Marine jellies, not sponges, were the first animals to evolve on Earth, according to a new study that unravels the mystery that has baffled zoologists for the last decade.

Researchers, including those from Vanderbilt University in the US, devised a new approach designed specifically to settle contentious phylogenetic tree-of-life issues.

They focused on 18 controversial relationships (seven from animals, five from plants and six from fungi) in an attempt to figure out why previous studies have produced such strongly contradictory results.

They got down into the weeds, genetically speaking, and began comparing the individual genes of the leading contenders in each relationship.

These analyses typically involve hundreds to thousands of genes. The researchers determined how much support each gene provides to one hypothesis (comb-jellies first) over another (sponges first).

They then labelled the resulting difference a “phylogenetic signal.” The correct hypothesis is the one that the phylogenetic signals from the most genes consistently favour.

Researchers determined that comb jellies have considerably more genes which support their “first to diverge” status in the animal lineage than do sponges.

“The current method that scientists use in phylogenomic studies is to collect large amounts of genetic data, analyse the data, build a set of relationships and then argue that their conclusions are correct because of various improvements they have made in their analysis,” said Antonis Rokas from from Vanderbilt University.

“This has worked extremely well in 95 per cent of the cases, but it has led to apparently irreconcilable differences in the remaining five per cent.

“In these analyses, we only use 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,” Rokas said.
“When you look at a particular gene in an organism, let's call it A, we ask if it is most closely related to its counterpart in organism B? Or to its counterpart in organism C? And by how much?” he said.

Due to their comparative simplicity, sponges were considered to be the earliest members of the animal lineage, researchers said.

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.

The study was published in the journal Nature Ecology and Evolution.