Up until now, habituation—a basic form of learning—was thought to occur only in more complex beings like worms, insects, birds, and mammals, which possess brains and nervous systems. However, recent research presents strong evidence that even minuscule single-celled organisms like ciliates and amoebae, as well as the cells within our own bodies, might display habituation similar to that of more advanced organisms with brains.
For instance, a dog learns to obey the command to sit, or a person gradually ignores the sound of a washing machine while reading. The ability to learn and adapt is crucial for evolution and survival.
Habituation, which is a simpler form of adaptation, refers to the reduced response to a stimulus after repeated exposure. For example, consider how you need more caffeine to stay focused as your body becomes accustomed to the initial boost from one espresso shot.
Previously, habituation was considered a trait limited to organisms with complex structures, such as those with brains and nervous systems.
However, a new study published on November 19 in Current Biology provides convincing evidence that even tiny single-celled organisms, like ciliates and amoebae, as well as our own cellular structures, can exhibit habituation similar to that found in more complex beings.
This research, conducted by scientists from Harvard Medical School and the Centre for Genomic Regulation (CRG) in Barcelona, indicates that solitary cells are capable of exhibiting behaviors that were previously underestimated.
“This discovery invites an intriguing question: How are brainless cells capable of such complex behaviors?” stated Jeremy Gunawardena, a senior author of the study and associate professor of systems biology at the Blavatnik Institute at HMS. He co-led the research with Rosa Martinez Corral, a former post-doctoral researcher in his lab who now leads a systems and synthetic biology research group at CRG.
The findings contribute to a burgeoning field of research. Earlier studies directed by Gunawardena revealed that a single-celled ciliate exhibited avoidance behavior, reminiscent of responses seen in animals faced with unpleasant conditions.
What the researchers found
Rather than observing cells in a conventional lab setting, the researchers employed sophisticated computer modeling to examine how molecular networks within ciliate and mammalian cells react to various stimulation patterns. They identified four networks that displayed signs of habituation typically found in animal brains.
These networks shared a key trait: each molecular network possessed two types of “memory” storage that recorded information gained from their surroundings. One type of memory faded considerably faster than the other, indicating a form of memory decay essential for habituation, the researchers explained. This leads to the conclusion that single cells can process and retain information over varying time durations.
Implications
Investigating habituation in single cells may enhance our understanding of how learning functions on a broader scale, according to the researchers. Additionally, this research reframes our perception of these basic organisms—they are not just molecular machines encased in minute forms, but are also entities capable of learning.
So, what could be the practical implications of this research?
The researchers remind us that these applications are still largely hypothetical for now. However, one intriguing possibility could be exploring the concept of habituation in the context of cancer and the immune response.
Cancerous tumors are skilled at dodging immune detection by misguiding immune cells into perceiving them as harmless entities. Essentially, it’s as if the immune cells have become habituated to cancer cells—they stop responding to the presence of the cancer, treating it like a familiar stimulus.
“It’s somewhat like a misconception. If we could understand how these false impressions form in immune cells, we might find ways to reprogram them so that they accurately recognize their surroundings, identify tumors as malignant, and launch an attack,” Gunawardena remarked.
“This is merely a dream at the moment, but it is a promising direction I would love to pursue in the future.”
