Afrotheria: plate tectonics meets genomics.

A frotheria is one of the most remarkable hypotheses in mammal evolution. It suggests that one-third of the orders of placental mammals form an ancient group that evolved on Africa when that continent was isolated from others through plate tectonics (1). Although this hypothesis has been predicted by molecular clock studies (2), evidence for it has emerged only in the last 3 years from phylogenetic analyses of DNA and protein sequence data (1, 3–6). Many mammalogists remain baffled and see no support from traditional sources of data such as anatomy (7). The recognition of Afrotheria splits apart other established groups of mammals, including ungulates and insectivores, 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 molecular data and find additional support for Afrotheria. The 4,700 species of living mammals are placed in about 20 orders, including such groups as the rodents (Order Rodentia), primates (Primates), and bats (Chiroptera) (9, 10). In systematics, taxonomic names often are treated as evolutionary hypotheses, implying that members of the group are more closely related to each other than to other species or groups. Afrotheria is a superorder (4) that contains six orders: the elephants (Order Proboscidea), sea cows (Sirenia), hyraxes (Hyracoidea), aardvark (Tubulidentata), elephant shrews (Macroscelidea), and golden moles and tenrecs (Afrosoricida) (Fig. 1). Some of the smallest (Lesser long-tailed tenrec, 5 g) and largest (African elephant, 5,000 kg) species of mammals belong to this group, and its members fill a diversity of ecological niches (11). In most classifications, elephants, hyraxes, and the aardvark are considered close relatives of other hoofed mammals (ungulates) such as horses, rhinos, and allies (Order Perissodactyla) and cattle, deer, and allies (Artiodactyla). With the exception of the ant-eating aardvark, ungulates typically are herbivores. Golden moles and tenrecs previously have been classified with other insectivores such as shrews and moles (Insectivora or Lipotyphla). The living elephant shrews, mouse-like in appearance, are insectivorous, but their ancestors were herbivores; morphological classifications usually place them with rabbits (Lagomorpha) and rodents (12). The elephant shrews were so named because of their elephant-like snout (Fig. 1) and not because of any presumed close ties to elephants. Although sirenians appear the most ecologically and morphologically divergent members of Afrotheria, their anatomy and fossil record have indicated a close relationship with proboscideans and hyraxes (9). In general, the fossil record (12) and distribution of afrotherians suggest an origin in Africa. The discovery of Afrotheria places more importance on plate tectonics in the early evolution of placental mammals (2, 13). However, this is another issue that is hotly debated (14). Molecular clocks derived from large numbers of genes have indicated that placental mammals not only were present deep in the Cretaceous ['100 million years ago (mya)] but were already diverging from one another into clades that eventually led to the present-day orders (2, 15). Today, Africa is connected to Europe and Asia, facilitating dispersal of mammals among these three continents. But in the early Cretaceous ('120 mya), Africa was connected to South America, with the two continents separating about 105 mya (16). Africa was relatively isolated between 105 and 40 mya (Fig. 2), and during this time afrotherians likely were evolving and adapting to different ecological niches. Around 30 mya, Africa began to collide with Europe and Asia, and since then these areas have been closely associated. van Dijk et al. (8) subject existing molecular data sets to additional scrutiny by searching for specific amino acid replace-