Animal-Like Embryos Evolved Long Before Complex Animals, Scientists Say

Complex animals evolved from single-celled ancestors, before diversifying into 30-40 distinct anatomical designs. When and how this major evolutionary transition occurred is the focus of intense debate. Now, an international team of researchers from the United Kingdom, China and Switzerland has found evidence that a key step in this transition occurred long before complex animals appear in the fossil record, in the fossilized embryos that resemble multicellular stages in the life cycle of single-celled relatives of animals.

Yin et al use X-ray microtomography to show that the 609-million-year-old Caveasphaera develops within an envelope by cell division, ingression, detachment, and polar aggregation in a manner analogous to gastrulation; together with evidence of functional cell adhesion and development within an envelope, this is suggestive of a holozoan affinity. The image shows a life cycle of Caveasphaera and a simplified phylogenetic tree of Holozoa. Image credit: Yin et al, doi: 10.1016/j.cub.2019.10.057.

Yin et al use X-ray microtomography to show that the 609-million-year-old Caveasphaera develops within an envelope by cell division, ingression, detachment, and polar aggregation in a manner analogous
to gastrulation; together with evidence of functional cell adhesion and development within an envelope, this is suggestive of a holozoan affinity. The image shows a life cycle of Caveasphaera and a simplified phylogenetic tree of Holozoa. Image credit: Yin et al, doi: 10.1016/j.cub.2019.10.057.

University of Bristol’s Professor Philip Donoghue, Dr. Zongjun Yin from the Nanjing Institute of Geology and Palaeontology and colleagues focused on the fossils of Caveasphaera, a multicellular organism that lived 609 million years ago (Ediacaran period) in what is now China.

Individual Caveasphaera fossils are only about half a millimeter in diameter, but X-ray microscopy revealed that they were preserved all the way down to their component cells.

“This study shows the amazing detail that can be preserved in the fossil record but also the power of X-ray microscopes in uncovering secrets preserved in stone without destroying the fossils,” said Dr. Federica Marone, a scientist with the Paul Scherrer Institute.

“We were able to sort the fossils into growth stages, reconstructing the embryology of Caveasphaera,” added Dr. Kelly Vargas, a researcher at the University of Bristol.

“Our results show that Caveasphaera sorted its cells during embryo development, in just the same way as living animals, including humans, but we have no evidence that these embryos developed into more complex organisms,” Dr. Yin said.

Caveasphaera had a life cycle like the close living relatives of animals, which alternate between single-celled and multicellular stages.

“However, Caveasphaera goes one step further, reorganizing those cells during embryology,” said Dr. John Cunningham, also from the University of Bristol.

Caveasphaera is the earliest evidence of this most important step in the evolution of animals, which allowed them to develop distinct tissue layers and organs,” added Dr. Stefan Bengtson, from the Swedish Museum of Natural History.

“I’m not totally convinced that Caveasphaera is an animal. It 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 Dr. Maoyan Zhu, from the Nanjing Institute of Geology and Palaeontology.

Caveasphaera shows features that look both like microbial relatives of animals and early embryo stages of primitive animals. We’re still searching for more fossils that may help us to decide,’ Professor Donoghue said.

“Either way, fossils of Caveasphaera tell us that animal-like embryonic development evolved long before the oldest definitive animals appear in the fossil record.”

The study was published in the journal Current Biology.

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Zongjun Yin et al. The Early Ediacaran Caveasphaera Foreshadows the Evolutionary Origin of Animal-like Embryology. Current Biology, published online November 27, 2019; doi: 10.1016/j.cub.2019.10.057

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