Mysterious fossils shed light on the origin of animals

Fossils older than 570 million years provide new secrets about the evolution of animals from a single cell. These fossils are neither strange bacteria, as some thought, nor animal cells, as others believed. But then what are they?

Images of one of the cell clusters fuels achieved by X-ray tomography A: three-dimensional representation for external. Six cells are visible. B: a cut in the cluster shows three nuclei. C: all nuclei are visible. The green is being divisive. Magnification: 4405 times. © Huldtgren et al., 2011, Science

Doushantuo is a region of southern China where fossil composed of cell clusters and aged 570 million years were discovered in 1998. For some researchers, they correspond to an embryonic form of the first metazoan animals . Indeed, the arrangement of fossilized cells resemble a morula, the first stage of embryonic development of an animal (the word, the Latin word meaning blackberry, and evokes the shape of this small cluster of cells). For others, these fossils could be bacteria . A study by Philip Donoghue and Stefan Bengtson, both paleontologists respectively at the University of Bristol and the Swedish Museum of Natural History , unveils publishing results undermining the two hypotheses in the journals Science
and PNAs . They are neither bacteria nor the embryos of animals.The new results were obtained using X-ray tomographic microscopy This method allows you to see inside fossils in three dimensions, without damaging them. About 450 fossils were analyzed. Fourteen cores were observed inside the fossilized cells. This result invalidates the hypothesis that these life forms are bacteria. Indeed, they are devoid of any nucleus. That same characteristic that earned them the name of prokaryotes (in contrast to eukaryotes, including ourselves, with plants, amoebae and some others).

Then are they animals? No. Cells were fossilized in the process of division when the nucleus cleave. However, the nuclear membrane the animal cells disappear during cell division. Therefore, the fossils do not contain animal cells. What is it then?

A single-celled ancestor of the animal kingdom

Some fossilized cell clusters observed in tomography are surrounded by a protective envelope. Cells located inside look like spores. A fossil even shows one of those ripped membranes with cells that escape. Compared with organisms living today, researchers have concluded that the fossils belonged to the group of Mesomycetozoea, part of the protists. This set is comprised mainly of single-celled organisms that are neither bacteria nor animals but can be considered as single-celled ancestors of the animal kingdom. In addition, species in this group produce large quantities of spores within a protective membrane over their reproduction. Thus, these old fossils 570 million years could account for the spores of the unicellular
ancestor of the animal kingdom. This information fills some holes that remain in the evolutionary history of animals.

According to the authors of this study, the results should be used to challenge existing theories about how the animals have learned to build multicellular structures from single cells. Some paleontologists, however, hold to clarify that it is necessary to take these results with caution because many fungi have characteristics similar to what was described by Philip Donoghue and colleagues. So, perhaps, other possible candidates. In science, everything is proved … until proven otherwise!

Abdullah Al Abbadi

Source: Planet Earth  published Dec 23, 2011


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