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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...

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marked as duplicate by Bryan Krause, kmm, David, Tyto alba, another 'Homo sapien' Jun 8 '17 at 2:53

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    $\begingroup$ It's not clear what you mean by "first"? Earliest in evolutionary time? $\endgroup$ – kmm Jun 5 '17 at 17:44
  • $\begingroup$ @kmm Hi kmm, I am looking for the 'most basic' one, but I think that acquiring ability to learn was so advantageous that it will be same ancestor for all of us that got this ability, so evolutionary 'first' should be the same... $\endgroup$ – Matas Vaitkevicius Jun 6 '17 at 1:39
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    $\begingroup$ When you guys say "the first", I hope you understand that the scala naturae is a completely wrong concept, that should be buried where it belongs: to the dark ages. I, you and a nematode worm have exactly the same evolutionary time, and no one (neither humans or nematodes) are the first. $\endgroup$ – user24284 Jun 6 '17 at 5:29
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    $\begingroup$ @Gerardo Furtado: Actually that nematode is argually more evolved than a human (or just about any vertebrate), because evolutionary time should be measured in generations, not years. $\endgroup$ – jamesqf Jun 6 '17 at 18:46
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    $\begingroup$ @GerardoFurtado good grief, relax! Please read what we are saying, my comment (and my answer in the linked post) are saying precisely that the whole idea of "more" or "less" evolved is nonsensical. However, if you were to use those silly terms, the only way they might make sense is in terms of generations. And with all due respect, get off your high horse. We're all experts here and have been working for just as long as you have. $\endgroup$ – terdon Jun 8 '17 at 12:29
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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.

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