Scientists have revealed that the origin of gills evolution is older than it previously though. According to a study conducted by the researchers, gills develop from the same embryonic tissue in both jawed and jawless vertebrates.
The term vertebrates is commonly used for an animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians and fishes.
Jawed vertebrates -- such as fish, birds and mammals -- make up 99% of all living vertebrates, including humans. Jawless vertebrates include the parasitic lamprey and scavenging hagfish: eel-like creatures that diverged from the ancestral line over 400 million years ago.
Previous research in this area included slicing thin sections of fish embryos to map organ growth. Through the results of the research the scientists came to know that gills were formed from different tissues. These different tissues involved the internal 'endoderm' lining in jawless vertebrates, and the 'ectoderm' outer skin in the jaw.
According to an old belief, the ancient jawed and jawless lines evolved gills separately after they split. The researchers explained 'convergent evolution'—where nature finds the same solution twice, to explain the ancient belief.
Content Source: https://www.sciencedaily.com/releases/2017/02/170209133427.htm