The evolution of pain systems across the animal kingdom provides crucial evidence for understanding which animals are capable of suffering and to what degree.
Pain neuroscience provides one of the most direct lines of evidence for welfare decisions about which animals matter and how much. The evolutionary conservation of nociceptive systems across taxa tells us that harmful stimuli detection is ancient and adaptive. The harder question - whether this detection involves conscious suffering - requires evidence about higher-level neural processing. In vertebrates, the evidence for pain experience is strong. In decapod crustaceans, it is increasingly persuasive. In insects, it is contested but growing. In molluscs other than cephalopods, it is weaker. The practical implication for welfare decision-making is a graded, evidence-sensitive precautionary approach: apply stronger welfare protections where evidence is stronger, and proportionate precaution where it is weaker.