First Animal Not What Scientists Have Long Thought

TUSCALOOSA, Ala. — The last common ancestor of all animal life was more like modern stem cells and not a clump of similar cells, as has long been thought.

The finding, published in Nature, will likely mean rewriting textbooks and encyclopedias as it changes the idea that the common ancestor of all animals about 800 million years ago was a simple ball of cells.

Instead, the first multicellular animal was likely a collection of cells that could transition into multiple types, much like modern stem cells that give rise to the specialized cells in animals, a genetic investigation involving a biologist from The University of Alabama argued.

“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,” said Dr. Kevin M. Kocot, UA assistant professor in biological sciences and curator of invertebrate zoology in the Alabama Museum of Natural History. “It’s important to test the traditional hypothesis.”

Kocot performed genetic data analysis for a team of scientists from University of Queensland in Australia, who led the research.

“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 Dr. Bernie Degnan, professor of biological sciences at UQ.

“Contrary to our prior understanding, 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,” he said. “The great-great-great-grandmother of all cells in the animal kingdom, so to speak, was probably quite similar to a stem cell.”

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. Thus, scientists reasoned, the first multicellular animal looked and behaved like a ball of choanoflagellates.

The UQ and UA scientists 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,” Kocot said. “They probably had complex cells with lots of genetic diversity that were able to differentiate into other cell types.”

The work means scientists do not know exactly what the first multicellular animal looked like, but they probably did not look like choanoflagellates, Kocot said.

“We’re taking a core theory of evolutionary biology and turning it on its head,” said Dr. Sandie Degnan, associate professor of biological sciences at UQ. “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.”