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Home , Aardvark, Afrotheria, Elephant shrew, Hyrax, Lipotyphla, Sirenia

Commentary

Afrotheria: Plate tectonics meets

S. Blair Hedges*

Department of Biology, Institute of Molecular Evolutionary Genetics, and Astrobiology Research Center, 208 Mueller Laboratory, Pennsylvania State University, University Park, PA 16802

frotheria is one of the most remark- Aable hypotheses in evolu- tion. It suggests that one-third of the or- ders of placental form an ancient group that evolved on when that continent was isolated from others through plate tectonics (1). Although this hypothesis has been predicted by molec- ular clock studies (2), evidence for it has emerged only in the last 3 from phylogenetic analyses of DNA and protein sequence data (1, 3–6). Many mammalo- gists remain baffled and see no support from traditional sources of data such as anatomy (7). The recognition of Afro- splits apart other established groups of mammals, including and , yet it is the most strongly supported grouping of mammalian orders in molecular phylogenies (4). In this issue of PNAS, van Dijk et al. (8) take a slightly different approach in analysis of molecu- lar data and find additional support for COMMENTARY . The 4,700 of living mammals are placed in about 20 orders, including such Fig. 1. Representatives of the six orders of mammals comprising the Superorder Afrotheria: (Upper Left) groups as the ( Rodentia), African forest (Loxodonta africana); (Upper Right) Golden-rumped elephant (Rhyncho- (Primates), and (Chiroptera) cyon chrysopygus); (Middle Left) ( afer); (Middle Right) Streaked (Hemi- (9, 10). In systematics, taxonomic names centetes nigriceps); (Lower Left) Eastern tree (Dendrohyrax validus); and (Lower Right) often are treated as evolutionary hypothe- (Dugong dugon). [Images of tenrec and dugong reproduced with permission from Andromeda Oxford ses, implying that members of the group are Limited (18); other images reproduced with permission from Jonathan Kingdon.] more closely related to each other than to other species or groups. Afrotheria is a superorder (4) that contains six orders: the insectivorous, but their ancestors were her- years ago (mya)] but were already di- (Order ), sea cows bivores; morphological classifications usu- verging from one another into that eventually led to the present-day orders (2, (), (Hyracoidea), aardvark ally place them with rabbits () 15). Today, Africa is connected to (Tubulidentata), elephant (Macros- and rodents (12). The elephant shrews were and Asia, facilitating dispersal of mammals celidea), and golden moles and so named because of their elephant-like among these three continents. But in the () (Fig. 1). Some of the smallest snout (Fig. 1) and not because of any pre- early (Ϸ120 mya), Africa was (Lesser long-tailed tenrec, 5 g) and largest sumed close ties to elephants. Although connected to , with the two (, 5,000 kg) species of sirenians appear the most ecologically and continents separating about 105 mya (16). mammals belong to this group, and its mem- morphologically divergent members of Africa was relatively isolated between 105 bers fill a diversity of ecological niches (11). Afrotheria, their anatomy and fossil record have indicated a close relationship with pro- and 40 mya (Fig. 2), and during this time In most classifications, elephants, afrotherians likely were evolving and adapt- hyraxes, and the aardvark are considered boscideans and hyraxes (9). In general, the fossil record (12) and distribution of afro- ing to different ecological niches. Around 30 close relatives of other hoofed mammals mya, Africa began to collide with Europe (ungulates) such as horses, rhinos, and allies therians suggest an origin in Africa. The discovery of Afrotheria places more and Asia, and since then these areas have (Order Perissodactyla) and cattle, deer, and been closely associated. importance on plate tectonics in the early allies (Artiodactyla). With the exception of van Dijk et al. (8) subject existing molec- evolution of placental mammals (2, 13). the -eating aardvark, ungulates typically ular data sets to additional scrutiny by However, this is another issue that is hotly are . Golden moles and tenrecs searching for specific amino acid replace- previously have been classified with other debated (14). Molecular clocks derived insectivores such as shrews and moles (In- from large numbers of have indicated sectivora or ). The living ele- that placental mammals not only were See companion article on page 188. phant shrews, mouse-like in appearance, are present deep in the Cretaceous [Ϸ100 mil- *E-mail: [email protected].

PNAS ͉ January 2, 2001 ͉ vol. 98 ͉ no. 1 ͉ 1–2 Downloaded by guest on September 25, 2021 Fig. 2. Plate tectonic reconstruction for the late Cretaceous (85 mya) at a time when Africa was separated from other continents, and afrotherians presum- ably were evolving in isolation. Land areas are orange, Africa is red, and current coastlines are included for reference (adapted from ref. 16).

ments that might support Afrotheria. As On the basis of the current composition eral have been poorly studied, and addi- expected, they identify a large number of of the group and limited knowledge of tional anatomical, developmental, and ge- sites in which an amino acid residue is relationships, the Cretaceous afrotherian netic studies are needed. The molecular uniquely shared (more or less) by afrother- ancestor likely was a small forest-dwelling evidence predicts that there are shared ians in three selected proteins. Next, they or possibly . What derived morphological characters that link collected sequence data from other afroth- morphological characters, if any, are together this odd collection of mammals. erian species to see whether they possessed uniquely shared by these mammals and Finding those characters would help us to these diagnostic amino acid residues. In all represent inheritance from that common understand better the evolutionary his- six sites that were selected, the additional ancestor? At first glance, the most obvious tory of this ancient of mammals. species were found to possess the diagnostic shared trait is the long snout possessed by residues. Finally, the data were subjected to several members of the group (Fig. 1). The some statistical analyses that are unique but sirenian snout is not long but is nonethe- are not necessarily an improvement over For permission to reproduce illustrations, I grate- current analytical methods. For example, less enlarged and is used for grasping vegetation (underwater) in a way vaguely fully thank Jonathan Kingdon for the elephant, the presence or absence of diagnostic sites , aardvark, and hyrax; Priscilla similar to the elephant’s trunk. Except for typically contributes to statistical support in Barrett and Andromeda Oxford Limited for the the widely used bootstrap method (17) and hyraxes, the snouts of all are tactile and dugong; and Dennis Ovendon and Andromeda with fewer assumptions. Nonetheless, van (except in tenrecs) mobile. Structurally, Oxford Limited for the tenrec. I thank Alan Dijk et al. present new sequence data for however, there is no evidence that these Walker and Sudhir Kumar for discussion and the African otter shrew and other species similarities are anything but superficial Anthony Geneva for assistance with graphics. that are important evidence in support of and are related to the ecology of the This work was supported by the National Aero- Afrotheria. . Nonetheless, afrotherians in gen- nautic and Space Administration.

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