Sponges regain their tag of most primitive animal on Earth
There was a mad rush to get to the base of animal tree of life in the past twelve months, in order to establish most primitive animal on our planet, with many phylogenomics studies contradicting each other. 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. This work is lead by famous Harvey Philippe in association with the group of Michael Manuel in paris and many other support labs.
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).
References:
Phylogenomics and the reconstruction of the tree of life.
Delsuc F, Brinkmann H, Philippe H.
Nat Rev Genet. 2005 May;6(5):361-75.
Phylogenomics: intersection of evolution and genomics.
Eisen JA, Fraser CM.
Science. 2003 Jun 13;300(5626):1706-7.
Philippe H, Delsuc F, Brinkmann H, Lartillot N:
Phylogenomics.
Annu Rev Ecol Evol Syst 2005, 36:541-562.
Phylogenomics.
Telford MJ.
Curr Biol. 2007 Nov 20;17(22):R945-6.
Phylogenomics Revives Traditional Views on Deep Animal Relationships.
Philippe H, Derelle R, Lopez P, Pick K, Borchiellini C, Boury-Esnault N, Vacelet J, Renard E, Houliston E, Quéinnec E, Da Silva C, Wincker P, Le Guyader H, Leys S, Jackson DJ, Schreiber F, Erpenbeck D, Morgenstern B, Wörheide G, Manuel M.
Curr Biol. 2009
UPDATE:
I am thankful to Jonathan Eisen for pointing out that its not correct to use the word “PRIMITIVE” for organisms , should only be used for characters.
This is simply wrong as the term primitive, which I avoid at all costs because it is so frequently misused, should only be used to refer to features of organisms not to the branching pattern in an evolutionary tree.
So in the article it should be “Sponges are the deepest branching in the group.” or “most basal” instead of primitive. sponges are typically considered to represent descendants of the earliest branching lineage. To be more clear, first animals probably were not very similar to sponges as we see them today , as representatives of a lineage that branched off first from those of other major animal groups and has been evolving for millions of years since diverging. Its quite possible that sponge evolved its simpler form from the more complex form during the course of hundred million years of evolution. To have more insights into the morphological changes ,its is very important to have data from fossil , as molecular information can only tell us about order of branching in tree of life.
calling organisms that branch deeply in a tree “primitive” is wrong not only because it is referring to an organism not a feature but also because deep branching does not imply ancestral features.
UPDATE II:
I found a very nice post by Ryan Gregory explaining the “Phylogenetic Fallacies: Early Branching Must Mean Primitive” -a post regarding cassey dunn work showing ctenophores as deepest branching group or most basal group
Specifically, it draws the false conclusion that a modern member of an early branching lineage is very similar to the distant ancestor that it shares with other lineages. In actuality, the species under consideration are all modern species whose lineages have been evolving for exactly the same amount of time since their divergence from a common ancestor. The comb jelly lineage may have branched first, but the common ancestor from which it and the other animals lineages diverged probably looked nothing like a comb jelly. It is entirely possible that comb jellies are highly derived (i.e., very different from their early ancestor), just as other animal lineages are…………….
“one cannot assume that a modern representative of an early branching lineage is the same as the ancestor from which it is descended” — RYAN GREGORY
For better understanding of phylogenetic trees read :
Understanding evolutionary trees.
Gregory, T.R. 2008b.
Evolution: Education and Outreach 1: 121-137.
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