Having a front end and a back end sure makes life easier. For one thing, it enables us to move with a sense of purpose and direction, with a clear sense of forwards and backwards. Plus it’s just nice to know that people can tell the difference between your head and your butt. Yet many early life forms lacked this basic arrangement, and researchers believe they may have just discovered the fossilised remains of the oldest creature to display such a layout.
Known as bilaterians, organisms with front and back ends are believed to have emerged during the Ediacaran Period, from 635 million to 541 million years ago. Until this point, the Earth was essentially populated by ancient buttheads, which had no clearly defined posterior or anterior and which are not related to any animals alive today.
However, writing in the Proceedings of the National Academy of Sciences, a team of researchers report the discovery of a 555-million-year-old worm known as Ikaria wariootia, which is the earliest known bilaterian and could well be the ancestor of all modern animals.
It has long been suspected that a series of fossilized burrows found in Ediacaran deposits in Nilpena, South Australia, were made by an early bilaterian, although scientists had until now been unable to find the remains of the creature that made them. Yet after noticing tiny oval impressions close to some of these burrows, the study authors were able to use three-dimensional laser scanning techniques to reveal the shape of several minuscule specimens, all showing a distinct head and tail.
Varying in length from 1.9 to 6.7 millimeters, the Ikaria display a grooved musculature that enabled it to burrow using a mode of locomotion called peristalsis, achieved by contracting its muscles in much the same way as modern worms do. This would have allowed for directed movement in a forwards direction as it searched out organic matter to feed on.
The displacement of sediment within its burrows reveals that it did indeed feed as it traveled, with evidence of a mouth at one end, an anus at the other, and some sort of gut connecting the two. Unlike many of the rudimentary life forms that existed at this time, Ikaria probably also boasted basic sensory apparatus that allowed it to detect the presence of food and aim its movement in that direction.
Study author Professor Mary Droser explained in a statement that this ancient bilaterian is “the oldest fossil we get with this type of complexity,” and that while researchers have always suspected that an animal like Ikaria wariootia must have existed, to actually find it fills in some major gaps in our understanding of how life on Earth evolved.
"This is what evolutionary biologists predicted," she said. "It's really exciting that what we have found lines up so neatly with their prediction."