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Hot evo­lu­tion­ary debate just ended: Sponges were the world’s first of the ani­mal tree

pub­lished 10 Decem­ber 2017 | mod­i­fied 10 Decem­ber 2017

demosponge maledivesWhich group of ani­mals evolved first? This prob­lem has become a bone of con­tention among biol­o­gists. An inter­na­tional research team is now con­fi­dent that the defin­i­tive answer is at hand: Sponges appeared before comb jellies.

Which came first — the sponges or the comb jel­lies? The con­sen­sus view among tax­on­o­mists has long been that the sponges (Porifera) rep­re­sent the old­est sur­viv­ing ani­mal phy­lum. How­ever, recent stud­ies of their genomes have sug­gested that this title rightly belongs to the comb jel­lies (Ctenophora). The issue has become one of the most hotly debated prob­lems in evo­lu­tion­ary biol­ogy, because the answer has pro­found impli­ca­tions for our under­stand­ing of the entire his­tory of ani­mal evolution.

Our results con­firm the clas­si­cal assump­tions con­cern­ing early ani­mal evo­lu­tion, and should help to put an end to the recent con­tro­versy over the ori­gin of mul­ti­cel­lu­lar animals.

Pro­fes­sor Gert Wörheide, co-​author, Chair of Palaeon­tol­ogy and Geo­bi­ol­ogy, Ludwig-​Maximilians Uni­ver­sität München (LMU)

Sev­eral stud­ies in recent years to answer the ques­tion have come to dis­cor­dant con­clu­sions: Some have sup­ported the con­ven­tional model, which favours the sponges, while oth­ers pointed to the comb jel­lies as being at the root of the ani­mal king­dom. Researchers led by Pro­fes­sor Gert Wörheide (LMU) and Pro­fes­sor Davide Pisani (Bris­tol Uni­ver­sity, UK) have now used a refined method to re-​examine the datasets employed in these inves­ti­ga­tions, and their results are unequiv­o­cal: The sponges form the ear­li­est branch on the ani­mals’ fam­ily tree. The new study sup­ports the idea that the results which sug­gested oth­er­wise were based on the use of inad­e­quate — and hence mis­lead­ing — ana­lyt­i­cal meth­ods. The results are pub­lished online on 30 Novem­ber in the jour­nal Cur­rent Biol­ogy.

Sponges revealed as sister-​group to all other ani­mals
This means that the last com­mon ances­tor of all ani­mals was most likely very sim­ple organ­ism, much like a sponge.

(Source: Uni­ver­sity of Bris­tol YouTube chan­nel)

All biol­o­gists accept that sponges and comb jel­lies are very ancient groups, which emerged more than 600 mil­lion years ago. Sponges are com­par­a­tively sim­ple mul­ti­cel­lu­lar organ­isms, which lack clearly defined tis­sues, such as mus­cles, and organs like the brain. Comb jel­lies are struc­turally much more com­plex. They use so-​called cilia to pro­pel them­selves through the oceans, and they pos­sess nerve cells and mus­cle cells. “If Ctenophora were indeed the old­est ani­mal phy­lum, one would have to assume either that pre­cur­sors of these organ sys­tems were already present in the com­mon ances­tor of all ani­mals but were lost prior to the emer­gence of the sponges, or that nerves and mus­cles evolved inde­pen­dently at least twice,” Wörheide points out. “That would neces­si­tate a com­plete revi­sion of our pic­ture of the evo­lu­tion of the organs and the ner­vous sys­tem. That is the rea­son why it is so impor­tant to cor­rectly define the sequence of ani­mal emer­gence early in evolution.”

In order to elu­ci­date the genetic rela­tion­ships between ani­mal phyla, biol­o­gists com­pare the amino acid sequences of mod­ern species with one another, and con­struct fam­ily trees based on the degree of dif­fer­ence between them. The most likely phy­loge­nomic tree should then faith­fully reflect the order in which the dif­fer­ent phyla diverged from their com­mon ances­tor and from each other. “Two of the impor­tant datasets used in the mod­el­ling stud­ies which iden­ti­fied the comb jel­lies as the old­est extant ani­mal group were made up of very het­ero­ge­neous data. We have now shown that none of the con­ven­tion­ally used sta­tis­ti­cal meth­ods is capa­ble of ade­quately analysing such datasets,” Wörheide explains. To min­imise the com­po­si­tional het­ero­gene­ity of the data, the authors of the new study employed a pro­ce­dure in which the amino acid sequence data were first divided into groups based on the bio­chem­i­cal func­tions of the pro­teins they form, and then sub­jected to the mod­el­ling process. The analy­sis of these datasets — includ­ing those derived from the data that had pre­vi­ously ranked the comb jel­lies as the older phy­lum — demon­strated with high sta­tis­ti­cal con­fi­dence that the sponges are in fact the old­est of the ani­mal groups. “Our results con­firm the clas­si­cal assump­tions con­cern­ing early ani­mal evo­lu­tion, and should help to put an end to the recent con­tro­versy over the ori­gin of mul­ti­cel­lu­lar ani­mals,” Wörheide concludes.

(Source: LMU press release, 01.12.2017)

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