Spotted at about 600 million years ago. From ScienceDaily:
One of the longest-running controversies in evolutionary biology has been, ‘What was the oldest branch of the animal family tree?’ Was it the sponges, as had long been thought, or was it the delicate marine predators called comb jellies? A powerful new method has been devised to settle contentious phylogenetic tree-of-life issues like this and it comes down squarely on the side of comb jellies.
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For nearly a century, scientists organized the animal family tree based in large part on their judgement of the relative complexity of various organisms. Because of their comparative simplicity, sponges were considered to be the earliest members of the animal lineage. This paradigm began to shift when the revolution in genomics began providing vast quantities of information about the DNA of an increasing number of species. Evolutionary biologists started to apply this wealth of information to refine and redefine evolutionary relationships, creating a new field called phylogenomics. In most cases, the DNA data helped clarify these relationships. In a number of instances, however, it gave rise to controversies that intensified as more and more data accumulated.
Comb jellies took the lead in 2008 but sponges struck back in 2017.
It seemed so obvious: Sponges are simpler. However, some data offered “apparently irreconcilable” differences:
The researchers determined how much support each gene provides to one hypothesis (comb-jellies first) over another (sponges first). They labeled the resulting difference a “phylogenetic signal.” The correct hypothesis is the one that the phylogenetic signals from the most genes consistently favor.
In this fashion, they determined that comb jellies have considerably more genes which support their “first to diverge” status in the animal lineage than do sponges. Paper. (paywall) – Xing-Xing Shen, Chris Todd Hittinger, Antonis Rokas. Contentious relationships in phylogenomic studies can be driven by a handful of genes. Nature Ecology & Evolution, 2017; 1: 0126 DOI: 10.1038/s41559-017-0126 More.
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See also: Sponges back in the ring with comb jellies for “oldest” title fight
Comb jelly files: Complex features do not each emerge once
Comb jelly DNA sequence offers “unintuitive facts” about evolution…
and
Researchers: The sponge is the oldest animal phylum after all (2015)
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