Scientists from France,Germany and Canada have recently put forward a new tree of life based on the most comprehensive phylogenomics study to date ,investigating 128 genes from a total of 55 species – including nine poriferans, eight cnidarians, three ctenophores and the single known species of placozoans. This study was mainly done to overcome the confusions prevailing at the base of the tree of metazoans and also to establish relationships between some of these very early animal groups with a high degree of confidence.

This new study revives traditional views on deep animal relationships placing Sponges back at base of the tree (important thing to remember here is that first animal group to diverge from common ancestor may not resemble present day Sponge,as they have undergone evolution for such a long time since separation. Sponges are representatives of the early branching lineage ,They are the deepest branching in the group.) Remember previous two phylogenomic studies argue against conventional view of sponges as descendants of earliest diverging lineage and predicted different organisms at basal position: Cassey Dunn et al had Ctenophores and Schierewater et al placed Placozoa at the base of tree.

The study shows for the first time that all sponges descended from a unique sponge ancestor, who in turn was not the ancestor of all other animals,which in a way tells us that humans did not descend from a sponge-like organism ,as some earlier molecular studies repeatedly claimed.

Bilateria, which include worms, mollusks, insects, and vertebrates, did not descend directly from this “spongy” ancestor.This proposition generated a lot of attention in the past. But our results clearly disagree with it.– Gert Wörheide (LMU Munich, Germany)

Another important outcome of the study was that the nervous system only evolved once in animal history,unlike famous study regarding ctenophores which suggests that nervous system developed several times independently in animal history, or lost in sponges and placozoans. There is ample evidence to suggest that part of the genetic toolkit responsible for building the nervous system in other animals was already present in sponges,it was just used in a different manner or/and may be with the help of some new protein members, in lineages which diverged subsequently after sponges during evolution (eg ; Bilaterians)

Nervous system OK !!! But what about origin of Hox genes ???

This tree nicely explains the origin of nervous system ,but if one takes a look at Hox gene origin ,it gets complicated. I was rather surprised authors fail to mention even a single sentence about Hox genes ,may be they might come up with data about Hox genes from the study in the form of another article. According to new tree Sponges form deepest branching group ,later placozoa separated and then Ctenophores / Cnidaria.
If we look at the data regarding Hox genes or closely related Para Hox,)in these early brabching phylas ,Sponges have only Nk genes and lacks both para hox and Hox genes . Trichoplax (Placozoa) have one Parahox in Trox-2 , Then Ctenophores again like sponges have only NK genes and so far no trace of any Hox or Para Hox gene . Even though the genome sequence of Ctenophores is awaited, but still many attempts from various groups to fish out Hox genes turned futile. However its sister group ,Cnidarians posses true hox genes. The overall picture we get is that Hox genes or Hox like genes appeared in Placozoa and then lost in Ctenophores but only to reappear again in Cnidarians. Its looks rather unlikely to me that Ctenophores has lost all Hox and parahox genes. I would be glad if someone could help me in understanding this aspect . Ctenophores as most basal suits well for Hox origin but this study contradicts that aspect. Lets wait for the authors to publish(if they) the details about Hox and related genes from the study,which might help us to better understand the origin and evolution of Hox genes.

Reference:
Phylogenomics revives traditional views on deep animal relationships.
Hervé Philippe Romain Derelle, Philippe Lopez, Kerstin Pick, Carole Borchiellini, Nicole Boury-Esnault, Jean Vacalet, Emmanuelle Renard, Evelyn Houliston, Eric Quéinnec, Corinne Da Silva, Patrick Wincker, Hervé Le Guyader, Sally Leys, Daniel J. Jackson, Fabian Schreiber, Dirk Erpenbeck, Burkhard Morgenstern, Gert Wörheide, Michaël Manuel.
Current Biology, 2009; DOI: 10.1016/j.cub.2009.02.052

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