Model of a 31-celled embryo, illustrating cleavage within the first animals.
Credit: Sean Werle/University of Massachusetts Amherst
WASHINGTON: Evidence of cell differentiation has been found in fossil embryos more than 550 million years old, offering insights into the formation of ancient embryos, according to a new international study.
"We're learning something about how the very earliest multicellular animal formed embryos and how the embryos developed," said co-author Rudolf Raff, of both Indiana University in the U.S. and the University of Sydney in Australia.
"This gives us an enormous and entirely surprising look at half-a-billion-year-old embryos in the act of cleaving. What a window on the past; we've had no prior idea what they might have done," he said.
A team of 15 scientists from 5 countries were involved in the study, which is published this week in the U.S. journal Science.
The researchers examined cells from the embryos of ancient animals that have been preserved in the Doushantuo Formation, a fossil site in South China. The work was carried out using x-ray imaging technologies that produce higher resolutions than hospital-CT scans.
As well as evidence of cell differentiation - which is the process by which cells specialise for specific functions - the scientists also identified what appeared to be cells about to divide.
They examined 162 "relatively pristine envelope-bound and spheroidal embryos in which recurrent biological structures and cleavage patterns could be distinguished from inorganic artifacts," according to the paper.
"We digitally extracted each cell from the embryos and then looked inside the cells," said co-author Shuhai Xiao of Virginia Tech in the U.S. "We found some kidney shaped structures within the cells which could be nuclei or other subcellular structures.
"It is amazing that such delicate biological structures can be preserved in such an ancient deposit."
In some four-cell embryos, each cell had two kidney shaped subcellular structures, which suggested they had been caught in the process of splitting during cell division, Xiao added.
James Hagadorn of Amherst College in Massachusetts, USA, a member of the international research team, said the findings cast serious doubt on previous claims that these embryos represent more derived or advanced groups of animals, for example, bilaterally symmetrical animals.
"Rather, all the available evidence suggests that they represent relatively simple forms, akin to sponge ancestors," he said.
The international research team included scientists from Australia, the USA, England, China, Sweden and Switzerland.
