If i was to define the simplest, most foundamental, TIB, fixed eyeknot, knotting module, that would be the Harness core, in the sense that it is not feasible to be simplified to a fixed loop with a lower level of complexity.
A TIB pactitioner might argue, within the meaning of harness reduction to a parent, simpler knotting object with the lowest nesting level, the simple noose. That's all very well but the noose is not a fixed eyeknot.
I use the term "core" because the harness is a structure with pseudo symmetrical components, therefore, it features four equivalent topologies with the potential of progressing from one to the other with just core maneuvering and none eye manipulation employed.
These formations comprise 1050, 1051, and their eye leg,end, swapped forms, thus inline TIB 1010 and TIB 1010 respectively. Once one is formed, the other three can be easily obtained.
Τhe exploration of elementary TIB, fixed loop objects, requires in-line conversion because there are additional, elementary, TIB modules with no symmetrical status, for example, TIB cowboy and flash bowline, which is another primary TIB module pair, with fixed eye characteristics, where the transition from one to the other is not so obvious, requiring eye manipulation.
Moreover, a topological correlation between the aforementioned (cowboy) core and the harness can be tracked, of course, as long as there would be no unknotted line in the transformation proccess.
Also the M core that was analyzed in previous replies, is another primary TIB, asymmetrical scheme, with a low level of maneuvering for its formation, which might also point to the harness core through a TIB route.
Many are those who might consider the butterfly core as the ultimate, TIB, fixed eye module, being the "queen" of all knot structures.
Indeed, it is stable and symmetrical,but it requires more complex components (overhands) and it is feasible to simplify it to a fixed eye form, with a lower nesting level.
Of course, the TIB enthusiast would suspect that there is a TIB pathway between the butterfly and the harness core.
In conclusion, would the bowlinesque, harness core, happen to be the most important TIB, fixed eye module, parent, king and queen to all TIB knots?