Researchers have discovered the first ancestor on the family tree that contains most modern-day animals, including humans, a finding that sheds more light on the origins and evolution of the animal kingdom.
The tiny, worm-like creature, named Ikaria wariootia, is described in the journal PNAS, as the earliest bilaterian, or organism with two symmetrical sides -- a front and back -- and openings at either end connected by a gut.
According to the scientists, including those from the University of California (UC) Riverside in the US, in spite of its relatively simple shape, Ikaria was complex compared to other fossils from this period.
They said the animal burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, indicating it may have had rudimentary sensory abilities.
Ikaria's depth and curvature, according to the researchers, represent clearly distinct front and rear ends, supporting the directed movement found in the burrows.
They said the burrows also preserve crosswise, "V"-shaped ridges, suggesting Ikaria moved by contracting muscles across its body like a worm.
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The study found evidence of sediment displacement in the burrows, and signs that the organism fed on buried organic matter, suggesting that Ikaria may have had a mouth, anus, and gut.
Comparing Ikaria with other early animals, the scientists said the earliest multicellular organisms on the Earth such as sponges and algal mats, had variable shapes.
They said these were collectively known as the Ediacaran Biota -- a group containing 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 study noted.
The development of bilateral symmetry, the researchers said, was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organise their bodies.
A multitude of animals, from worms to insects to dinosaurs to humans, are organised around this same basic bilaterian body plan, they said.
Biologists studying the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs, the UC Riverside scientists said.
But, they added that preserving and identifying the fossilised remains of such an animal was thought to be difficult, if not impossible.
In earlier studies scientists had noted that fossilised burrows found in 555 million-year-old Ediacaran Period deposits in South Australia, were made by bilaterians.
But, the researchers said, there was no sign of the creature that made these burrows, leaving the scientific community with nothing but speculation.
Assessing these deposits, the scientists then noticed miniscule, oval impressions near some of these burrows.
They then used a three-dimensional laser scanner to analyse these deposits which revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature.
According to the study, the animal ranged between 2-7 millimeters (mm) long and about 1-2.5 mm wide, with the largest the size and shape of a grain of rice
"We thought these animals should have existed during this interval, but always understood they would be difficult to recognize," said Scott Evans, study co-author and a recent doctoral graduate from UC Riverside.
"Once we had the 3D scans, we knew that we had made an important discovery," Evans said.
The researchers said Ikaria wariootia was named to acknowledge the original custodians of the land, with the genus name Ikara, meaning "meeting place" in the Adnyamathanha language, and the species name coming from Warioota Creek, which runs from the Flinders Ranges to Nilpena Station in Australia.
"Burrows of Ikaria occur lower than anything else. It's the oldest fossil we get with this type of complexity," said Mary Droser, another co-author of the study from UC Riverside.
"This is what evolutionary biologists predicted. It's really exciting that what we have found lines up so neatly with their prediction," Droser added.
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