Who Might Have Laid the First Egg?

With the first multicellular animals, the first egg appeared.

And the earliest-known ones found till now, in 1998, are the thousands of 600-million-year-old spherical tiny baseballs embryo microfossils from the Doushantuo Formation, a fossil deposit in South China.

In 2000, the researchers discovered who might have been the parents: a tubular coral-like animal, called Megasphaera ornata.

In February the team will present findings about the intermediary stage that linked the embryo to the supposed adults. There was a mystery, but now the researchers discovered 80 intermediate stage fossils, that have traits in common with both groups. The discovery seems to provide the missing link between egg and adult versions of one of the Earth's earliest multicellular animals.

At the exterior, the early and intermediate stage embryos look the same. They have the same size (about 0.02 inches or 0.05 mm wide or about as big as a grain of sand).

The intermediary phase presents an envelope like that of an earlier embryonic stage, harboring the coiled tubular embryo inside. The envelope has a groove on the surface, made of three clockwise coils.

The researchers employed microfocus X-ray computed tomography (microCT) to investigate bellow the envelope the embryo inside. The tubular inner embryo follows the pattern of the envelope, being coiled, with three clockwise coils.

Some specimens exposed signs of uncoiling. "This is further evidence that these embryos would have grown into the tubular organisms," said Shuhai Xiao, associate professor of geosciences at Virginia Tech and lead investigator. "? if this possibility holds up to further testing, the new fossils may bridge the developmental gap between two previously described Doushantuo forms."

"The discovery of additional intermediary stages and even more advanced specimens would be the ultimate test," added Xiao.

But the fossilization conditions might not have enhanced the preservation of more developed stages and life forms. And future more advanced imaging techniques could help us see a primordial step in animal evolution investigating the first eggs ever to appear.

Photo credit: Shuhai Xiao An embryo with envelope partially removed, an intermediate state and Doushantuo rocks as background