The earliest fossil animal with a head and a rear end – as characterizes the family tree of animals – and bilateral symmetry has been found in Australia, more than a decade after its putative burrows were identified in rock over half a billion years old.
The almost miraculous discovery of Ikaria wariootia was described Monday in the Proceedings of the National Academy of Sciences.
The wormlike critter could be the missing link between the simpler Ediacaran proto-animals that had no mouths, guts or anus, and the ancestral animal from which we descended, the team postulates.
Living about 560 to 551 million years ago, Ikaria was moving vertically and horizontally in wormlike fashion through the sandy seafloor in the Ediacaran era and is, to date, the earliest bilaterian (organism with a front and tail end) to be found in Australia.
That isn’t to say it’s the earliest bilaterian ever: There are other candidates, including the possible ancestors of jellyfish. But the specimens of Ikaria, 108 of them, were found far – more than 100 meters (330 feet) – below fossils of Kimberella, the other serious contender for first bilaterian, the paper explains.
Bilaterians today include most of the animals we know, and are characterized by having clear anterior and posterior differentiation: a mouth leading to a gut leading to an anus.
Is Ikaria the earliest? When and where life began is unknown. Some claim that layered rocks found in Australia, dating to 3.5 billion years ago, and in Alberta, Canada, 3.8 billion years ago, are fossil microbial mats – vast colonies of primordial bacteria that created dense blankets on the seafloor.
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Others contend that the layered rocks could be, well, layered rocks. However, in Quebec’s Nuvvuagittuq Supracrustal Belt, structures were found inside quartz from 3.8 billion years ago that scientists say look like filaments and tubes formed by bacteria.
The next huge argument is over the beginning of multicellular organisms. One team claims to have found indirect evidence: unnatural tunnels bored 2.1 billion years ago in Gabon, Africa. If these tunnels are indeed not a geological phenomenon, then a creature several millimeters in length was moving there and it wasn’t a giant ameba.
Whenever complex multicellular life arose, by the Ediacaran era, starting around 635 million years ago, the seafloor became populated by weird, multicellular creatures. The Ediacarans lacked basics of animals as we know them, and are unlike anything known today and may have left no descendents: It seems that the non-bilaterian ones among them died out at the start of the great Cambrian Explosion of speciation.
Yet among them was this early example of the classic animal format (and Kimberella, apparently later).
“The development of bilateral symmetry was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organize their bodies,” the UC Riverside team explains.
“Burrows of Ikaria occur lower than anything else. It’s the oldest fossil we get with this type of complexity,” said co-author Prof. Mary Droser. “Dickinsonia [an Ediacaran] 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.”
The geologists were delighted and surprised at their discovery. Fossils of animals with skeletons or shells are rare enough. Fossils of soft tissue animals by the tracks they made, going back more than half a billion years, is practically miraculous.
Like in the case of the Gabon proto-worms (speculative), at first no “smoking fossil” of the animal was found – only tiny tunnels, 15 years ago. Paleontologists thought the tunnels may have been made by early bilaterians, with a head that gave them direction.
And then Scott Evans, a recent doctoral graduate, and Droser, a geology professor, noticed tiny oval impressions near some of these burrows, got funding from NASA’S exobiology endeavor, and scanned the suspect blobs.
The blobs turned out to look like grains of rice, 2 to 7 millimeters in length and 1 to 2.5 millimeters in width. Their size is “just right” to have made the mysterious burrows, the team says.
Moreover, high-tech inspection of the tunnels showed ridging, suggesting that Ikarians moved by contracting their muscles across the body like a worm; they ate organic matter with their mouths, and relieved themselves.
It seems that the ancestor of all the complex animals as we know them arose, if so, before the Great Cambrian Explosion of speciation that would spell the end of the Ediacaran era after tens of millions of years of dominion.