Sponges regain their tag of most primitive animal on Earth

By | May 7, 2021
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In April 2008, Cassey dunn et el published their famous phylogenomics work in Nature, which placed Ctenophores at the base of the tree making them most primitive animal on Earth. Before that work Sponges were considered as first animals to branch off from rest of the animals and hence occupied a most basal position in the tree of animals for long time.


However, some months later another phylogenomics study carried out by Bernd Schierwater et al ; changed things dramatically by placing Trichoplax as most primitive animal replacing Ctenophores. Now a new phylogenomic study published in online section of Current Biology is making headlines , where Sponges regain their position as most primitive animal on Earth.

So what makes this work from Micheal Manuel and colleagues different from other phylogenomics studies done earlier ? What makes it more credible? But before getting answers to these questions, we will try to understand a general view of what phylogenomics is all about ?

A phylogenetic tree is a graphical representation of the evolutionary relationship between taxonomic groups.The term phylogeny refers to the evolution or historical development of a plant or animal species, or even a human tribe or similar group. The idea of phylogenetic tree is not something of modern era,in fact it is used for the first time by Charles Darwin in his most famous book “On the origin of species” .The diagram of Tree of life happens to be only illustration in his famous book ,to represent evolutionary relationships among species, in the form of a phylogenetic tree.

As all the extant species on earth share a similar history in terms of their common ancestry and this makes understanding of phylogenetic trees becomes a prerequisite of almost any evolutionary
study. In today’s molecular era thanks to revolutionizing progress in DNA sequencing technology,there is ever increasing numbers of whole genome sequences and collections of ESTs.The wealth of generated by sequencing various species from different phyla lead to the origin of new branch in “Phylogenomics”(portmanteau word for phylogenetics and genomics) ,which is considered as intersection of evolution and genomics , using phylogenetic principles to make sense of genomic data. The first use of word phylogenomics can be traced back to writings of Jonathan A. Eisen @ The Institute of Genome Research (TIGR) in the context of an “approach to the prediction of gene function” for genome-scale data. The contribution of Phylogenomic studies in the use of data at the genomic scale to reconstruct the phylogeny of organisms is immense.

Anyhow coming to the main story of the day regarding phylogenomics study by Harvey.P. et al involving 128 gene data set including newly generated sequence data from ctenophores, cnidarians, and all four main sponge groups. Prior phylogenomics studies provided valuable insights for for intrabilaterian relationships,but things related to basal phyla still remain a puzzle. Deciphering true relationships among early branching animals is extremely vital in understanding origins of defining features of animal body plans, such as symmetry, nervous system, and the mesoderm.

So what makes this study different ?

Authors are of the opinion that previous phylogenomics studies of basal metazoan relationships are supported by poor supported trees and produced contradicting results.This study is mainly done to clarify the basal metazoan relationships, involving more comprehensive sampling of all the major early branching animal lineages and the effort is seen in as they used 128 different protein-coding genes (30,257 unambiguously aligned positions) for 11 outgroup species and 44 metazoans, including 9 sponge species, 3 ctenophores, 9 cnidarians, the placozoan Trichoplax.

The two important outcomes from the study are:
1) Sponges (Porifera) are monophyletic and not paraphyletic as repeatedly proposed , thus undermining the idea that ancestral metazoans had a sponge-like body plan. (This is the first phylogenomic data set to include all four main sponge lineages)
2) Ctenophora is together with the cnidarians.
3) Sponges is the earliest diverging animal group(most primitive animal) and followed by Placozoa (Trichoplax).

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