[jtsnow]

Wouldn't modifying current behavior be sufficient to justify the existence of pain? The mechanism can be a useful adaptation even if a single organism cannot learn. Maybe the default state of bacteria is pain and they can only relieve it temporarily. The ones that survive to multiply are the ones best adapted to relieving pain.

[kevin_thibedeau]

The entire purpose of experiencing externally induced pain is to cause a reaction before the threshold of permanent damage is reached. Given our imprecise biological systems, that threshold needs to be crossed with a safe margin of error. It stands to reason that reactions to avoid threats to the the continuation of life are valid, regardless of the presence of nociceptors.

Internal pain (headaches, etc.) may require some manner of cognition to associate behaviors with the result, assuming they have a cause other than flawed biology.