Slime mould - Physarum polycephalum - are capable of problem solving without neurons or a brain. Photo: Adobe Stock Images
The brain is an evolutionary marvel. By shifting the control of sensing and behaviour 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 neighbours (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 behaviour 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 (sensitising) or dialling down (habituating) one’s response with repeated exposure.
Further along is associative learning, in which a cue is reliably tied to a behaviour. 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 specialised 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.