Evolutionary biologist Thibaut Brunet was studying single-celled organisms called choanoflagellates when he noticed something strange: Microbes are typically rigid, but when trapped in a confined space, they begin to move like The drop (see video, above). In his laboratory at the University of California (UC), Berkeley, he observed his external flagella disappear in the form of a whip; parts of their bodies began to come out, forming bubbles called blisters; and they were able to slip into new spaces, like jelly making their way through a maze.
Because choanoflagellates are close relatives of animals, the finding suggests that complex movements evolved first in the ancestors of both groups. It also supports the idea that animals evolved from an ancestor that resembled choanoflagellates, says Maja Adamska, an evolutionary developmental biologist at the Australian National University who was not involved in the work. “The finding is so clear, it just makes one 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, even placing them in chambers with narrow and wide areas. Each time, the microbes turned into droplets that moved to escape, the team reports this week in eLife. The choano could even easily switch between crawling and swimming to get out of a squeeze in its watery environment.
These two behaviors are reminiscent of those seen in today’s animal life. Animals depend on two basic types of tissue organization. One is a flat sheet of epithelial cells that have an upward and downward orientation, like the cell of a swimming choanoflagellate, that has a distinct top and bottom (see video to the right). The other form is 3D and includes more free-form cells that crawl during development, settling in specific locations to become organs. The new work shows that the choano can be of both types, going from its normally rigid cell to the deformable one under stress.
This ability to switch from one place to another may have been essential when the first animals began to explore new environments. Eventually, organisms developed the ability to form 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, Brunet and King suggest.
Researchers have debated which came first: the ability to become an organism with many cells or the ability to produce different types of cells. This new flexibility in choanoflagellates suggests that “this ability to switch between cell states predates multicellularity,” says King. 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.”