The tree of evolution is imperfect. It has been drawn by scientists over the last 150 or so years based on fossil finds, living organisms and available genetic evidence. That tree must be constantly tweaked as new specimens are found and new evidence emerges.

Until recently it had been thought that the evolution of complex cells such as muscles only happened once, early in the evolutionary process. The complexity of these cells, the argument goes, indicates that the mutation which caused them could not have happened more than once.

A recent study by a team of researchers from the National Human Genome Research Institute (NHGRI) has cast doubt on this view. The study involves a whole-genome sequencing of a comb jelly, a ctenophore species native to the coastal waters of the western Atlantic Ocean.

Whole-genome sequencing involves comparing the order of all of the chemical bases of DNA. In the case of the comb jellies that means 150 million base pairs. In humans there are three billion base pairs. It differs from simple DNA profiling in that profiling seeks to determine if genetic material came from a particular individual.

The results of the whole-genome sequencing show that the comb jellies split from the rest of the animals before sponges did. Sponges do not have the complex cells such as muscles. Comb jellies have complex cells and a basic nervous system called a nerve net. This implies that complex cells were either gained and lost multiple times or ctenophores evolved these cells independently.

“Having genomic data from the ctenophores is crucial from a comparative genomics perspective, since it allows us to determine what physical and structural features were present in animals early on. These data also provide us an invaluable window for determining the order of events that led to the incredible diversity that we see in the animal kingdom,” said Andy Baxevanis, Ph.D., senior author of the study and senior scientist in NHGRI’s Division of Intramural Research in a statement.

The research will require some re-drawing of the evolutionary family tree as well as some rethinking of the likelihood of similar structures evolving more than once in different species.

“Our study demonstrates the power of comparative genomics research having an evolutionary point of view, probing the interface of genomics and developmental biology. The data generated in the course of this study also provide a strong foundation for future work that will undoubtedly lead to novel findings related to the nature of animal biology,” said co-author Jim Mullikin, Ph.D., NISC director.