If you look at both quadrupeds and bipeds, you'll see around the time they evolved an interrim period in which creatures existed that were not really quadrupeds or bipeds.
Evolutionary success creates only the illusion of lines, none really exist.
You take the word 'line' too literally here, it just means a boundary.
Reality has very few lines.
Using an unnecessarily strict definition for line, you'll find there is almost nothing in the universe that is perfectly distinct:
There are no species, no genders, no living things, dead things, no discrete individuals, not even any discrete objects, no colors. Nothing is high, nothing is low, nothing is dark, nothing is bright; nothing is anything.
We can obviously draw a boundary between quadrupeds and bipeds -- we're doing it the second we use the words. Whether that boundary is absolute is not valuable.
The interesting question isn't "Can we argue for a continuum of awareness between insects and humans?" That part is easy.
The interesting part is, "Do we know enough about consciousness to describe a meaningful boundary between them?"
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