Minimal organization could illustrate humble origins, or it might represent the extreme simplification of a more complex ancestor.
The Placozoa
Trichoplax adhaerans (a name which means: ‘hairy flat sticky thing’) was discovered way back in 1883, but it was only given its own phylum in the 1970s when it was realized just how weird it really is. (Over a dozen species of Trichoplax have now been identified.)
- The animal is flat and around one millimeter in length. It has three components to its body – a squishy fibrous middle layer held between two ciliated layers of single cells, one above and the other below.
- As its scientific name suggests Trichoplax tends to stick to things. It uses its cilia to move about slowly, and its lower surface absorbs any microbes it encounters.
- When present in a marine aquarium it will occasionally be seen as a little pinkish blob creeping slowly up the glass – but remember it is very small and very slow-moving!
Utter Simplicity
- Trichoplax has only four types of cell; ciliated ones above, very similar ciliated ones below, digestive cells on the underside, and strange ‘syncytial’ cells in the middle. The whole body consists of no more than 1,000 cells.
- There are only six pairs of chromosomes which carry less than 12,000 genes – this is the smallest animal genome known.
There are two ways to guess at how animals might be related to one-another. Either compare the details of their structural organization, or study their DNA and biochemical composition.
- Structural analysis is almost a non-starter for the Placozoa. They are so simple that they either represent a very ‘primitive’ phylum, or maybe they are secondarily simplified members of some ‘higher’ group. Whichever is correct – there is too little present to help clarify the matter.
- DNA studies in 2008 revealed remarkably complex capabilities and led some to conclude that Trichoplax is somehow related to the sponges – maybe even the sole survivor of a group of advanced sponges that were close to the ancestors of most living animals.
Taxonomists try to classify animals in ways that reflect their relatedness to one-another. When they get it right then animals sharing the same group (say sister species within the same genus) had a common ancestor from which they all evolved.
- There is high confidence in the phylogenetic (showing relationships) accuracy of modern classification where large, complex, animals are concerned. Classification based on detailed study of anatomy is usually confirmed by genomic studies (looking at the DNA).
- When we are dealing with very simple animals there is less to go on. Older classifications – based on an observation of body forms – were often ‘best guesses’. Genomic studies can throw up surprises here, as illustrated in the case of the placozoans.
Just how an obscure pink blob might be related to other animals seems at first glance to be an extremely arcane question, but in order to answer it adequately the detailed biochemistry of the animal needs to be fully researched. This will lead to a much better understanding of what is actually going on, and since many of the DNA sequences of the Placozoa are found in other animals (including humans!) there could well be significant additions to our knowledge.
It is easier to study the details of a simple system than of a complex one – understanding how the Placozoa function will help understand systems in higher animals, including humans.
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