Chinese fossils reveal a primordial surge in animal evolution

A remarkable collection of fossils from China reveals that animal life in Earth's ancient seas diversified millions of years earlier than previously thought, with a range of forms including ancient representatives of the group that later gave rise to vertebrates, including humans.

Paleontologists have unearthed about 700 fossils of individual small soft-bodied animals that lived about 546 to 539 million years ago, during the Ediacaran period, showing a dramatic change in the animal world at the time. Many of them are unusual and barely recognizable as animals to the untrained eye.

The fossils, discovered in China's Yunnan province, are collectively known as the Jiangchuan biota. They were preserved in the form of so-called carbon film, a dark, two-dimensional layer of carbon that remains after an organism is compressed during the fossilization process. This has allowed anatomical details such as the intestines to be preserved, as well as structures for feeding and locomotion, Reuters reports.

The discovery is particularly significant because it shows that rapid diversification of the animal world was already underway during the Ediacaran, before the well-known burst of evolutionary innovation that occurred in the following Cambrian period. By the end of that Cambrian explosion, early representatives of most of the major animal groups that still exist today had appeared.

"We have found a fossil site that gives us new information about the emergence of complex animal life, before the Cambrian explosion. We have discovered evidence of animal groups that are normally only found around 520 million years ago - after the peak of the Cambrian explosion - that existed as early as the late Ediacaran, more than 20 million years earlier," said paleontologist Frankie Dunn from the University of Oxford, one of the authors of the research published in the journal Science.

During the Ediacaran, Earth was a very different place than it is today. The planet was recovering from a deep global ice age known as the "Snowball Earth," the continents were arranged very differently, and oxygen levels in the atmosphere were significantly lower, Reuters reports.

Amidst all this, the first forms of animal life were emerging in the seas. The oldest undisputed animal fossils date back to about 574 million years ago and belong to creatures that resemble fern fronds or feathers. Other previously known Ediacaran animals included sponges and cnidarians, a group that includes jellyfish and corals.

"If you could peer into the Cambrian, you would recognize much of the animal world around you, but that's not the case with the Ediacaran, where recognizable animals would likely be rare and few in number. Our new site shows a world in transition, moving toward the complex, animal-dominated world we see around us today," Dan said.

Among the fossils of the Jiangchuan biota, researchers have discovered the oldest known members of a large group known as deuterostomes. This is significant because vertebrates - including humans - are deuterostomes, although they are anatomically very different from those that lived in the Ediacaran seas.

Fossils from the Jiangchuan biota included bilateral animals, or those whose bodies could be divided into two equal halves. Most animals today are bilateral, but this was a revolutionary innovation during the Ediacaran.

Among the fossils were animals with U-shaped bodies, attached to the seabed by a stalk and with a pair of tentacles on their heads to catch food. They were early representatives of the animal group that today includes starfish and acorn worms.

There was also a creature that researchers called a "trumpet worm" because of its vague resemblance to that musical instrument, with a worm-like body attached to the seabed and a trunk that could be turned outward.

"When we were collecting fossils in the field, we were all surprised by how diverse the fauna was and how many fossils there were," Dan said.

"We expected to see increasing evidence of animals in the Ediacaran, but animals like the trumpet worm tell us that not all of these forms will be what we might predict based on today's animal diversity or even the Cambrian explosion," Dan said.

"It shows us that we still have a lot to learn about the spread of animal life and the nature of the Ediacaran-Cambrian transition."