Washington: Scientists have found fossils that shed light on the evolutionary origin of animals from single-cell ancestors.
The fossils preserve stages in the life cycle of an amoeba-like organism dividing in asexual cycles, first to produce two cells, then four, eight, 16, 32 and so on, ultimately resulting in hundreds of thousands of spore-like cells that were then released to start the cycle over again.
The pattern of cell division is so similar to the early stages of animal, including human, embryology that until now they were thought to represent the embryos of the earliest animals.
The researchers studied the microscopic fossils using high energy X-rays at the Swiss Light Source in Switzerland, revealing the organisation of the cells within their protective cyst walls.
The organisms should not have been fossilized, they were just gooey clusters of cells, but they were buried in sediments rich in phosphate that impregnated the cell walls and turned them to stone.
“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!” John Cunningham, the study co-author, said.
This X-ray microscopy revealed that the fossils had features that multicellular embryos do not, and this led the researchers to the conclusion that the fossils were neither animals nor embryos but rather the reproductive spore bodies of single-celled ancestors of animals.
“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 ten years is flat wrong. Our colleagues are not going to like the result,” Philip Donoghue, said.
The study has been recently reported in Science.