A great majority of humanity remained illiterate just a few generations ago. It's absurd to posit they weren't conscious.

Hunter-gatherer and nomadic lifestyles are complex and require deep thought and planning and coordination. You can't do it purely reactively or carrying out simple scripts.

Its not so clear to me if it's the chicken or egg. So many things are good candidates for the key ingredient.

I personally think that consciousness is social theory of mind applied to the self, or something like simulating and being concerned with how the tribe sees you and interprets your intentions etc, forcing you to analyze yourself similarly to how you analyze others. This would mean that solitary animals are less conscious than social ones.

You're conflating "written language" with "language". While it's still far from impossible to find people who can't read or write, it's exceptionally difficult to find adults with no grasp of any spoken language.

Illiterate isn't the same as having no language, just (sic) saying.