Perhaps the first animals appeared on Earth were the gelatinous ctenophores

Mertensia ovum, a species of ctenophores (Photo courtesy Arctic Exploration 2002, Kevin Raskoff, MBARI, NOAA/OER)
Mertensia ovum, a species of ctenophores (Photo courtesy Arctic Exploration 2002, Kevin Raskoff, MBARI, NOAA/OER)

An article published in the journal “Nature Ecology & Evolution” offers an answer to the arguments about the most ancient group of animals appeared on Earth. According to a team of evolutionary biologists at Vanderbilt University and the University of Wisconsin-Madison ctenophores were the first animals and not sponges, as many believe. This conclusion was reached by comparing a set of genes of 18 branches of the living beings’ family tree using 6 of animals, plants and fungi as a control branches.

Ctenophores (Ctenophora) are a phylum with a small number of species existing today such as the comb jelly – species Mertensia ovum – but they’re very diversified. These marine animals are composed almost entirely of water and from this point of view resemble cnidarians (Cnidaria), members of the phylum that includes jellyfish. Because of their soft and gelatinous body, ctenophore fossils are very rare but some found in the Burgess Shale are really ancient as they date back about 500 million years ago.

The problem in understanding which group of animals was the first to appear on Earth has long been a source of debate. Traditionally, the animals family tree has been built based on the evaluation of their complexity. The simplicity of the sponges convinced many that they were the first animals but the increasing use of genetic testing has become increasingly important in biological research helping among other things to understand the various relationships in a discipline called phylogenomics.

In 2008 one of the first filogenomic studies suggested that ctenophores could be the first animals appeared on Earth and not sponges. It was the beginning of a debate in which several researchers expressed their reasons in indicating one or the other group as the oldest. A few weeks ago an article published in the journal “Cell” indicated sponges as the oldest and claimed that indicating ctenophores was a mistake due to their rapid molecular evolution that created the impression of relationships that actually don’t exist.

Antonis Rokas, professor of biological sciences at Vanderbilt University developed a new method of filogenomic analysis together with Xing-Xing Shen and assistant professor Chris Todd Hittinger of the University of Wisconsin-Madison. According to them it’s a method that has worked very well in 95% of tested cases but led to apparently irreconcilable differences in the remaining 5%.

The researchers decided to focus on 18 controversial relationships to understand the motives of those results at least apparently wrong. To do so, they started comparing individual genes of the leading candidates in each of the potential relationships. It’s a type of research that usually affects hundreds of thousands of genes and the researchers determined how much each of them supports the hypothesis of ctenophores or sponge as the first animals. The result is that ctenophores have far more genes that indicate them as the first to diverge from the others than sponges.

It’s difficult to think that the results of this research will be accepted without further discussion because the topic is so controversial and the recent article published in “Cell” gave opposite results. The advances in genetic techniques and in disciplines such as phylogenomics will certainly lead to more research, who knows with what results.

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