In the distant past, fleas adopted a parasitic lifestyle feeding on the blood of animals. Their relationship to free living insects is controversial, with most evidence pointing to the Mecoptera (scorpionflies) as the closest relatives. Fleas are parasites of mammals, and most extant species feed on rodents. Fleas may incidentally feed on reptiles and birds, but they are not suitable for flea development. Fleas have many adaptations to feeding on mammals that greatly alter their appearance compared to free living insects. The body is laterally flattened to resist removal by the host. The mouthparts are adapted for piercing the host tissue and feeding on blood. Some adaptations to a parasitic lifestyle makes some features more similar to features of unrelated parasites than closer free living relatives. These drastic morphological changes present difficulties in reconstructing phylogenies. In these groups, DNA evidence can be informative.
A group of scientists* gathered DNA sequences from 16 of the 18 extant flea families. The DNA evidence suggests that the extant flea families last had a common ancestor around 95 million years ago and the hosts were marsupials and mammals. The data suggest that fleas abandoned free living to become parasites of the expanding group of animals that are ancestral to mammals. The evidence does not support a link between fossils of parasite found in association with reptiles and fleas. DNA data is increasingly important in resolving controversies when the morphological data is sparse and limited to a few fossils.
Qiyun Zhu, Michael Hastriter, Michael Whiting, Katharina Dittmar. 2015. Fleas (Siphonaptera) are Cretaceous, and Evolved with Theria. Molecular Phylogenetics and Evolution. 90: 129–139.