In: Biology
Structures in organisms (when no longer adequate for their original purpose) often become secondarily adapted for a different use. In the course of their evolutionary history, vertebrates have utilized three different "urinary" systems. Today, however, we only use one. What has happened to the other two? Why were they no longer adequate for mammalian needs? What do you think would happen to such vestigial structures if a secondary "use" did not arise? Why?
The vertebrate urinary system has undergone very rapid and discrete changes during the evolution. Based upon the ambient environmental conditions, the urinary system developed three types of kidneys or nephrons as discussed below:
1. Pronephrons: These are most primitive type of nephrons which contain a simple structure with direct connectivity to the urethra. Owing to their simple structure, they are least efficient in water retention in the body.
2. Mesonephrons: The are intermediate type of nephrons with a slightly extended type of tubule with them. They show improved water retention in the body.
3. Metanephrons: These are the most developed types of nephrons found in contemporary vertebrates with high efficiency of water and ionic retention.
Due to change in habits and habitats and lowered water content in the environment, the vertebrates required a system which could hold maximum water in the body with purification. Hence, they slowly shifted from pronephrons to mesonephrons and then finally utilized the metanephrons. All the vertebrates on earth today utilize metanephrons for urinary system.
As a result, the declined use of these primitive type of nephrons lead to their slow extinction from the vertebrate systems and now emerge as vestigial in only some lower vertebrates.