Study finds proof of origin of single-cell ancestors
Scientists in China, Sweden and the United Kingdom have discovered evidence of the single-celled ancestors of animals, dating from the interval in Earth's history just before the emergence of multicellular animals. The proof is found in rocks from south China, around 570 million years old. Presented in the journal Science, the study received partial funding under the EU's Seventh Framework Programme (FP7).
Past studies suggested that life evolved from a single-celled universal common ancestor, and at various points throughout the Earth's timeline, single-celled organisms meshed to emerge into larger and multicellular organisms. The wide variety of the animal kingdom is an example of this. But finding proof of these major evolutionary transitions has not been a successful endeavour... until now.
Led by the Swedish Museum of Natural History, researchers from China, Sweden, Switzerland and the United Kingdom have identified how the fossils preserve stages in the life cycle of an amoeba-like organism dividing into asexual cycles. One cell produces 2 cells, then 4, 8, 16, 16, 32 and so on. The outcome is hundreds of thousands of spore-like cells that were then released to relaunch the cycle. According to the researchers, the cell division pattern is very similar to the early stages of animal embryology, including that of the human, that scientists used to think represented the embryos of the earliest animals.
Putting the fossils under the microscope, and using high-energy X-rays, the team succeeded in revealing the organisation of the cells within their protective cyst walls. The researchers say the organisms should not have been fossilised. But not only were they not gooey clusters of cells, but they were buried in sediments rich in phosphate that had impregnated the cell walls and turned them to stone, they add.
Commenting on the findings of the study, lead author Therese Huldtgren from the Department of Palaeozoology at the Swedish Museum of Natural History says: 'The fossils are so amazing that even their nuclei have been preserved.'
Adds co-author Dr John Cunningham from the School of Earth Sciences at the University of Bristol in the United Kingdom: 'We used a particle accelerator called a synchrotron as our X-ray source. It allowed us to make a perfect computer model of the fossil that we could cut up in any way that we wanted, but without damaging the fossil in any way. We would never have been able to study the fossils otherwise!'
Using the X-ray microscopy helped the researchers show how the fossils had features that multicellular embryos do not possess. This led them to suggest that the fossils were neither animals nor embryos, but rather the reproductive spore bodies of single-celled ancestors of animals.
Co-author Professor Philip Donoghue, also of the University of Bristol, comments: 'We were very surprised by our results — we've been convinced for so long that these fossils represented the embryos of the earliest animals — much of what has been written about the fossils for the last 10 years is flat wrong. Our colleagues are not going to like the result.'
For his part, co-author Professor Stefan Bengtson says: 'These fossils force us to rethink our ideas of how animals learned to make large bodies out of cells.'
Experts from Stockholm University in Sweden, the University of Bristol in the United Kingdom, the Institute of Geology and Geophysics at the Chinese Academy of Geological Sciences, the Swiss Light Source at the Paul Scherrer Institute, and the Institute for Biomedical Engineering of the University of Zurich and of the Swiss Federal Institute of Technology Zurich in Switzerland contributed to this study.