WASHINGTON — The earliest known ancestor of most animals may have been a minute creature shaped like a flattened helmet and barely visible to the naked eye, according to a new fossil discovery.
The findings should help researchers understand how complex life evolved and may offer clues to the curious proliferation of new animals known as the Cambrian Explosion.
The ancestors of most animal lineages first appeared during this episode of rapid evolution approximately 540 million years ago. The new fossils, however, lived around 55 million years earlier.
Despite these animals’ minute size, their biology was relatively complex. Thus, animals with a fairly sophisticated “genetic toolkit” may have existed well before the Cambrian Explosion. What led them to diversify so dramatically in the Cambrian is still an open question.
The researchers report their discovery in the journal Science, published by AAAS, the nonprofit science society.
Why sponges don’t shake hands
Like most modern animals, the animals that emerged during the Cambrian Explosion had two-sided body plans instead of circular ones. That is, they had a left and right side, a top and bottom, and a mouth and anus. This type of body plan is known as “bilateral symmetry.”
In contrast, many sponges and cnidarians such as corals have “radial symmetry,” which means that cutting the shape in half — in any direction — produces two sides that are mirror images of each other.
The fossils that David Bottjer of the University of Southern California and his colleagues from China, the United States and Taiwan discovered in China’s Doushantuo Formation may be the earliest known examples of bilaterally symmetric animals.
“This discovery helps us to learn more about the murky origins of bilaterian animals, which are most of what we see on Earth. You and I would have to go scuba diving to see cnidarians and sponges,” Bottjer said.
The researchers named the new animal “Vernanimalcula,” or “small spring animal,” referring to the fact that it lived after a “wintry” period of extensive glaciation.
A long fuse?
The genetic programming that produces a bilaterally symmetric body plan is relatively complex, which is one of the things that makes the sudden “explosion” of many bilaterians at the start of the Cambrian such a puzzle.
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One of the key questions about early animal evolution has been the relationship between complexity and size. Did one develop before the other? Did they go hand in hand?
Most of the bilaterian animals appearing in the Cambrian were substantially larger than the microscopic creatures discovered by Bottjer and his colleagues. The Vernanimalcula fossils suggest that “maybe complex animals were around beforehand, and it was just the ability to grow large that caused the Cambrian Explosion,” Bottjer said.
“The Cambrian Explosion may have had a really long fuse,” he added.
Which came first: ‘Chicken’ or eggs?
For a while now, scientists have been discovering signs of animal life around 20 to 30 million years before the Cambrian Explosion. These fossils, known as the “Ediacaran fauna,” were relatively large but primitive.
“These fauna were soft — a lot of them were just big flat sheets with compartments,” Bottjer said.
This has confused people somewhat as to how to classify them, but most researchers would agree that the majority of them were primitive cnidarians, according to Bottjer.
“There were probably some bilaterians around. We just don’t have much of a record. Then you go back farther in time and we don’t have any record of any of these Ediacaran animals. That’s where our fossils come in,” he said.
The 580 million-year-old to 600 million-year-old Doushantuo Formation, where the Vernanimalcula fossils were found, has already yielded some tantalizing signs of animal life before the Ediacaran. Scientists previously discovered tiny eggs and embryos in this sedimentary rock layer, although it wasn’t certain whether the “chicken” that laid these eggs was a bilaterian.
Little vacuum cleaners
The new fossils, which may indeed be related to the embryos found nearby, have many of the bilaterian characteristics that could make them early ancestors of most modern animals.
In order to identify the fossils and then distinguish their mouths, internal organs and other structures, the researchers sliced off paper-thin sections of the rock and studied them under a microscope.
The researchers then used a computer program to reconstruct a three-dimensional image of what the animal may have looked like.
“They were probably little vacuum cleaners, with suctionlike mouths. Basically just a little guy scooting along the ocean bottom, probably sucking up microbes,” Bottjer said.
He thinks the animals used suction for feeding because of the signs of muscles around the mouths in the fossils. The “scooting” part is speculation, however; the fossils don’t reveal how the animals moved.
Fossils few and far between
Only a handful of the fossils the researchers identified in their rock slices were actually Vernanimalcula specimens. The others were small bits of sponges and cnidarians, as well as eggs and embryos.
Bottjer thinks that’s because preserving the soft tiny animals with their internal organs intact requires an unusual set of conditions in which phosphate — which we have in our bones and teeth — speedily works its way into the cellular structures.
In fact, the lack of known Precambrian fossils is one of the reasons that scientists still have so many questions about how the earliest animals evolved. Bottjer is optimistic, however, that he and his colleagues will find more specimens to study.
“People have only really been looking for the last 10 years. Lots of times people think there’s nothing more to find but soon they’re saying, ‘Wow, we found something else!’ The fossil record is better than we think it is,” he said.
© 2013 American Association for the Advancement of Science