On an Alternate Universe Earth, 3rd Impact occurred, the entire Human race became a single, gelatinous entity, a vast lake of LCL.
An inter-dimensional rift opened, swallowing the earth and rending it asunder, the Lake survived, deposited somewhere on the other side of the Galaxy, long into the distant past.
Countless years passed, until eventually, something stirred within the lake of LCL, humanoid shapes, created by new AT-Fields, began to emerge, they found they could manipulate their AT-Fields to assume nearly any shape with enough practice. Hunted, feared, they began to mistrust those who could not change shape. Changelings the others called them, they began their own plans, creating a race of genetically engineered soldiers known as the Jem'Hadar.
The Changelings vowed that they would bring order to the chaotic Solids, even if they had to perform Instrumentality on all of them, so that they too would know the unity of the Great Link.