Ordinarily, we might think about sensation, perception, and behavior in animals requiring networks of neurons, performing complicated neural calculations to promote their survival. At least that's my bias as a neuroscientist.
Amoebae and certain other single-celled organisms can sense and move through their environments without an elaborate nervous system, using a process called chemotaxis. Beyond this, the ameobic slime mold Dictyostelium discoideum does something special. It can assemble into a multicellular organism when needed.
'Dicty' have sensors all over their outer membranes, and they perform a biochemical computation inside the cell that can reconstruct their actin cytoskeleton to flow toward or away from chemical stimuli, dragging the cell with it via extracellular linkages. They live in moist earth in three dimensions, although in the lab they are usually studied on a flat surface. Their biochemical network has lessons for studying adaptive function in nervous systems.
In Dicty, networks of sensors transduce signals into the cytoplasm, where networks of excitation and inhibition perform something like a 'Mexican hat' function that attempts to identify the direction of external chemical gradients. This amounts to analog decision-making in space, and the tiny animal flows its cytoplasm in the apparently desirable direction.
When food becomes scarce, a local population of these ameobae start to secrete a diffusable messenger, which they use to aggregate into a mass that organizes into a worm-like creature. That worm-like creature crawls to a suitable location, plants its nose on a physical substrate, lifts its tail up, and forms a 'fruiting body' that explodes with airborne spores that might migrate to greener pastures.
This system has been studied extensively. Like sea sponges, this system gives us insights into the origin of tissues, organs, and organisms from spontaneous aggregations of single cells.
I am continually surprised at how few scientists know about this living link to how primordial multicellular life forms may have originally emerged. Tell your friends.
The author's affiliation with The MITRE Corporation is provided for identification purposes only, and is not intended to convey or imply MITRE's concurrence with, or support for, the positions, opinions or viewpoints expressed by the author.