Comb jellies were the earliest animals: Study

The delicate marine predators called comb jellies were the earliest animals -- not sponges as had long been thought, claims a new genetic analysis.

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 or was it the comb jellies?'

The new study, published in the journal Nature Ecology & Evolution, suggests that comb jellies were the first branch of the animal family tree.

With their analysis involving hundreds to thousands of genes, the researchers determined that comb jellies have considerably more genes which support their "first to diverge" status in the animal lineage than do sponges.

The researchers believe that the findings could have a major impact on scientists' thinking about how the nervous system, digestive tract and other basic organs in modern animals evolved.

For nearly a century, scientists organised 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.

The researchers decided to focus on 18 of these controversial relationships (seven from animals, five from plants and six from fungi) in an attempt to figure out why the studies have produced such strongly contradictory results.

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

"In these analyses, we only use genes that are shared across all organisms," said one of the researchers Antonis Rokas, Professor at Vanderbilt University in Nashville, Tennessee, US.

"The trick is to examine the gene sequences from different organisms to figure out who they identify as their closest relatives. 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," Rokas added.

Their analysis showed that comb jellies have considerably more genes which support their "first to diverge" status in the animal lineage than do sponges.

--IANS

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