Sunday, April 20, 2008

Comb jellies

I just thought it was sort of fascinating that comb jellies (also known as Ctenophorae), which aren't true jellyfish but are closely related, are now thought to be the oldest extant linage of the animal kingdom, usurping the title from sponges.

What's especially strange (though hardly inexplicable) is that comb jellies have some features that sponges lack, such as differentiated tissues and a nervous system.

The First Animal On Earth Was Significantly More Complex Than Previously Believed
A new study mapping the evolutionary history of animals indicates that Earth's first animal -- a mysterious creature whose characteristics can only be inferred from fossils and studies of living animals--was probably significantly more complex than previously believed.

Using new high-powered technologies for analyzing massive volumes of genetic data, the study defined the earliest splits at the base of the animal tree of life. ...

Among the study's surprising findings is that the comb jelly split off from other animals and diverged onto its own evolutionary path before the sponge.

How could it be that comb jellies are notably more complex than sponges, in spite of having appeared earlier?
Dunn says that the comb jelly could only have achieved its apparent seniority over the simpler sponge via one of two new evolutionary scenarios:

1. the comb jelly evolved its complexity independently of other animals, after it branched off onto its own evolutionary path; or
2. the sponge evolved its simple form from more complex creatures -- a possibility that underscores the fact that "evolution is not necessarily just a march towards increased complexity," says Dunn. "This scenario would provide a particularly dramatic example of that principle."

The evidence that comb jellies emerged earlier than sponges comes from intensive analysis of genetic data. The general idea is that detailed computer analysis comparing the DNA of different species indicates which species emerged earlier or later. As the researcher explains:
"Even though we looked at fewer than 100 species, they were sampled in such a way that they inform the relationships of major groups of animals relative to each other. Therefore, this study, and others like it, will have implications for the placement of far more species than just those that are sampled."

A somewhat earlier announcement is slightly more specific about what the researchers did:

Tree Of Animal Life Has Branches Rearranged, By Evolutionary Biologists
A study led by Brown University biologist Casey Dunn uses new genomics tools to answer old questions about animal evolution. The study is the most comprehensive animal phylogenomic research project to date, involving 40 million base pairs of new DNA data taken from 29 animal species.

Speaking of jellyfish-like creatures, I highly recommend Jelly Music.

Further reading:

Shock: First Animal on Earth Was Surprisingly Complex – another news report of the research

Phylogenetic fallacies: "early branching equals primitive" – blog post that cautions against certains misinterpretations of genomic data

Genomicron: Phylogenetic fallacies: “early branching equals primitive” – another blog post, elaborating on the preceding one

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