Q:

What is the most basic organism to respond to Classical conditioning (capable of learning)?

I have been doing some reading of Classical conditioning which is considered to be an indication of animal's ability to learn...
Before this I considered insects to be machine like - devoid of any sort of 'thinking' just doing things that evolution 'made' them to do to survive...
However I have found that cockroaches and even fruit flies could be conditionally trained... 
Which made me wonder What would be the least complex species that respond to classical conditioning?
I have also read this question about single-celled organisms capable of learning and I don't think that this 'cuts it'... this is nothing more than chemical response... however classical conditioning is different and in my opinion certainly does indicate ability to learn...

A:

Very simple nervous systems are capable of learning  C. elegans, with only 302 neurons, is reportedly capable of both learning and memory.  There is a fairly large literature on the subject, as C. elegans is a popular research subject - a Google Scholar search gives about 40K hits, e.g. http://learnmem.cshlp.org/content/17/4/191.long
We have to remember that small is not necessarily primitive: the C. elegans nervous system (like every other one in existence today) has been optimized by millions of generations of evolution to do what C. elegans needs in order to survive.
Actual evidence of learning in nervous systems in the distant past is rare.  It's behavior, and behavior doesn't fossilize well :-)  Actual nervous systems apparently don't fossilize all that well, either.  The earliest well-preserved example I could find was this http://www.cam.ac.uk/research/news/520-million-year-old-fossilised-nervous-system-is-most-detailed-example-yet-found from 520 million years ago.  But since learning would give a creature a survival advantage, I'd guess that it evolved fairly early.

