Learning Without a Brain: A Closer Look at Evolutionary Cognition

Monday - October 16 2023, 22:15 UTC - 1 year ago

Learning is any change in behavior as a result of experience, and it comes in many forms. Recent research on the brainless has refuted the notion that complex brains are necessary for sophisticated cognition. Jellyfish, sea anemones, and even single-celled organisms like slime molds have been shown to possess notable learning ability.

The brain is an evolutionary marvel. By shifting the control of sensing and behavior to this central organ, animals (including us) are able to flexibly respond and flourish in unpredictable environments. One skill above all—learning—has proven key to the good life.

But what of all the organisms that lack this precious organ? From jellyfish and corals to our plant, fungi, and single-celled neighbors (such as bacteria), the pressure to live and reproduce is no less intense, and the value of learning is undiminished.

Recent research on the brainless has probed the murky origins and inner workings of cognition itself, and is forcing us to rethink what it means to learn.

Learning About Learning .

Learning is any change in behavior as a result of experience, and it comes in many forms. At one end of the spectrum sits non-associative learning. Familiar to anyone who has "tuned out" the background noise of traffic or television, it involves turning up (sensitizing) or dialing down (habituating) one’s response with repeated exposure.

Further along is associative learning, in which a cue is reliably tied to a behavior. Just as the crinkling of a chip packet brings my dog running, so too the smell of nectar invites pollinators to forage for a sweet reward.

Higher still are forms like conceptual, linguistic, and musical learning, which demand complex coordination and the ability to reflect on one’s own thinking. They also require specialized structures within the brain, and a large number of connections between them. So, to our knowledge, these types of learning are limited to organisms with sufficient "computing power"—that is, with sufficiently complex brains.

The presumed relationship between brain complexity and cognitive ability, however, is anything but straightforward when viewed across the tree of life.

This is especially true of the fundamental forms of learning, with recent examples reshaping our understanding of what was thought possible.

Who Needs a Brain? .

Jellyfish, jelly-combs, and sea anemones stand among the earliest ancestors of animals, and share the common feature of lacking a centralized brain.

Nonetheless, the beadlet anemone (Actinia equina) is able to habituate to the presence of nearby clones. Under normal circumstances it violently opposes any encroachment on its territory by other anemones. When the intruders are exact genetic copies of itself, however, it learns to recognize them over repeated interactions, and contain its usual aggression.

A recent study has now shown box jellyfish too are avid learners, and in an even more sophisticated manner. Though they possess only a few thousand neurons (nerve cells) clustered around their four eyes, they are able to associate changes in light intensity with tactile (touch) feedback and adjust their swimming accordingly.

This allows for more precise navigation of their mangrove-dominated habitats, and so improves their odds as venomous predators.

No Neurons, No Problem .

Stretching our instincts further, evidence now abounds for learning in organisms that lack even the neuronal building blocks of a brain.

Slime molds are single-celled organisms that bumble about in search of food. When exposed to a chemical gradient they will shift their crawling direction in response and remember the experience, adjusting their future progress with experience.