SCIENCE

Jellies or sponges? No, not the kitchen items—the animals. Maybe you haven’t been debating that last issue for decades, but evolutionary biologists have. Now one group says they’ve got an answer: It’s the jellies. For now. (Popular Science)

Where are the jellies on the Tree of Life? Where are the sponges?

Discussion Ideas

• A new study supports the placement of ctenophores, or comb jellies, as the earliest branch on animals’ phylogenetic tree. What is phylogenetics?
  • Phylogenetics is the study of how organisms relate to each other as they develop over time.
  • According to Understanding Evolution (definitely one of our go-tos around here), “all life on Earth is united by evolutionary history; we are all evolutionary cousins—twigs on the tree of life. Phylogenetic systematics is the formal name for the field within biology that reconstructs evolutionary history and studies the patterns of relationships among organisms.”

• What is the newest evidence for ctenophores being the first animals to evolve?
  • Genetics. “In looking at the genes connecting marine jellies (ctenophora), sponges (porifera), and the group of all other animals (yes, it’s a giant group), they found most of the key genetic relationships supported a closer link between sponges and the ‘everything else’ group than between marine jellies and everything else. That makes jellies the outlier. And in a phylogenetic tree, the outlier is the early-comer. As the animal kingdom evolved from a common ancestor, ctenophora branched off to do its own thing first, then later sponges branched off from the remaining group.”

• The article says this study “almost certainly isn’t the final call in this debate.” Why might this finding be contentious among evolutionary biologists? What evidence might support sponges being the older species?
  • Sponges are much simpler animals. They have no body symmetry, no organs, no nervous system, no digestive system, no circulatory system.
    • Ctenophores also lack digestive and circulatory systems. They do have a form of radial symmetry, a simple nervous system, and musculature.
  • Ctenophores being the “sister group” of all other animals suggests two weird scenarios.
    • It could indicate that the comb jelly evolved its complexity independently of other animals, after it branched off onto its own evolutionary path.
    • Or, the ancestors of sponges were more complex and evolved to a simpler form.

• The great Popular Science article claims that ctenophores were the first animals to evolve. The scientific paper, however, says the comb jellies are “the earliest-branching metazoan phylum.” What are metazoans?
  • Animals! All metazoans are animals, and all animals are metazoans.
    • Backstory: In the 19th century, scientists divided animals into two big groups: the multicellular metazoans and the single-celled protozoans. Today, protozoans are not considered animals at all, but part of the big group of diverse organisms known as protists. So, only us multicellular metazoans are left in the animal kingdom.

TEACHERS TOOLKIT

Popular Science: Suck it, sponges: Marine jellies were the first animals to evolve

Nat Geo: Circle of Life study guide

University of California Museum of Paleontology: Understanding Evolution—Phylogenetic systematics, a.k.a. evolutionary trees

(extra credit!) Nature Ecology & Evolution: Contentious relationships in phylogenomic studies can be driven by a handful of genes