Pre-Cambrian species shared many characteristics with organisms living today

Animals who lived in an ancient period, which until the mid-20th century lacked evidence in the fossil record, shared much of the same complexity and similarity as do living things today, the journal "Nature" recently reported.

In her news feature published Oct. 28, Nature journalist and associate editor, Traci Watson, described the Fractofusus, an organism that lived about 560 million years ago and was a "revolutionary animal" that "lived and died in the muck."

"This was the earliest-known animal to show unequivocal evidence of two momentous innovations packaged together: the ability to roam the ocean floor, and a body built from segments," Watson wrote in her news feature. "It was also among the oldest known creatures to have clear front and back ends, and a left side that mirrored its right. Those same features are found today in animals from flies to flying foxes, from lobsters to lions."

The Fractofusus and other creatures like it lived in a period that for decades was not reflected in the fossil record but were thought to exist. 

Famed Naturalist Charles Darwin wrote in his "Origin of Species," published in 1859, about the perplexing lack of fossils in the strata of the pre-Cambrian period, the Ediacaran, and postulated that vast amounts of life must have existed, although he couldn't then prove it. These missing fossils were one of the main problems with his theory of common descent by means of natural selection, a matter that later came to be called "Darwin’s Dilemma."

Darwin himself dismissed the difficulties by writing in "Origin of Species" that for the "periods prior to the Cambrian system, I can give no satisfactory answer" and he left the dilemma there.

It would not be until the 1930s and 1940s that the dilemma would be addressed. It was then that imprints were found in rocks around the world of life that came during the Ediacaran. Organisms that lived during the Ediacaran period are called "Ediacarens."

Today most scientists are describing Ediacarans as "a grab bag of disparate life forms" instead of part of a self-contained group, Watson wrote.

"Many scientists – although not all – are also signing on to the idea that some fraction of the Ediacaran organisms were probably animals, including some that don’t look like any animal alive today," she said.

"That idea dovetails with genetic evidence that animals, or metazoans, first appeared more than 600 million years ago, well before the Ediacaran. There are no definitive fossils to illustrate the dawn of the animals, but the early metazoans were probably small, soft, simple things, including ancestors of modern creatures such as sponges and corals."