Re-evaluating The Base Of The Animal Kingdom

The traditional view of animal evolution is that sponges came first. All animals are multicellular so the first animals evolved from cooperating colonies of cells: sponges are the most-colonial of all the animals. They consist of genetically identical cells, but the cells do not form true tissues and when broken apart are capable of acting autonomously. Additionally they posses nothing in the way of a nervous system or muscles - all hallmarks of more complex animal groups. The simplicity of sponges placed them logically at the base of the animal kingdom. Recent molecular studies, however, challenge this evolutionary view, claiming that ctenophores are the most primitive.

A ctenophore, more commonly known as a comb jelly

The ctenophores are unfamiliar to most people. More commonly known as comb jellies, their bodies are spherical or ellipsoid, covered in rows of tiny hair called cilia. These cilia beat in unison to propel these bizarre creatures through the seas. The molecular evidence placing ctenophores at the base of animal phylogeny is surprising as the group are without a doubt more complex than sponges, possessing true tissues and a nervous system. This surprising rearrangement is cemented by two possible explanations.

The first, and more unlikely scenario, is that the ctenophores were once simple, but developed anatomical and morphological complexity independently of the higher animal groups. The second scenario is that sponges were once more complex but gradually lost this, shedding the hallmarks of more complex groups. Neither scenario is particularly appealing, but the general inconvertibility of molecular data is such that the origin of ctenophores prior to the sponges gained widespread notice among biologists. A study published just a few days ago, however, shows that even molecular data can occasionally produce erroneous results. This new study supports the traditional view that the simple sponges is indeed the oldest animals.

The study, conducted by an international team of researchers led by Dr Davide Pisani from the University of Bristol, evaluated the statistical methods used to analyse the genomic data for sponges and ctenophores in the papers which supported the 'ctenophore-first' model of animal evolution. 'The analytical methods used in the original reports were not the most appropriate for the task at hand, and this led to systematic errors in the modelling of sequence evolution,' said Dr Gert Wörheide from Ludwig-Maximilians-Universitaet.

'Using elaborate calculation procedures, we were then able to demonstrate that the placement of ctenophora at the base of the evolutionary tree of animals is artefactual. When the more powerful models are applied to these datasets, one finds that the sponges are indeed the earliest diverging animal group,' concluded Wörheide. Simply put, sponges do display the simplest morphologies of the all the animal groups. Recognition of this is what led evolutionary biologists to place them at the base of the animal kingdom in the first place. Having molecular data to conclusively back this up strengthens a model of evolution which is in turn supported by evidence from comparative anatomy and the fossil record.