the burrows. 

Then UC Riverside’s Scott Evans and Mary Droser noticed miniscule oval impressions near some of these burrows. 

With funding from a NASA exobiology grant, they used a 3D laser scanner to reveal the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature.

The animal was 2–7 millimetres long and about 1–2.5 mm wide. The largest individuals were the size and shape of a grain of rice – just the right size to have made the burrows. 

“We thought these animals should have existed during this interval, but always understood they would be difficult to recognise,” says Evans. 

“Once we had the 3D scans, we knew that we had made an important discovery.”

The research team, including the South Australia Museum’s James Gehling, named the creature Ikaria wariootia to acknowledge Nilpena’s original custodians. 

The genus name comes from Ikara, which means “meeting place” in the Adnyamathanha language. It’s the Adnyamathanha name for the Flinders Ranges mountain feature known in English as Wilpena Pound. The species name comes from Warioota Creek, which runs from the Flinders Ranges to Nilpena Station.

“Burrows of Ikaria occur lower than anything else,” says Droser.

“It’s the oldest fossil we get with this type of complexity. Dickinsonia and other big things were probably evolutionary dead ends. We knew that we also had lots of little things and thought these might have been the early bilaterians that we were looking for.”

In spite of its relatively simple shape, Ikaria was complex compared to other fossils from this period. It burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, indicating rudimentary sensory abilities. 

The depth and curvature of Ikaria represent clearly distinct front and rear ends, supporting the directed movement found in the burrows.

The burrows also preserve crosswise, “V”-shaped ridges, suggesting Ikaria moved by contracting muscles across its body like a worm, which is known as peristaltic locomotion. Evidence of sediment displacement in the burrows and signs the organism fed on buried organic matter reveal Ikaria probably had a mouth, anus and gut.

“This is what evolutionary biologists predicted,” says Droser. “It’s really exciting that what we have found lines up so neatly with their prediction.”