Jellies: Our Original Ancestor?

Researchers have long struggled to determine what animal is our oldest ancestor. For almost a century, many experts believed it was the sea sponge, largely because of its ultra-simple genetic structure, but now the ocean jelly has taken its place.

Researchers from Vanderbilt University and the University of Wisconsin-Madison recently published a paper in the journal Nature Ecology & Evolution outlining how their new genetic analysis reveals that the comb jelly (a ctenophore) evolved first.

The researchers explained that their new technique “consistently supports ctenophores [marine jellies] as the sister group to all other metazoans,” or multicellular animal life.

Scientists previously tried to work out the relationships among different animals by collecting large amounts of data, analyzing it, building a possible family tree, and arguing that their results were correct. “This has worked extremely well in 95% of the cases,” said Antonis Rokas, PhD, a researcher on the team from Vanderbilt. “But it has led to apparently irreconcilable differences in the remaining 5%.”

The research team instead decided to focus on 18 of these controversial relationships—7 from animals, 5 from plants, and 6 from fungi. They compared the individual genes of the leading contenders in each relationship and only used genes that are shared across all organisms.

"The trick is to examine the gene sequences from different organisms to figure out who they identify as their closest relatives,” Dr. Rokas said. “When you look at a particular gene in an organism, let's call it A, we ask if [that gene] is most closely related to its counterpart in organism B? Or to its counterpart in organism C? And by how much?"

According to the researchers, the earlier the animal emerged on Earth, the earlier it likely diverged into new species. Because this animal would have more related species as a result, it would have a higher phylogenetic signal.

After studying thousands of genes and seeing how much support each gene provides to comb jellies and sponges, the researchers discovered that the genes consistently favored comb jellies as Earth’s first species.

The research team used this same method to examine whether crocodiles were more closely related to birds or turtles—74% of the shared genes favored that crocodiles and birds were sister lineages while turtles were only close cousins.

Based on their results, the researchers concluded that 1 or 2 “strongly opinionated genes” cause the problem in contentious cases because the methods that are currently being used are highly susceptible to the influence of these genes. This means that removing even 1 of these genes can completely flip the results of a similar study.

"We believe that our approach can help resolve many of these long-standing controversies and raise the game of phylogenetic reconstruction to a new level," Dr. Rokas said.