A team of researchers from the University of Queensland and the University of Alabama has found that the first multicellular organisms probably weren’t like the modern-day sponge cells, but were more like a collection of cells that could transition into multiple types, much like modern stem cells that give rise to the specialized cells in animals. Their work appears in the journal Nature.
Choanocyte-derived archaeocytes are capable of generating new choanocyte chambers (dotted line). Scale bar – 10 μm. Image credit: Sogabe et al, doi: 10.1038/s41586-019-1290-4.
“Scattered throughout the history of evolution are major transitions, including the leap from a world of microscopic single-cells to a world of multi-celled animals,” said University of Queensland’s Professor Bernie Degnan, co-lead author of the study.
“The great-great-great-grandmother of all cells in the animal kingdom, so to speak, was probably quite similar to a stem cell.”
“It’s one of those things that has always been in the textbooks, and people have always assumed, but no one has ever really tested. It’s important to test the traditional hypothesis,” said co-author Dr. Kevin Kocot, a researcher at the University of Alabama and curator of invertebrate zoology in the Alabama Museum of Natural History.
The closest relative to animals are single-celled organisms called choanoflagellates that look like microscopic tadpoles swimming in the ocean but lack many defining traits of animals. They look a lot like a type of cell in a sponge, the most ancient lineage of all animals.
It was thought the similarities between choanoflagellates and these sponge cells, choanocytes, meant they evolved from a common ancestor with this appearance.
In the study, the researchers looked at the genes active in the choanoflagellates, different types of sponge cells and other single-celled organisms.
Unexpectedly, they found choanoflagellates and the sponge’s choanocyte cells are not alike. In fact, they are the least alike of all the sponge’s cells.
Instead, they found choanoflagellates had the most in common with sponge cells called archeocytes, which function similarly to stem cells in more complex animals.
The first animal cells, then, were able to transition between multiple states, forming a more complex multicellular animal than previously thought.
“The first multicellular ancestor of animals weren’t just a simple ball of cells,” Dr. Kocot said.
“They probably had complex cells with lots of genetic diversity that were able to differentiate into other cell types.”
“We’re taking a core theory of evolutionary biology and turning it on its head,” said co-lead author Sandie Degnan, a biologist at the University of Queensland.
“Now we have an opportunity to re-imagine the steps that gave rise to the first animals, the underlying rules that turned single cells into multicellular animal life.”
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Shunsuke Sogabe et al. Pluripotency and the origin of animal multicellularity. Nature, published online June 12, 2019; doi: 10.1038/s41586-019-1290-4
