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Error in the Genealogy of Human, Daniel Chourrout
New research findings show one entire group of small marine animals is more closely-related to human than initially thought. Other changes in the tree of animal life may take place in the future.
This new research is presented in an article from the journal Nature. Daniel Chourrout from the University of Bergen has contributed to the research of Herve Philippe's group at the University of Montreal which concludes that the respective positions of two animal groups distantly related to human should be inversed.
At first glance, the finding does not seem so significant: just a change of order between two branches in the tree. The lancelets or cephalochordates (such as amphioxus) must surrender their place to a group of simpler and generally smaller animals named tunicates (such as ascidians, salps and appendicularians). The importance here is that among these two groups, the simplest one is our closest relative.
Failure of older approaches
“Just a few years ago, such a modification of the tree was inconceivable” explains Daniel Chourrout, Director of the Sars Centre, Unifob, at the University of Bergen.
Scientists tend to view evolution from simple to complex. For the relatives of humans and vertebrates, the current classification had received support from morphologists, paleontologists, but also more recently from rather straightforward molecular analyses. These analyses are based on sequence information from one gene family (ribosomal RNA genes), which permits the construction of so-called phylogenetic trees supposed to approximate the real genealogy of species and groups of species.
However, a classical problem with this standard molecular approach is that certain groups evolve faster than others. These groups tend to be erroneously rejected near the base of the tree, as if they were more ancient than they really are.
Slow and Fast evolving animal groups:
Indeed, research at the Sars Centre has shown that Tunicates have been evolving faster than other related groups. To avoid errors caused by unequal speed of evolution, several precautions must be taken. In the study published in Nature, Herve Philippe’s group has incorporated many more species and sequence information than usual. These include information from two distantly related tunicates. The recently sequenced genome of Oikopleura dioica, model system established at the Sars Centre, has been used here. When the information is treated with various tree construction methods, the results show: our closest relatives are the simple tunicates, and not the more complex cephalochordates.
“The former classification was an artefact due to the utilisation of standard methods. Tunicates were misplaced because they have evolved rapidly. Cephalochordates were grouped with human and other vertebrates because they evolve slower. Through the change of genealogy, we learn that the common ancestor of the three groups may have looked more like an amphioxus than like a tunicate. The evolution has been simplifying in the tunicate branch, but has increased complexity in our own branch,” explains Chourrout.
Other research now clearer
“When we realise that the old tree was incorrect, the dominos begin to fall in place” explains Chourrout.
We can now see that several recent findings are also consistent with the new tree. For example, research at the Sars Centre had shown that the tunicate Oikopleura has lost quite a few developmental genes. This finding corresponds with the idea of a simplification process. Other research at the University of Maryland has recently shown that tunicates possess specialised cells of the so called neural crest, a structure so far considered specific of vertebrates and not identified in cephalochordates. The tunicates may have these cells because they are more related to vertebrates.
Next Animal Model Systems:
The finding that tunicates emerge closer to human than first believed makes the Sars Centre research on Oikopleura even more relevant. Knowing more about our ancestry is in itself a quite exciting happening, but finding that some animals are after all fairly close to us creates opportunities for tunicates as model systems, for example in medical research.
“Today there are a few very widely used model systems which are convenient to use because they are considered anatomically simple, such as the fly or the roundworm C. Elegans. The appealing difference here is that Tunicates are not only simple, but also much closer to human in evolution” explains Chourrout