Relationship of aardvark to elephants, hyraxes and sea cows from α-crystallin sequences

Abstract

One of the great enigmas of mammalian phylogeny is the genealogical relationship of the aardvark (Orycteropus afer), the only living representative of the order Tubulidentata. Although now generally grouped close to the ungulates and paenungulates^1–4, and thought to be derived from a common condylarthran stock^5–7, it still holds an isolated position among eutherian mammals. The evidence of the evolutionary relationships of the aardvark may be obscured at the morphological and anatomical level by the pronounced adaptations to its ant-eating and burrowing habits, and may be better preserved in its protein molecules, as direct copies of the genetic material. We have therefore now studied the amino acid sequence of the eye lens protein α-crystallin A of the aardvark, and compared it with the corresponding sequences from species representing 15 mammalian orders. Unique similarities were observed among the α-crystallin A sequences of aardvark and the paenungulates manatee, hyrax and elephant, suggesting that the closest relatives of the Tubulidentata are found in the paenungulate orders Sirenia (sea cows), Hydracoidea (hydraxes) and Proboscidea (elephants). Moreover, these orders do not seem to be derived from a common stem group with the ungulates, but might well be one of the first offshoots of the placental mammalian line.