Evolutionary biologist Thibaut Brunet was studying single-celled organisms called choanoflagellates when he noticed something strange: The microbes are typically rigid, but when they got trapped in a tight space, they started to move like The Blob (see video, above). In his lab at the University of California (UC), Berkeley, he watched as their external whiplike flagella disappeared; parts of their bodies began to push out, forming bubbles called blebs; and they were able to squeeze into new spaces, like jelly pushing through a maze.
Because choanoflagellates are close relatives of animals, the find suggests complex movements first evolved in the ancestors of both groups. It also lends support to the idea that animals evolved from an ancestor that resembled choanoflagellates, says Maja Adamska, an evolutionary developmental biologist at Australian National University who was not involved with the work. “The finding is so clear—it just makes you wonder why no one looked before.”
After Brunet made his initial observation, he, UC Berkeley evolutionary biologist Nicole King, and their colleagues put the choanoflagellates through more workouts. They used different ways to confine the “choano,” as Brunet has nicknamed them, including placing them in chambers with both narrow and wide areas. Each time, the microbes became blobs that shimmied to escape, the team reports this week in eLife. The choano could even readily switch between crawling and swimming to get out of a tight squeeze in their watery environment.
These two behaviors are reminiscent of those seen in animal life today. Animals rely on two basic types of tissue organization. One is a flat sheet of epithelial cells that have an up-and-down orientation—like the cell of a swimming choanoflagellate, which has a distinct top and bottom (see video, right). The other form is 3D and includes more free-form cells that crawl around during development, settling into specific spots to become organs. The new work demonstrates that choano can be both types, switching from its usually stiff cell to the deformable one under stress.
This ability to switch back and forth may have been critical as early animals began to explore new environments. Eventually, organisms evolved the ability to form the different types of cells at the same time in different parts of the body. That set the stage for complex multicellular organisms, and eventually, humans to arise, Brunet and King suggest.
Researchers have debated which came first: the ability to develop into an organism with lots of cells, or the ability to produce different cell types. This new-found flexibility in choanoflagellates suggests “this ability to alternate between cell states predated multicellularity,” King says. By studying choanoflagellates, she and her colleagues hope to learn about the organism that gave rise to both choanoflagellates and animals. “We are seeing a much more nuanced and detailed view of the last common ancestor.”