The genealogy of genealogy of neurons

Abstract

Two scenarios of neuronal evolution (monophyly and polyphyly) are discussed in the historical timeline starting from the 19th century. The recent genomic studies on Ctenophores re-initiated a broad interest in the hypotheses of independent origins of neurons. However, even earlier work on ctenophores suggested that their nervous systems are unique in many aspects of their organization and a possibility of the independent origin of neurons and synapses was introduced well before modern advances in genomic biology.

Keywords: ctenophora; evolution; genome; mnemiopsis; neurons; neurotransmitters; phylogeny; pleurobrachia.

Figure 1.

Two alternative scenarios of neuronal evolution (Ctenophora-basal hypothesis). The polyphyly or multiple origins of neurons as the example of convergent evolution (left). Monophyly or the single-origin hypotheses implies multiple loss of neural systems in sponges and placozoans as well as massive loss of many molecular components involved in neurogenesis and synaptic functions (right). The monophyly hypothesis still implies independent recruitment of other molecular components involved in neural and synaptic functions—the situation which still suggests the extensive parallel evolution of neural organization in ctenophores. Here, ctenophores are considered as sister to other animals (modified from⁵). However, even the classical view of the animal phylogeny (sponges are sisters to other animals, see Fig. 2) still implies the parallel evolution of neurons and neural signaling in the animal kingdom.

Figure 2.

Two alternative scenarios of neuronal evolution (Porifera-basal hypothesis). The polyphyly or multiple origins of neurons as the example of convergent evolution (left). There is a primary absence of neurons in sponges; independent origins of neurons in both ctenophora and cnidaria-bilateria clade. Monophyly or the single-origin hypotheses implies (as in Fig. 1) loss of neural systems in ctenophores and placozoans as well as massive loss of many molecular components involved in neurogenesis and synaptic functions (right). The monophyly hypothesis also implies the independent recruitment of novel molecular components involved in neural and synaptic functions in Ctenophora - the situation which still suggests the extensive parallel evolution of neural organization in this animal lineage. Here, ctenophores are considered as the second branching animal clade; whereas sponges are viewed as sister to other animals (see Fig. 1 for the alternative phylogeny).