The World of Minibeasts

How we are Related to Earthworms

When the arthropods evolved from their predecessors, the Annelidae, they inherited the body segments but continued to evolve.

In insects for example, some previously equally-sized segments joined to form three larger portions: the head, the chest and the abdomen. On larger insects such as bees or beetles you can clearly recognize the segments in the abdomen with the naked eye. In the arachnids, the segments have fused to form only two sections: the head (equivalent to the combined head and thorax in insects) and the abdomen.

The reason why insects have three pairs of legs while spider-likes have four, is, that in insects three segments holding a pair of legs each joined together, whereas in spider-likes there were four to join.

Segmentation of the body is not limited to arthropods and Annelidae. Many animals, including humans, have similar features. The segmentation of the human body, most clearly seen on our spine, suggests that we are related to arthropods and Annelidae.

On one hand, by the existence of our spine we differ essentially from the Annelidae and from the arthropods.

In evolutionary terms, spines developed in animals more recently than segmented bodies and legs. The spine, from which all other bones developed, means that we belong to the phylum of vertebrates.

On the other hand, the fact that the spine is made up of segments, or vertebrae,  reflects that the humans (and all other animals with backbones) evolved from segmented animals like arthropods and Annelidae.

Therefore a single vertebra, combined with a pair of ribs, which are joined to that vertebra, corresponds to a single body segment. So we, the humans, also show the archaic feature of body segmentation, which may be an evidence that all creatures that have evolved ever since the Annelidae are related.

Which is what was thought until several years ago. There is another theory which says that segmentation developed several times during evolution, that is, in different phylums. In that case, not all creatures  that show segmentation are related. 

Anyway, it does show, that segmentation is a very useful principle in body construction of the higher developed creatures on our planet. 

Text by Maria Pfeifer