University of Bristol – November 27,
2019 -- A new study by an international team of researchers, led by scientists
from the University of Bristol and Nanjing Institute of Geology and
Palaeontology, has discovered that animal-like embryos evolved long before the
first animals appear in the fossil record.
Animals evolved from single-celled
ancestors, before diversifying into 30 or 40 distinct anatomical designs. When
and how animal ancestors made the transition from single-celled microbes to
complex multicellular organisms has been the focus of intense debate.
Until now, this question could only be
addressed by studying living animals and their relatives, but now the research
team has found evidence that a key step in this major evolutionary transition
occurred long before complex animals appear in the fossil record, in the
fossilised embryos that resemble multicellular stages in the life cycle of
single-celled relatives of animals.
The team discovered the fossils named Caveasphaera
in 609 million-year old rocks in the Guizhou Province of South 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.
Kelly Vargas, from the University of
Bristol’s School of Earth Sciences, said: “X-Ray tomographic microscopy works
like a medical CT scanner, but allows us to see features that are less than a
thousandth of a millimeter in size. We were able to sort the fossils into
growth stages, reconstructing the embryology of Caveasphaera.”
Co-author Zongjun Yin, from Nanjing
Institute of Geology and Palaeontology in China, added: “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.”
Co-author Dr John Cunningham, also from
University of Bristol, 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,
reorganising those cells during embryology.”
Co-author Stefan Bengtson, from the
Swedish Museum of Natural History, said “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”.
Co-author Maoyan Zhu, also from Nanjing
Institute of Geology and Palaeontology, said he is not totally convinced that Caveasphaera
is an animal. He added: “Caveasphaera looks a lot like the embryos
of some starfish and corals – we don’t find the adult stages simply because
they are harder to fossilise
Co-author Dr Federica Marone from the
Paul Scherrer Institute in Switzerland said “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.”
Co-author Professor Philip Donoghue,
also from the University of Bristol’s School of Earth Sciences, said “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.
“Either way, fossils of Caveasphaera tell
us that animal-like embryonic development evolved long before the oldest
definitive animals appear in the fossil record.”
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