The brain is an incredible evolutionary feat that allows animals, including humans, to adapt and thrive in unpredictable environments. Learning, in particular, has proven to be crucial for a good life. However, what about organisms that lack a brain? From jellyfish and corals to plants, fungi, and bacteria, the pressure to survive and reproduce is still intense, and the value of learning remains significant.
Recent research on brainless organisms has explored the origins and inner workings of cognition, challenging our understanding of what it means to learn. Learning is defined as any change in behavior resulting from experience and can take various forms. Non-associative learning involves sensitizing or habituating one’s response to repeated exposure. Associative learning occurs when a cue is consistently tied to a behavior. Higher forms of learning, such as conceptual, linguistic, and musical learning, require complex coordination and reflection on one’s own thinking, and are limited to organisms with complex brains.
However, the relationship between brain complexity and cognitive ability is not straightforward across the tree of life. Fundamental forms of learning have been observed in organisms without brains, reshaping our understanding of what was previously thought possible.
For example, the beadlet anemone can habituate to the presence of nearby clones, recognizing them over time and containing its aggression. Box jellyfish, despite having only a few thousand neurons clustered around their eyes, can associate changes in light intensity with touch feedback and adjust their swimming accordingly. This enhances their navigation abilities in mangrove habitats.
Even organisms without neurons can exhibit learning. Slime molds, single-celled organisms belonging to the protist group, have shown cognitive abilities such as remembering routes to food and using past experiences for future foraging. Plants like Venus flytraps can remember and tally up touches from living prey before closing their traps for digestion. The shameplant can habituate and learn to ignore repeated false alarms, while the garden pea can associate a gentle breeze with the presence of essential sunlight.
These findings challenge our notions of sensation, thought, and behavior in biology. They also have ethical implications, such as our understanding of pain perception in animals without complex brains. If these organisms can learn and feel, it raises questions about how we treat them in various aspects of life.
Overall, the diverse and curious forms of life without brains demonstrate the power of adaptive evolution. They encourage us to reflect on our position in the tree of life and emphasize the value of studying, appreciating, and conserving lives different from our own.
