Ok, headcrabs aren't the origin of the Dark Falz because the Profound Darkness is... but have you considered that perhaps headcrabs are the origin of the Profound Darkness? Thus making them the Grand Origin of Dark Falz.
Or maybe headcrabs were the origin of Photoners or even Xion, after all we have that weird sea like effect in the mothership and what lives in the sea? Crabs! Remember that Xion used to just be an ocean planet that gained a consciousness, what can be found living in the ocean? Crabs!
Well I thought headcrabs could already be an extension of the Profound Darkness, but it wasn't until interaction with humans that a humanoid entity was born!