I think there are several lines. Phase changes happen relatively suddenly, when a system or subsystem reaches a critical threshold. The experience of "qualia" certainly involves many such phase changes as a complex, dynamical system grows in complexity while maintaining stability.

A sufficiently complex organism lacking eyes but having light-sensitive organs still experiences qualia if you define it the right way. But do they experience heartbreak like I do? It isn't an all-or-nothing situation, even if we don't yet know where these lines are.

This supports the idea that subjective consciousness emerges from complexity in systems that have sensory feedback loops. The simpler the system, the smaller the qualia space.