New study conducted by Schierwater, B. et al. have shown that the Triploblastic bilaterians and diploblastic animals ( Placozoa,Sponges,Ctenophores and Cnidarians) animals , probably separated very early, at the very beginning of animal evolution and independently evolved their complex body plans, including body axes, nervous system, sensory organs, and other characteristics. In the present study authors analysed morphological and genetic information from 24 metazoan lineages and the resultant phylogeny tree shows a early divergence of simple animals from complex ones ,during the metazoan evolution . One line of the new tree goes to the placozoa, sponges and jellyfish and the other goes to the more complex animals . The point where these lines meet is at placula – the most ancient ancestor of all animals and the closest living relative of which is the placozoan Trichoplax adhaerens.
The work published in latest issue of Plos Biology is quite admirable as the authors did a decent job in resolving tree of life and also came up with some astounding result that all animals are divided into two clades . The new tree of animals suggest that Bilateria and diploblasts (Porifera, Ctenophora, Placozoa, and Cnidaria) are sister groups. The real surprising one was Placozoa being place at the root of diploblastic group.
Caption: This is a new metazoan tree of life presented in Schierwater, et al. 2009.
Credit: AMNH
It was only from last year’s phylogenomics work conducted by Dunn et al ; placed Ctenophores at base of animal tree , but unfortunately that work didnot include placozoan genome data. The new tree goes against the conventional principles in evolutionary biology that complex life forms derive from more primitive ancestors. Its always believed that it has long complex animals, like the Bilateria, arose from simpler (diploblastic) organisms such as the cnidarians (corals, polyps, and jellyfishes). But the new study indicates that the first major event in animal evolution split bilateral animals from all diploblasts , quite early in animal evolution and later they went on their paths to evolved in parallel.
But the paper in Plos Biology also disappoints on various counts . If the results displayed in the study are true then Nervous system has evolved twice during animal evolution and many biologists are finding it difficult to buy this theory .Placozoa and sponges don’t have a nervous system, but the related cnidarians do like all bilaterians and the large number of molecular similarities between the nervous systems of cnidarians, Drosophila and vertebrates , makes it difficult to believe that nervous system in bilaterians and diploblasts evolved independently . If that was the case then we might expect them to be formed in different manner using quite different set of genes and we all know it is not the case.
Many people would have appreciated the work if a more balanced discussion of either possibilities was taken into account -what if Trichoplax is reduced, and not the ancestral diploblast ? Trichoplax is believed to be quite derived organism by some researchers and explaining the loss of nervous system in trichoplax would be more logical in this scenario . If we believe that the common ancestor of all animals had nervous system and subsequently Trichoplax and sponges lost it . It makes sense also that it would be rather easier to evolve nervous system once( in common ancestor of all animals) and lose twice ( in Trichoplax ,Sponges ) than evolving it twice altogether. If nervous system was evolved only once and lost in Trichoplax and sponges then one might expect to see remnants of nervous system in the genome of these two organisms . This is what exactly found from the genome sequence. Sponges and Trichoplax share many genes with bilaterians used for forming nervous systems
Read an excellent article by Todd Oakley dealing with two animal nervous system

David Miller of James Cook University in Townsville, Australia is also wary of of the way in which these trees are calculated. He believes that small alterations in the settings of some of these analysis tools can make major differences to the outcomes and this certainly isn’t the last word on the scheme of animal evolution. Hox gene evolution also gets a bit complicated with the new tree , as hox genes are found in all bilaterians and placozoa ( Trox-2 ,a para hox gene ) indicates that common ancestor of all animals also had Hox genes. Subsequently Hox gene is lost in sponges ,only to be regained in cnidarians . This new tree of life poses more new questions instead of solving older questions and its for the leaders in the field to decide which study is more credible ( which can explain many aspects of evolution of animals) between the phylogenomics study of dunn et al and this new study.

Reference :
Schierwater B, Eitel M, Jakob W, Osigus H-J, Hadrys H, et al. (2009) Concatenated analysis sheds light on early metazoan evolution and fuels a modern ‘‘Urmetazoon’’ hypothesis.
PLoS Biol 7(1): e1000020. doi:10.1371/journal.pbio.1000020
Image Credit :
Trichoplax – W. Jakob

Related Posts:

• New tree of animals suggests nervous system evolved only once in animal history
• Sponges regain their tag of most primitive animal on Earth
• Meet the most primitive animal group – Ctenophores
• Functional equivalence of proteins among animals
• Early Antennapedia gene Family : Evidence from Trichoplax genome

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