Since the days of Charles Darwin, researchers are interested in reconstructing the "Tree of Life", and in understanding the development of animal and plant species during their evolutionary history. In the case of vertebrates, this research has already come quite a long way. But there is still much debate about the relationships between the animal groups that made their apparation very early in evolutionary history, probably in the late Precambrian, some 650� million years ago. An international research group led by LMU Munich Geobiology Professor Gert Wörheide and colleagues from France and Canada has now managed to explain the relationships between some of these very early animal groups with a high degree of confidence. In the most comprehensive study of its kind, the researchers show that all sponges descended from a unique sponge ancestor, who in turn was not the ancestor of all other animals. That means that humans did not descend from a sponge – like organism either, as some scientists have put forward. Moreover, the results also suggest that the nervous system only evolved once in animal history.
The most ancient animal groups (phyla) include the Porifera (sponges), Placozoa, Cnidaria, and Ctenophora (comb jellies). The sponges are extremely simply built, and have no organs. The placozoans also have a very simple structure. They have a flat, disk-shaped body, and no organs either. Comb jellies, the ctenophores, are life forms that resemble jellyfish. The true jellyfish, however, are part of the cnidarians, a phylum that also includes corals and sea anemones. The exact relationships among these early animal groups are still controversial, as different research groups have often obtained conflicting results. In particular, results from morphological studies, which look for structural similarities between different organisms, frequently contradict the results from molecular biological studies. The latter explore the functions of genes, and deduce phylogenetic relationships from gene sequences.
Aiming to resolve these controversies, a group of international scientists led by Hervé Philippe (Université de Montréal, Canada), Gert Wörheide (LMU Munich, Germany) and Michael Manuel (University of Paris, France) performed the most comprehensive study to date and investigated 128 genes from a total of 55 species – including nine poriferans, eight cnidarians, three ctenophores and the single known species of placozoans. Their analyses were based on a relatively new approach called phylogenomics, which determines the evolutionary relationships of life forms by comparing large datasets of gene sequences. Together with biochemists, evolutionary and computational biologists from Germany, France and Canada, the team analyzed more than 30,000 amino acid positions. Using computer analyses, the researchers then estimated a phylogenetic tree that displays how related the studied animals are.
One of the most significant outcomes of this study is new evidence that all species of sponges are descendants of a single ancestor. On the other hand, Bilateria, which include worms, mollusks, insects, and vertebrates, did not descend directly from this "spongy" ancestor. "If the ancestral animal would have had a sponge-like organization or body, as some earlier molecular studies repeatedly claimed, then we would all be descendents of such sponge-like organisms," explains Wörheide. "This proposition generated a lot of attention in the past. But our results clearly disagree with it." The analyses also revealed that ctenophores and cnidarians most likely belong to a common group. "This group, the "coelenterates", is most closely related to the bilaterians," explains Wörheide. "Our results support, after much controversy, a hypothesis that was already formulated back in 1848."
My head's going to pop here.
Implications?
4 comments:
It doesn't really mean sponges are not our relatives. It just means they are a lot more basal on the whole animal family tree than anyone thought. Anything is related to anything if you go back far enough. The only real way sponges could not be our relatives is if life on Earth evolved more than once.
There has been talk of multiple origination events from the most basal eukaryotes. The plants and animals are two that are talked about most. There's fungus and others as well. However, there's also talk that the Ediacarans/Vendians might be something completely separate from animals, plants, or whatever.
The most interesting question, which isn't answered in this summary, is if choanoflagellates were included. Choanoflagellates are single-celled or colonial "protists" which very closely resemble the sponge cells called Choanocytes.
Choanoflagellates are clearly the closest-relations to metazoans regardless, but if they form a monophyletic group with sponges, it would, depending upon the nomenclature you use, either require choanoflagellates become metazoan, or mean metazoa is polyplyletic, meaning it's an invalid clade/sponges aren't metazoan.
If as stated it is true that the nervous system evolved here only once, say vs the eye which has evolved numerous times, is any intelligence whatsoever in Earth's biota a fluke? Yes, I know that datasets of one aren't very informative.
Post a Comment