What organisms are the oldest animals on the Earth? Recently, scientists seemed to take a big step toward finding the answer to that question. Based on their study of fossils of embryos around 600 million years old in southwest China's Guizhou Province, the paleontologists from China and the United States believe that the adult form of these embryos represents the oldest animal found on the planet so far.

The February issue of Geology, the journal of the Geological Society of America, carried a cover story on the research of these paleontologists. It said that the scientists revealed how the ancient animal embryos developed into mature adult forms. The article entitled "Rare Helical Spheroidal Fossils from the Doushantuo Lagerstatte: Ediacaran Animal Embryos Come of Age?" was written by Shuhai Xiao, James W. Hagadorn, Zhou Chuanming and Yuan Xunlai, who have conducted related research for a long time.

About a decade ago, the paleontologists from both sides of the Pacific discovered thousands of spheroidal microfossils of 600-million-year-old embryos in the Doushantuo Formation, a fossil deposit near Weng'an, Guizhou Province. Later they unearthed fossils of a tubular coral-like animal, which they named Megasphaera ornata for its appearance, and found them to be the grown-ups of the embryos found earlier.

There are thousands of early-stage embryos, but only 80 pieces have been recovered that have traits in common with both groups, said Yuan, a member of the Chinese Academy of Sciences who participated in the Sino-U.S. embryo study and one of the co-authors of the article in Geology.

These intermediary stage embryos have an "envelope," or outer covering, similar to that of the earlier embryonic stage, and they are about the same size-about 0.02 inches wide. They have a coiled tubular embryo within the envelope. Their envelope has a groove on the surface, consisting of three clockwise coils.

According to Yuan, each embryo, about the size of a grain of sand, is composed of two to nearly 1,000 cells. The embryos cloaked themselves inside an envelope with tiny holes in a pattern similar to stitches on a baseball that they use to transport, store or metabolize molecules, he said.

Scientists used X-ray computed tomography and other scanning equipment to peel off the envelope and discovered that the embryo cells are dividing and unfurling.

"Previously we discovered that some cells were clustered together, but they showed no signs of dividing," Yuan said.

Helped by advanced imaging technology, the scientists also found that the tubular embryo is coiled, with three clockwise coils.

Some of the intermediate embryos also appeared to be unfurling, encouraging the speculation that if the process had continued, the embryos would distend like a stretched slinky or a flattened fusilli noodle into the tubular adult form.

In the article in Geology, the researchers state, "... if this possibility holds up to further testing, the new fossils may bridge the developmental gap between two previously described Doushantuo forms."

Zhou, a researcher with the Nanjing Institute of Geology and Paleontology, said that the new fossils could provide the missing link between the egg and adult of one of the Earth's earliest animals.

He said the discovery could provide a hint of how ancient animals developed and reveal the unending evolution of the Earth's life forms. But scientists added that they need other intermediary stages to complete the developmental journey of the ancient animals.

The conditions that preserved the ancient embryos may not have been favorable for preserving or fossilizing more-developed life forms, researchers note. Connecting the first moments of animal evolution will likely require more use of advanced imaging techniques, they said.

But from the cell fission process of Megasphaera ornata, its adult form is of the same size as its embryo, which raises another question: How can it lay an egg of a size similar to its own? Did these animals become extinct after a failed evolution? Up to now, these questions had no certain answers. But Yuan said he believes this animal did exist prior to 600 million years ago.

Scientists said that Megasphaera ornata has provided a new clue for researching and deciphering the origin and evolution of life on the Earth. Only when you find the origin, can you determine the trend of evolution and forecast the future, and all the information on the development of biology can be found in fossils, Yuan said.