From simple metazoan animals with radial symmetry evolved a group which designated part of its body as “front” and became bilaterally symmetrical. Sensory structures such as eyes and chemosensory cells accumulated at this front end, creating a head, and increased clusters of nerve cells to process this sensory information developed into the first brains.

Ancestral bilaterans evolved a number of additional traits which are shared in modern bilaterans. These include advances in the nervous system (multiple neuronal shapes, axons and dendrites, wider use of neurotransmitters and neuropeptides, glial cells, neurosecretory cells similar to those of the hypothalamus, longitudinal nerve cords, and prominent eyes), muscular system (such as peristalsis, titin, troponin, and tropomyosin), digestive system (including an anus and the mucociliary mechanism), and reproductive system (such as follicular cells around the ova in higher worms). Many molecular pathways are shared among bilaterans, including pathways for programmed cell death and expansion of immunoglobulin and transcription factor families.

The gene sequences of bilaterans group them as a clade in a nested hierarchy which contradicts the creationist prediction of representing equally unrelated organisms. Worms possess some, but not all, of the components of the more complex systems of higher animals which undermines the “irreducible complexity” argument of intelligent design.