It appears that multicellular organisms do not have a monopoly of learning and recalling experiences. The single-celled slime mold can do the same, according to a new study.
For the first time, scientists showed that an organism devoid of a nervous system — particularly the slime mold or Physarum polycephalum — can perform type of learning known as habituation. This sheds light on the evolutionary process of learning, contradicting the belief that it requires neurons.
“Evidence for learning in non-neural multicellular organisms is scant, and only a few unequivocal reports of learning have been described in single-celled organisms,” wrote the authors.
But the thinking is slowly changing as scientists venture into the abilities of brainless animals. The slime mold, for instance, is an amoeba-like, single-celled creature with multiple nuclei, inhabiting shady and cool areas and feeding on bacteria and fungi for hundreds of millions of years now.
Yet the slime mold has exhibited a number of astonishing intellectual abilities, such as avoiding traps, solving a maze, or tweaking its nutrition. And it can get more surprising.
In a nine-day experimental study, researchers from France’s Toulouse University challenged various groups of this species with bitter yet harmless substances they should pass through to get to delicious oats. One group face a bridge filled with quinine or caffeine, while the other crossed a non-challenged bridge.
Gradually and after being initially reluctant to pass through the bitter substances, the molds in the first group realized that the implanted items were harmless. They traveled through them in an increasingly quick fashion, behaving the same way as the second group after six day.
It’s called habituation, or when one sheds the fear of a specific behavior toward something after being confronted with it several times. This type of learning, previously demonstrated in invertebrates known as sea hares, exists in all creatures but had never been seen previously in non-neural ones.
The slime mold’s habituation, however, emerged as specific to a given substance, as those habituated to quinine showed a distrust of caffeine and vice-versa. Also after a couple of days’ rest to likely forget about the bitter taste experience, the molds reached to a quinine or caffeine-laced bridge as if it was the first day they encountered them, forgetting that they had deemed the bitter bridges safe.
According to lead author and master’s student Romain Boisseau from Paris’ Ecole Normale Superieure this mechanism may have appeared very early in the history of life itself.
“Probably learning abilities evolved first, before the evolution of neurons and nervous systems,” he said.
The findings were published April 27 in the journal Proceedings of the Royal Society B.