Time to Fall Back—New Fossil Sets Evolutionary Clock Back 60 Million Years

Documenting their findings in this week's issue of the journal Nature, Virginia Tech geobiologist Shuhai Xiao and a team of researchers from the Chinese Academy of Sciences have found a multicellular fossil that predates the earliest of our known ancestors by nearly 60 million years; setting the evolutionary clock back quite a bit. At first glance, the microscopic spherical ball of fossilized cells appears to be a blastocyst frozen in time, but under further inspection the researchers have found that it is not quite like anything they've seen before.

Found in 600 million year old Ediacaran fossils from China, the multicellular ball is one of many mysteries to paleontologists today. Originally believing that multicellular/skeletal animals appeared only 542 million years ago during a period of branching life on Earth known as the Cambrian Explosion, the microscopic ball shatters the current evolutionary timeline and reveals a potential 60 million year gap in the fossil record.

"Our work shows evidence that this organism developed multiple kinds of cells 600 million years ago" lead researcher Shuhai Xiao says. "This [discovery] opens up a new door for us to shine some light on the timing and evolutionary steps that were taken by multicellular organisms that would eventually go on to dominate the earth in a very visible way."

Investigating how, and when, multicellularity actually came to be derived from single-celled ancestors that predominated the pre-cambrian epoch, Xiao and his team in international researchers analyzed fossilized remains trapped in calcium phosphate rocks from the Doushantuo Formation in Guizhou, China. Able to recover a nearly perfectly preserved three-dimensional fossil of a spherical ball of conjoined cells, the researchers were able to discover that the 600 million year old fossil showed all signs of multicellular life: differentiation, cell-to-cell adhesion, and programmed cell death, all of which are necessary components within our own bodies.

But even with the question of when answered, the researchers have yet to have been successful in determining exactly what the fossil may be, in that the Ediacaran fossils appear unlike anything currently alive today.

"Fossils similar to these have been interpreted as bacteria, single-cell eukaryotes, algae and transitional forms related to modern animals such as sponges, sea anemones, or bilaterally symmetrical animals" Xiao says. "[But] this paper lets us put aside some of those interpretations."

While the researchers cannot exactly rule out what the fossil may have been 600 million years ago to the equivalent in modern life, they are confident that the microfossils are not likely bacterium as they share much more similarities with multicellular organisms... leading researchers to believe this may have been the cells actually responsible for multicellular life: an embryo.

"We have not proven that these are animal embryos" Xiao says, " although it remains one of two possibilities and certainly narrows down the options."