Resolution criteria

This market resolves based on the scientific consensus regarding which phylum is the sister group to all other animals. Resolution will be determined by examining major peer-reviewed phylogenomic studies and reviews published in high-impact journals (Nature, Science, PNAS, Molecular Biology and Evolution, etc.) The market resolves to:

• Porifera if and when the preponderance of evidence strongly supports sponges as sister to all other animals

• Ctenophora if and when the preponderance of evidence strongly supports comb jellies as sister to all other animals

• None of the above if a third group is identified as sister to all other animals

Background (warning: AI generated)

Sponges were traditionally considered the sister group of all other animal lineages, consistent with the view that early animals were morphologically simple. However, genomic data has sparked tremendous controversy as some phylogenomic studies support comb jellies taking this position.

The debate centers on methodological differences in phylogenetic analysis. Porifera tends to emerge as the sister group when site-specific amino acid differences are modeled, whereas Ctenophora emerges as the sister group when they are ignored. A 2023 Nature study using chromosome-scale gene linkage (synteny) found deeply conserved syntenic characters uniting sponges with bilaterians, cnidarians, and placozoans to the exclusion of comb jellies—placing ctenophores as the sister group to all other animals. However, other analyses find that Ctenophora-sister is recovered across a full range of examined conditions, while Porifera-sister is recovered in some analyses under narrow conditions.

Considerations (warning: AI generated)

The resolution of this question has major implications for understanding early animal evolution. Ctenophora are comparatively more complex animals with rotational symmetry, possessing muscles, a through-gut, and a nervous system, while Porifera are simple multicellular animals that lack both body symmetry and true tissues and organs. The longer ctenophore stem branch has led some to suggest that Ctenophora-sister could be an artifact of long-branch attraction to outgroups. Different phylogenetic methods continue to yield conflicting results, making this an active area of research with no definitive consensus yet established.