The most diverse animal group on Earth remains a puzzle of evolutionary history

Nature · Published 29 September 2026 Swipe more like this →

In short: Panarthropoda encompasses the vast majority of animal life, including arthropods, velvet worms, and water bears. While these creatures share distinct features like segmented bodies and ladder-like nervous systems, their exact family tree remains a subject of intense scientific debate, with new genetic studies constantly shifting our understanding of their origins.

Panarthropoda represents a massive clade containing the greatest diversity of animal groups. Its primary members are the extant phyla Arthropoda, Tardigrada, and Onychophora. The group also includes extinct marine legged worms known as lobopodians, which are considered a paraphyletic group where the basal members and last common ancestors of the extant phyla likely emerged. Recent additions to this complex puzzle include Sialomorpha, a microinvertebrate genus discovered in Dominican amber in 2019, which exhibits a baffling mix of tardigrade and mite-like traits.

The defining physical characteristics of panarthropods include a segmented body, paired appendages aligned with those segments, and a ventral nervous system arranged like a ladder. Historically, these features led scientists to group them with annelids under the name Articulata. However, modern phylogenomic studies have largely overturned this, placing panarthropods instead within the Ecdysozoa, closer to cycloneuralians like nematodes. Annelids, meanwhile, are now classified among the Spiralia, suggesting that their segmented bodies evolved independently through convergent evolution.

Determining how these three phyla relate to one another remains a significant challenge. Three main hypotheses exist: Tactopoda, which links arthropods and tardigrades; Antennopoda, which pairs arthropods with onychophorans; and a sister relationship between onychophorans and tardigrades. Evidence for these theories is often contradictory. For instance, while Tactopoda is supported by neuroanatomical and mitochondrial data, other studies suggest that shared features like head appendages in Antennopoda might be the result of convergent evolution rather than shared ancestry. As recently as December 2024, new genetic research has continued to challenge existing models, supporting a sister relationship between Onychophora and Arthropoda.

While the monophyly of Panarthropoda is widely accepted by neuroanatomical, phylogenomic, and palaeontological research, the internal relationships remain fluid. Some studies even place tardigrades closer to nematodes than to other panarthropods. This ongoing uncertainty highlights the difficulty of mapping the deep evolutionary history of the most successful animal lineage on the planet.

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