Some single-celled organisms are capable of adopting a hierarchical response to environmental conditions, trying certain options only when others have been found to fail. The observations vindicate claims that have been mocked for more than a century, and demonstrate the capacity for autonomy to exist even without cells specialized for thinking.
A century ago, Herbert Spencer Jennings described the single-celled protist Stentor roeseli's response to unpleasant stimuli. S. roeseli usually attaches itself to algae and beats its cilia to bring food particles close enough to consume. Jennings reported that when exposed to an infusion of carmine powder, S. roeseli will usually bend out of the way of the noxious stream, as if hoping it will quickly pass and things will return to normal.
However, when he repeatedly applied the same chemical, Jennings found the protist would try alternative approaches, either reversing the direction in which the cilia beat to repel what was troubling it or either contraction or detachment. In the last case, the protist would float off to seek some more attractive perch.
For humans, or even advanced animals, this sequence looks normal, with minimal responses that escalate only if necessary. It's much more unexpected in something lacking a central nervous system. How does something without a brain develop a memory, let alone the capacity to apparently “decide” the threat is ongoing enough to require relocation?
Jennings' account was consequently met with skepticism, and when others failed to replicate his work, it was dismissed. Yet 70 years after his death, Jennings has been vindicated.
In Current Biology, a team led by Harvard's Dr Jeremy Gunawardena describe firing sodium azide-carrying polystyrene beads at some S. roeseli under a microscope and recording the response. Video evidence of exactly the sequence Jennings described is there for all to see – first evasion, then attempts to repel the threat, and finally contraction or escape when it was clear the balls were not going away.
"We consider the behavior hierarchy as a form of sequential decision making," the paper notes, "in the sense that, when given similar stimulation repeatedly, the organism 'changes its mind' about which response to give."
Gunawardena thinks the reason those who sought to replicate Jennings failed was that they were using a different and more mobile species of Stentor, having not been able to get hold of S. roeseli. Nevertheless, the authors acknowledge not all the S. roeseli exposed to the bead bombardment followed the neat order Jennings described. On a group basis, this pattern was typical, but many specimens opted to try to beat away the balls as their first option.
The mystery of how some single-celled organisms can change their minds in response to new information is only matched by the puzzle of why so many people apparently refuse to do the same.