Volume 2, Issue 17 December 2005/January 2006
Our Single-Celled Ancestors
Biologist Nicole King studies tiny creatures that may be the closest living relatives to our single-celled ancestors.
Six-hundred million years ago, a pivotal turning point in the history of life occurred. In the ancient sea, multicellular organisms evolved that are now recognized as the world's first animals. But what was the biology of the single-celled organism that made the transition? And how did it become the common progenitor of all animals? To answer these questions, UC Berkeley biologist Nicole King studies tiny creatures called choanoflagellates that may be the closest living relative to our single-celled ancestors.
"These early organisms are not preserved in the fossil record, so we don't know very much about how multicellularity first evolved," says King, a professor in the Departments of Integrative Biology and Molecular and Cell Biology. "But choanoflagellates might provide insight into that transition."
Choanoflagellates are one-celled protozoans that live in fresh water and the ocean. Resembling sperm, the tiny organisms are approximately 10 microns across—nearly 100 would fit on the head of a pin. While choanoflagellates have long been suspected to be relatives of animals, studying their basic biology has historically been difficult. In recent years though, the genomics revolution has spawned techniques that are enabling King and her colleagues to look closely at the cellular secrets inside the organisms.
Propelled by their flagella, choanoflagellates move through water collecting bacteria on a collar of tentacles at the base of the cell body. (photo by Melissa Mott)
"When I first stumbled upon choanoflagellates, I was dumbfounded," says King, who in September received a prestigious MacArthur Foundation "genius award." "As a scientist interested in the cellular bases for animal development, I couldn't believe what a goldmine these organisms might be. Once I realized that we could apply genomic tools to study them, this project really opened up."
King's earliest experiments helped confirm that choanoflagellates are indeed closely related to animals. Next, she and her colleagues surveyed the organism's genes at a high level and quickly discovered that it contains genes that were previously thought to only exist in animals. The big surprise was that two of those genes are actually used by animals to express proteins for cell adhesion and cell communication. In other words, a single-celled animal is making proteins that are seemingly essential only to multicellular animals.
"It's amazing." King says. "We interpret that as evidence that some of the protein machinery for multicellularity actually evolved before the origin of animals, before multicellularity itself. The proteins predated their current function in animals."
According to King, this "a classic example of co-option," an evolutionary process in which an existing biological structure or system is adapted for a new function.
"Right now, we're very interested in understanding how the proteins function in choanoflagellates and to use that as a tool in investigating what they might have been doing in the common ancestor," she says.
Choanoflagellates were first considered to be close relatives of animals in the late nineteenth century. (courtesy the researchers)
As one of only a handful of laboratories around the world studying the choanoflagellates using methods from molecular and cell biology, King's research group is developing most of their techniques from scratch. Meanwhile, they're collaborating with scientists from the Department of Energy's Joint Genome Institute, who are sequencing the whole genome of a choanaoflagellate. Once completed, the code will enable the King and her colleagues to reconstruct the minimal genome of the last common ancestor and seek out the hidden details of its evolutionary history.
"I was surprised to learn that so much of animal biology was in place before the origin of animals," King says. "And I think that's what motivates most scientists--not learning that you were right, but learning that you were wrong."