From Erica Tennenhouse at New Scientist:
Most life forms on Earth lack neurons, and yet they frequently manage to behave in complex ways. Previously, we have chalked this up to innate responses refined over generations, but it is beginning to look as if some of these humble non-neural organisms can actually learn. While that’s left some scientists scratching their heads, others are busy investigating how this ability could offer new approaches to fighting diseases and designing intelligent machines. (paywall) More.
As Tennenhouse points out, even slime molds can learn, but they are a collection of individual life forms acting as one.
New Scientist is probably looking for some explanation that does not see intelligence as immaterial. But it is immaterial, as neurosurgeon Michael Egnor’s experience shows. (*They remaned normal even as he was removing parts of their brain.)
As we all know, all life differs from non-life in its drive to remain in existence as a life form. Perhaps we must start there. (?)
See also: Can slime molds think? Computer scientists say maybe.
Does intelligence depend on a specific type of brain?
Animal minds: In search of the minimal self