The very first lifeforms were single-celled and remained so for the first billions of years of evolution. It was only around 600 million years ago that life made the jump into complex multicellularity. Today, we tend to think of single-celled organisms as primitive and rather daft, but we should know better considering the history of the tree of life.
Single-celled organisms are actually pretty smart. Change my mind
In the early 20th century, an American zoologist and geneticist, by the name of Herbert Spencer Jennings, published a study in which he described a never before seen behavior.
Using a microscope and a pipette, Jennins stimulated a freshwater single-celled organism called Stentor roeselii. The microscopic creature is relatively large and features a trumpet-shaped body lined with hairlike projections called cilia, which it uses to swim in fluid and feed. At one end of its body, S. roeselii secrets a holdfast -- a sticky substance that allows the single-celled organism to attach to debris in order to remain stationary while feeding.
Jennings meticulously documented how S. roeselii responded to carmine powder, which has irritant properties. The scientist noted that the organism bent its body to avoid the powder but if the exposure was continuous, it would reverse the movement of the cilia to expel particles away from the feeding orifice. If both these tactics failed, the organism would contract, coiling itself around the holdfast. If this too didn't work, S. roeselii would detach its holdfast and simply swim away.
This is a striking display of complex behaviors for a single-celled organism. They form a hierarchy of decisions, which are carried out based on ranked preferences.
At the time of the study's publication (in 1906) everyone was stunned. But the enthusiasm didn't last long when no other group was able to replicate the findings -- until now.
More than a century after publishing his landmark paper, Jennings has been vindicated by a recent study authored by scientists at the Harvard Medical School.
"Our findings show that single cells can be much more sophisticated than we generally give them credit for," said corresponding study author Jeremy Gunawardena, who is an associate professor at the Harvard Medical School.
Gunawardena became interested in the subject when he learned that one 1967 study that presumably refuted Jennings' findings didn't actually use S. roeselii, but rather a different species, Stentor coeruleus, which could swim but did not attach to feed. Well, no wonder they weren't able to replicate the findings, the scientist thought to himself.
In time, the researcher became fascinated with Jennings' original experiment. As a mathematician by training, Gunawardena could not try replicating the study by himself but each time he would bring the subject up with biologist colleagues, the discussion didn't seem to lead anywhere. No one was really interested.
"I kept bringing up this idea at my lab group meeting, saying that it tells us something about the capabilities of single cells. We don't think this way about how cells work anymore," he said. "And, unsurprisingly, no one was interested. It's ancient history, it's descriptive biology—all the things young, bright trainees wouldn't touch."
But eventually, Gunawardena found some help. He piqued the interest of Sudhakaran Prabakaran, now a group leader at the University of Cambridge in England, and Joseph Dexter, now a fellow at the Neukom Institute for Computational Science at Dartmouth.
The three embarked on a project that took more than eight years to complete. There was no grant money whatsoever -- they did it all in their own free time, as an unpaid side project. "It wasn't anyone's day job," Gunawardena said.
Finding S. roeselii wasn't easy, but they eventually got lucky and found a supplier in England. They then replicated Jenning's original experiment using modern tools such as video microscopy. Instead of using a pipette, the researchers employed a micropositioning system that accurately delivered the irritant near the mouth of the tiny organism.
Initially, they used carmine powder as the irritant, but the response wasn't impressive. They eventually found that microscopic plastic beads elicited the hierarchic response to the irritant stimulus -- although it wasn't that straightforward.
There was a lot of variation among different subjects -- one might alternate between bending and contracting, while others might bend and alter their cilia before contracting. But a statistical analysis uncovered a common pattern.
When an irritant is first introduced, S. roeselii will generally start bending and alter its cilia. If the stimulus continues to be present, it will contract or detach and swim away.
"They do the simple things first, but if you keep stimulating, they 'decide' to try something else. S. roeselii has no brain, but there seems to be some mechanism that, in effect, lets it 'change its mind' once it feels like the irritation has gone on too long," Gunawardena said.
"This hierarchy gives a vivid sense of some form of relatively complex, decision-making calculation going on inside the organism, weighing whether it's better to execute one behavior versus another," he said.
Gunawardena says that it all makes sense considering S. roeselii's ecological role.
"Organisms like S. roeselii were apex predators prior to multicellular life, and they are extremely widespread in many different aquatic environments," he said. "They have to be 'clever' at figuring out what to avoid, where to eat and all the other things that organisms have to do to live. I think it's clear that they can have complex ways of doing so."
It's incredibly fascinating to see modern research reviving such important but overlooked science. Going forward, the fact that single cells are capable of highly complex behavior could have major implications in biology, including cancer research.
"I think this experiment forces us to think about the existence of, very speculatively, some form of cellular 'cognition,' in which single cells can be capable of complex information processing and decision-making in response," he continued. "All life has the same underpinnings, and our results give us at least one piece of evidence for why we should be broadening our view to include this kind of thinking in modern biology research."
"It also illustrates how, sometimes, we tend to ignore things not because they don't exist, but because we don't think it's important to look at them," he added. "I think that's what makes this study so interesting."
The findings appeared in the journal Current Biology.