Early central nervous system evolution: an era of skin brains?

In the late nineteenth century, there were numerous attempts to show how the central nervous system (CNS) evolved. However, no consensus was reached and the subject went out of style until around 20 years ago, when rapid progress in molecular biology began to impact ideas about animal evolution.The nervous system is difficult to consider in a phylogenetic context, because many of its properties are emergent novelties. A CNS is defined as a place where neurons are more concentrated than elsewhere in the body, but this definition blurs the distinction between the CNS and the peripheral nervous system of basal invertebrates.It is not known whether the common ancestor of the protostomes and deuterostomes — the two main groups of bilaterally symmetrical animals — had a biphasic life history, in which a swimming larva undergoes metamorphosis to become a benthic adult. However, when proposing homologies for CNS regions, it is better to consider adult rather than larval characters.Most current phylogenetic analyses place the cnidarians, which comprise organisms like hydras and jellyfish, just basal to the bilateral animals. Cnidarian polyps have several relatively independent nerve nets. These nets tend to be especially condensed near the oral opening, where they are usually termed nerve rings.There are half a dozen hypothetical evolutionary schemes for transforming an anthozoan-like creature (with a solid or hollowed-out endoderm) into a bilateral animal with a through gut. Most begin with an individual non-colonial ancestor, although one — the colonial scenario — proposes that an ancestral colony of polyps individuated into a single, bilaterally symmetrical animal.Until recently, it was assumed that the ancestral nerve net became localized as all or part of the CNS in the basal groups of bilaterally symmetrical animals. However, in the enteropneust hemichordate Saccoglossus, expression of homologues of vertebrate CNS anteroposterior patterning genes is not limited to the nerve tracts, but extends widely throughout the ectoderm, implying that the CNS of this animal includes the entire basiepidermal nerve net.