Did neurons evolve twice? The mystery of animal origins

Nature · Published 2 October 2026 Swipe more like this →

In short: The evolutionary tree of animals has a massive split at its base. Did complex features like nervous systems appear once in a common ancestor, or did they emerge independently in different lineages? New evidence from chromosomal linkages suggests one group may have branched off before the others even developed.

Scientists are currently debating two primary scenarios regarding the earliest branches of animal evolution: the ctenophore-sister hypothesis and the sponge-sister hypothesis. The former suggests that ctenophores (comb jellies) diverged first, while the latter posits that sponges (poriferans) took that early path. This distinction is not just academic; it fundamentally changes our understanding of how complex traits like neurons, muscles, and specific gene regulatory networks first appeared in nature.

The debate is difficult to resolve because the fossil record is largely silent on these Precambrian lineages. Furthermore, morphological characters—such as the choanocytes of sponges—are often unreliable for determining true homology, as they can be easily lost or arise through convergent evolution. While some phylogenomic studies have united bilaterians, cnidarians, and placozoans into a monophyletic clade called Parahoxozoa, results remain sensitive to how sequence evolution is modeled.

A new approach using chromosomal synteny—the physical linkage of genes on chromosomes—offers a more stable metric. Because interchromosomal translocations are rare and effectively irreversible in large populations, these patterns act as 'shared derived features.' Recent research shows that ctenophores share ancestral gene linkages with unicellular eukaryotes, while Parahoxozoa (bilaterians, cnidarians, and placozoans) are united by shared derived synteny patterns. This suggests that if the ctenophore-sister hypothesis is correct, neurons may have either been lost in sponges or evolved twice independently.

The implications are profound for evolutionary biology. If Parahoxozoa represents a single unified radiation of complex animals, it implies a linear progression toward complexity. However, if ctenophores branched off first with their own unique nervous systems, it suggests that the 'blueprints' for animal life were far more diverse and experimental than our current models might assume.

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