Scientists Have Proposed a New Ancestor for All Animal Life

A team of scientists say they have found a new candidate for the origins of animal life, according to a study published in the journal Nature.

It has long been thought by many biologists that the first multi-celled animals evolved from single-celled organisms resembling choanocytes—microbes which are found in the internal chambers of modern-day sponges.

These choanocytes look very similar to other single-celled organisms called choanoflagellates, which are considered to be the closest living relative of animals.
"Most biologists for decades have believed this theory to be true, as sponge choanocytes look so much like single-celled choanoflagellates," Sandie Degnan, an author of the study from the University of Queensland, Australia, said in a statement.

But the Queensland researchers have challenged this idea, suggesting instead that these original animals evolved from another type of cell that can change into many different forms, much like the stem cells found inside our own bodies.
"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," Bernie Degnan, another author of the study from Queensland, said in a statement. "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," he said.

Using new technology, the Queensland team tested the idea that the first animals resembled choanocytes and choanflagellates by comparing the transcriptomes—a set of genetic molecules—of different types of cells.

The organisms the team investigated included three three cell types found in sponges—including choanocytes—choanoflagellates and other single-celled microbes belonging to the group holozoans. For each organism, they look for certain transcriptome "signatures" in order to understand more about their evolutionary history.

"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," Sandie Degnan said. "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."

"We're taking a core theory of evolutionary biology and turning it on its head," she said. "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."

Scanning electron microscopy image of an Amphimedon queenslandica intact chonanocyte chamber in which many individual choanocytes are visible. Rebecca Fieth