Animal-like embryos developed from single-cell creatures long before animals themselves, according to a study which assessed the fossilised remains of an organism found in 609-million-year-old rocks in South China's Guizhou Province. The researchers, including those from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), analysed the tiny fossils of the multicellular organism of Caveasphaera, which measured about a half-millimetre in diameter, and were preserved down to their component cells.
The study, published in the journal Current Biology, used X-ray microscopy to assess the various fossils displaying different stages of Caveasphaera development - from a single cell to a multicellular organism. "X-Ray tomographic microscopy works like a medical CT scanner, but allows us to see features that are less than a thousandth of a millimetre in size. We were able to sort the fossils into growth stages, reconstructing the embryology of Caveasphaera," said study co-author Kelly Vargas from the University of Bristol in the UK.
The researchers said that until now, sufficient fossil records had not been available to understand when and how animal ancestors made the transition from single-celled microbes to complex multicellular organisms. So far, they said, this question could only be addressed by studying living animals and their relatives.
In the current study, the researchers found evidence that a key step in this evolutionary transition happened long before complex animals appeared in the fossil record. They said this evidence could be found in the fossilised embryos resembling multicellular stages in the life cycle of single-celled relatives of animals.
"We were able to sort the fossils into growth stages, reconstructing the embryology of Caveasphaera," Vargas said. According to the researchers, Caveasphaera had a life cycle like their close living relatives which alternate between single-celled and multicellular stages.
"However, Caveasphaera goes one step further, reorganising those cells during embryology," said John Cunningham, study co-author from the University of Bristol. The study noted that Caveasphaera sorted its cells during embryo development in a similar way the process worked in living animals, including humans.
"But we have no evidence that these embryos developed into more complex organisms," said Yin Zongjun, study co-author from NIGPAS.
However, the researchers are unsure if Caveasphaera was an animal by itself, or just an important step in animal evolution.
"Caveasphaera looks a lot like the embryos of some starfish and corals - we don't find the adult stages simply because they are harder to fossilize," said Zhu Maoyan, co-author of the study from NIGPAS.