The author gives three hypotheses concerning the evolution of segmentation. These hypotheses involve the evolution of segments in three main phyla that include arthropods, annelids, and chordates. Hypothesis 1 states that each phylum acquired its segmentation following an independent process. Hypothesis 2 states that segmentation evolved both in an independent and homologous pattern depending on the phyla of the organism. In that case, the process of segment formation is different in certain organisms and similar in others depending on their phyla. Segmentation in annelids evolved through the development of stem cells from the front section towards the back of the organism. Stem cells form the founder cells of individual segments through imbalanced longitudinal divisions. Segmentation is carried out by specific genes that are unique to each organism. Hypothesis 3 states that the evolution of segmentation is similar throughout the three phyla. Therefore, differences in the evolution of segmentation should be non-existent. This hypothesis holds that segmentation is formed through a process of somitogenesis and is similar in all organisms. Genes that code for segmentation are localized on the front section of developing metameres. The genes only function after morphological segmentation and are supported in their function of gene expression by certain muscle cells contained in the somites. Hypothesis 2 and 3 also suggest that despite the loss of segmentation in certain phyla, the embryogenesis of organisms that do not possess segments should be characterized by evidence of past abilities to form segments by their phyla.
The author evaluates each hypothesis based on the evolution of segmentation in certain organisms belonging to each of the three phyla. For hypothesis 1, he evaluates the evolution of segmentation in insects. The evaluation of hypothesis 2 involves a discussion of the evolution of segmentation in annelids and evaluation of hypothesis 3 involves an exploration of segmentation in chordates. The author contradicts hypothesis 1 by comparing the process of segment formation in different species of arthropods. The facts provided reveal that segmentation is variable in each organism and follows a different progression. The author partially validates hypothesis 2 by comparing segment evolution in two species of leech (Helobdella triserialis and Theromyzon rude). He states that both species contain genes that play a role in segmentation. Finally, the author contradicts hypothesis 3 by stating that the process of somitogenesis that is responsible for segmentation in vertebrates is different from a similar process in chordates that occurs from the anterior to the posterior sections. In addition, he states that segmentally reiterated patterns during gene expression are different in various organisms within the same phyla.
At the end of the article, the author concludes that further studies need to be conducted in order to answer the question whether segmentation is homologous across all phyla. In order to find a conclusive answer, gene analysis involving organisms belonging to different phyla should be conducted. In addition, there is need for development of systems to study the functioning of genes involved in the evolution of segmentation. The article highlights numerous similarities that exist between segmentation patterns in organisms belonging to different phyla. However, the author warns that these similarities do not provide sufficient data to make the conclusion that segmentation is homologous across all phyla. The author finds the conclusion that segmental evolution is different across phyla more appropriate for further studies.