Scientists believe that simpler forms of organisms such as bacteria and single-celled creatures thrived on the Earth 600 million years ago, but bizarre-looking fossils that were discovered in calcium phosphate rocks in southern China provided evidence that multicellular organisms may have already existed during this period.
In a new study published in the journal Nature on Sept. 24, a group of researchers closely examined the fossils called Megasphaera using microscopy and found multiple cells that are cleaved together in sphere-shaped clusters.
They also observed that the cells differ from each other in size and shape, which could be a possible indication that these cells may have gone through a process called cell differentiation, wherein the cells developed into different types of tissue and assumed different functions.
Researcher Shuhai Xiao from the department of geosciences at Virginia Tech in Blacksburg said that these are signs of the complex structure of multicellular organisms that cannot be found in bacteria or single-celled protists.
"Their complex multicellularity is inconsistent with a phylogenetic affinity with bacteria, unicellular protists, or mesomycetozoean-like holozoans," Xiao and colleagues wrote. "We conclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely."
The researchers also noted clusters of smaller cells they called "matryoshkas," which they suspect are reproductive cells; this finding is crucial because the fossils possibly represent the transition between single cellular organisms and multicellular animals.
"The bottom line is that they are multicelled and that they have cellular differentiation and that they have separation of reproductive cells from sterile somatic [body] cells," Xiao said. "This is a big thing, because if you look at modern multicellular organisms, including animals, this is a critical step towards very complex multicellular organisms."
Jim Gehling, a paleontologist from the South Australian Museum, said that scientists were not exactly sure if multicellular creatures existed 635 million to 545 million years ago during the Ediacaran period. Large Ediacaran fossils were believed to be giant single-celled organisms without true internal organs, bones, shells and other hard parts, but the findings of the study indicate the presence of more complex organisms during the period.
"Without this sort of evidence we could never be sure that there were multi-celled creatures in the Ediacaran," Gehling said. "It's a very affirmative paper for those of us who believe that the roots to the animal tree of life lie within the Ediacaran period."
