So I guess it depends on how you want to define consciousness. It's a biological reality that once sperm meets egg, you will have all the genetic information required to create your central nervous system represented there. But in my mind, this represents
the potential for consciousness, not consciousness itself.
In my mind consciousness implies self-awareness and subjective experience. You might then be able to say that a phagocyte has some rudimentary form of consciousness, because it has a "behavior" of pursuing and destroying undesirable bacteria and particles in the body, so it must have some level of "perception" over which entities are to be targeted, and which to ignore. But at that level you can trace the behavior to chemical signaling and sub-cellular machinery, and it's possible to trace the phagocyte's behavior to simple cause-effect relationships in terms of physics and biochemistry. So I think it's hard to imagine that a phagocyte has "experience" in any real way.
I think when we talk about human consciousness, the content on which our subjective experience operates includes things like sensory information, verbal information and memories. And these types of information have neural representations which are quite clearly understood. And even for the task of consciousness, which is to interpret and contextualize all the sensory information coming into the brain, you can point to a brain region like the thalamus, which contains inputs from your senses, along with almost every other brain region, and make a reasonable argument that this might represent the physical manifestation of what we call consciousness.