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For generations, textbooks have had an account of where animals came from, with the earliest forms being something like sea sponges. A comparison of the genes of cells that make up sponges and those from single-celled organisms has thrown this into doubt. Instead, the authors propose, we are descended from something more like a stem cell, a single ancient cell capable of shifting into many forms.
Sponge cells called choanocytes have a tail or flagellum that they use to beat water to create a flow of nutrients for them to absorb. The standard story is that animals came about because choanocyte-like cells formed colonies, with some of them eventually developing specialist roles until none could get by without each other.
Professor Bernie Degnan of the University of Queensland set out to test this by sequencing the genes in choanocytes, but also other components of sea sponges. What he found didn't support the dominant theory at all. Choanocytes' transcriptome signatures turn out to be very different from the single-celled organisms they are supposed to have descended from. In the best part of a billion years, some big changes are certainly possible, but the differences are not what was expected.
Instead, Degnan told IFLScience, our closest unicellular relatives more closely match archaeocytes, the highly flexible sponge cells that resemble our own stem cells in their capacity to take on different forms as the need arises. “All organisms have life cycles,” Degnan said. “These are dictated by external environmental conditions like the presence of food. Single-celled organisms can change over the course of their lives into different forms to take advantage of this.”
Degnan's work was published in Nature. He admitted the theory still has “a lot of handwaving” when it comes to explaining what drove these flexible cells to come together to form multi-celled animals and this was “beyond the scope of our study”.
"The great-great-great-grandmother of all cells in the animal kingdom, so to speak, was probably quite similar to a stem cell,” Degnan said in a statement. "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."
Degnan and co-authors compared the genes from choanocytes, mesenchymal archaeocytes, and a third type of sponge cell, epithelial pinacocytes with choanoflagellates, considered the closest single-celled organisms to animals surviving today, and therefore likely candidates to resemble what we evolved from.
Not only are archaeocytes from Amphimedon queenslandica, the sponge chosen for the treatment, the closest match, but choanocytes have the least genetic similarity, despite the visual resemblance, reinforcing that the last common ancestor of all animals was far more flexible than we have imagined.
