Evolution and evolvability · Article

Origin and evolution of neurons and nervous systems

The origin of neurons is disputed in part because the branching order of the earliest animals is disputed. Moroz and Kohn (2016) report that genome studies placed Ctenophora as sister to all other animals, and they argue that synapses arose more than once.⁵⁶ Arendt and colleagues (2015) instead describe a basal nerve net built from simple reflex arcs.³ Fábregas-Tejeda and Sims (2025) ask basal cognition research to weigh convergence more heavily.⁹

Earliest held: 2010, Domazet-Loso T, Tautz D
Most discussed in: Independent origins of neurons and synapses…, 2016
In the library: 19 passages in 5 works
Rewritten: 2026-10-21

Tree of early animals

Domazet-Loso and Tautz call the early branching of the Metazoa particularly critical for their cancer-gene results. They (2010) took the classical view, with Porifera as a basal metazoan clade and Cnidaria and Bilateria branching later.¹ They also tested an alternative in which Porifera and Cnidaria branch first. General phylogenomic analyses do not support that view, and their phylostratigraphic pattern was not much changed by it.² Moroz and Kohn (2016) describe a different dispute. An early Mnemiopsis genome project did not confirm the ctenophore-first hypothesis. The Mnemiopsis and Pleurobrachia teams both later recognized Ctenophora as sister to all metazoans.⁵

Nerve net and ancestry

Arendt and colleagues (2015) asked what cell types made up the first nerve net. They suggest that its core units were local two- to three-cell sensory-effector reflex arcs that innervate effector cells directly.³ They propose that bilateral symmetry improved feeding on a mucociliary sole, and that animals at this grade were close to the last common ancestor of cnidarians and bilaterians.⁴ In their account, later diversification of cell types produced the neuron types of today. Moroz and Kohn (2016) present the independent origins of neurons as a hypothesis. As a corollary, their analyses suggest that both electrical and chemical synapses evolved more than once.⁶

Precursors before neurons

Newman (2019) notes that planar cell polarization may have arisen in the common multicellular ancestor of the metazoans. Key components were lost in most sponge groups and placozoans.⁷ Ctenophores have the pathway only incompletely. Fábregas-Tejeda and Sims (2025) point to work finding neuropeptide homologs in choanoflagellates. This points to a pre-metazoan onset of these signalling functions. They call it relevant for understanding how these functions were later co-opted in nervous system evolution.⁸ They also hold that nerve-net origins require study of non-neuronal organisms inside and outside the animal kingdom.⁸

Convergence and cognition

Fábregas-Tejeda and Sims (2025) argue that basal cognition researchers should give greater consideration to convergent evolution and homoplasy.⁹ They observe that in a recent double special issue on basal cognition, only one article has a section on convergent evolution. That section is restricted to animal neural systems.⁹ They suggest this infrequent treatment may come from a strong assumption of Darwinian continuity.⁹

SourcesEach quotation was checked word for word against the passage it opens.

In our analysis we took the classical view, where Porifera are a basal metazoan clade, whereas Cnidaria and Bilateria are branching off later.Domazet-Loso T, Tautz D, 2010 · Phylostratigraphic tracking of cancer genes suggests a link to the emergence of… · open at passage 15
We find that our phylostratigraphic pattern is not much affected by this different topology: gatekeeper genes are still significantly over-represented at ps5Domazet-Loso T, Tautz D, 2010 · Phylostratigraphic tracking of cancer genes suggests a link to the emergence of… · open at passage 15
Core units of the basal metazoan nerve nets appear to be local two- to three-cell sensory-effector reflex arcs [138] that directly innervate the effector cellsArendt D, Benito-Gutierrez E, Brunet T, Marlow H, 2015 · Gastric pouches and the mucociliary sole: setting the stage for nervous system… · open at passage 39
the transition to bilateral symmetry, which enabled directed movement, facilitated food search and avoided grazing the same area twiceArendt D, Benito-Gutierrez E, Brunet T, Marlow H, 2015 · Gastric pouches and the mucociliary sole: setting the stage for nervous system… · open at passage 5
both genome-wide studies recognized Ctenophora as the earliest branching animal lineage and sister to all metazoansMoroz LL, Kohn AB, 2016 · Independent origins of neurons and synapses: insights from ctenophores · open at passage 11
our analyses suggest that both electrical and chemical synapses evolved more than once.Moroz LL, Kohn AB, 2016 · Independent origins of neurons and synapses: insights from ctenophores · open at passage 1
The PCP pathway may have arisen in the common multicellular ancestor of the metazoans, but key components were lost in most sponge groups and placozoansNewman SA, 2019 · Inherency of Form and Function in Animal Development and Evolution · open at passage 18
recent work has found neuropeptide homologs in choanoflagellates, thus pointing to the pre-metazoan onset of their signalling functionsFábregas-Tejeda A, Sims M, 2025 · On the prospects of basal cognition research becoming fully evolutionary… · open at passage 32
it would be advantageous for scientists and philosophers interested in basal cognition to give greater consideration to convergent evolution and homoplasies more generallyFábregas-Tejeda A, Sims M, 2025 · On the prospects of basal cognition research becoming fully evolutionary… · open at passage 46

Linked ideas

Basal cognitionrelated to

Fábregas-Tejeda and Sims assess how far basal cognition research treats convergence and homoplasy, which bears directly on whether neurons are one origin or many.

Phylostratigraphy and the evolutionary origin of cancer genesrelated to

Domazet-Loso and Tautz tested their phylostratigraphic pattern against alternative early animal branching orders.

Inherency of form, mesoscopic materials and the physical origin of body plansrelated to

Newman discusses features present in basal metazoans, such as planar cell polarization, that preceded and shaped later animal organization.

Gap junctionsrelated to

Moroz and Kohn argue that electrical synapses evolved more than once, a claim that bears on how electrical coupling is compared across animals.

Cephalization and physiological dominance of the anterior regionrelated to

Arendt and colleagues link bilateral symmetry and directed movement to the stage set for nervous system evolution.

Neurotransmitters before neuronsrelated to

Arendt and colleagues suggest indolamine and neuropeptide release may have modulated early nerve net activity.

Where it is discussedPassages matching independent origins of neurons, ctenophore-first hypothesis, Ctenophora sister to all animals, basal metazoan

2016	Independent origins of neurons and synapses: insights from ctenophores · Moroz LL, Kohn AB	10
2019	Inherency of Form and Function in Animal Development and Evolution · Newman SA	3
2015	Gastric pouches and the mucociliary sole: setting the stage for nervous system… · Arendt D, Benito-Gutierrez E, Brunet T…	3
2025	On the prospects of basal cognition research becoming fully evolutionary… · Fábregas-Tejeda A, Sims M	2
2010	Phylostratigraphic tracking of cancer genes suggests a link to the emergence of… · Domazet-Loso T, Tautz D	1