A team of scientists from the University of Queensland used a new technology to study the evolution of multi-celled animals, and uncovered something surprising in the process. Their discovery has been published in Nature.
“We’ve found that the first multicellular animals probably weren’t like the modern-day sponge cells, but were more like a collection of convertible cells,” said Professor Bernie Degnan. “The great-great-great-grandmother of all cells in the animal kingdom, so to speak, was probably quite similar to a stem cell.”
“This is somewhat intuitive as, compared to plants and fungi, animals have many more cell types, used in very different ways — from neurons to muscles — and cell-flexibility has been critical to animal evolution from the start,” Professor Degnan explained.
This new discovery completely disproves a centuries-old theory that suggests all multi-celled animals evolved from a single-celled ancestor called choanocyte, which is similar to a modern sponge cell.
“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,” Professor Degnan said. “With multicellularity came incredible complexity, creating the animal, plant, fungi and algae kingdoms we see today. These large organisms differ from the other more-than-99-per-cent of biodiversity that can only be seen under a microscope.”
The team sequenced all the genes expressed in each individual cell studied, which allowed them to compare similar cell types over time. By potentially finding the “signatures” of each cell type, the team could then discover the evolutionary history of each type.
“Biologists for decades believed the existing theory was a no-brainer, as sponge choanocytes look so much like single-celled choanoflagellates – the organism considered to be the closest living relatives of the animals,” said senior coauthor Associate Professor Sandie Degnan. “But their transcriptome signatures simply don’t match, meaning that these aren’t the core building blocks of animal life that we originally thought they were.”
“This technology has been used only for the last few years, but it’s helped us finally address an age-old question, discovering something completely contrary to what anyone had ever proposed,” she continued. “We’re taking a core theory of evolutionary biology and turning it on its head.”
“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,” she concluded.
As Professor Bernie Degnan stated, this discovery could help scientists better understand our own human evolution and stem cells, which could lead to better understanding cancer.
Image Credit: University of Queensland