Determination of higher-level relationships among placental mammals using morphological evidence proved remarkably challenging 61011121314151617. Despite general agreement about subdividing placental mammals into orders, recognition of deeper nodes in the tree has been tentative at best (see for example 18), and morphological interpretations, such as the placement of colugos with bats in the grouping Volitantia 61011, frequently clash with the molecular evidence. The rapidly accumulating molecular evidence has yielded an entirely new perspective on placental mammal evolution. Phylogenetic reconstructions using comprehensive DNA datasets (see for example 19) have led to consistent recognition of four monophyletic superorders: Afrotheria, Euarchontoglires, Laurasiatheria and Xenarthra. Those superorders were confirmed by the most extensive analysis to date, generating a supertree combining results from more than 2,500 partial trees 20 (Figure 1). The superorder Euarchontoglires (alternatively known as Supraprimates 212223) is often divided into two subgroups:

Euarchonta (Dermoptera (colugos), Primates and Scandentia (tree-shrews)) and Glires (Lagomorpha (rabbits and hares) and Rodentia (rodents)). However, although the consensus supertree 20 portrays Euarchontoglires as monophyletic, internal relationships between tree-shrews, Glires and colugos+primates appear as an unresolved trichotomy (Figure 1). It should also be noted that tree-shrews and lagomorphs (usually linked to rodents in Glires) have emerged as sister groups in several individual studies: those of the ε-globin gene 24; exon 28 of the von Willebrand factor gene 25; complete protein-coding mitochondrial DNA sequences 26; and complete sets of tRNA and rRNA sequences from mitochondrial genomes 27. Figure 2 shows a version of this part of the tree with possible sister groups indicated.

The now widely recognized taxon Euarchonta is a radically pruned version of Gregory's Archonta, excluding not only elephant-shrews (now placed in Afrotheria) but also bats (Chiroptera, in Laurasiatheria). Strikingly, molecular evidence uniformly indicates a very deep separation between colugos and bats, soon after the common ancestor of extant placental mammals (see Figure 1). Similarities that led morphologists to recognize the Volitantia probably reflect convergent gliding adaptations in colugos and the (hitherto undocumented) precursors of bats.

Investigation of short interspersed nuclear elements (SINEs) in Euarchontoglires 2223 identified Euarchonta and Glires as monophyletic sister taxa, but left the relationship between colugos, tree-shrews and primates within the Euarchonta as an unresolved trichotomy (see Figure 2). SINEs originate from retroposition of small RNAs as localized insertions throughout eukaryote genomes. Potentially, they are highly informative phylogenetic markers because retroposition at exactly the same site in independent lineages (that is, convergent evolution) is highly unlikely. SINEs derived from 7SL RNA seem to be a shared derived feature of Euarchontoglires, subsequently leading to dimeric Alu sequences in primates, chimeric sequences in tree-shrews and B1 sequences in rodents 23. Moreover, a genome search revealed five independent retroposon insertions shared by tree-shrew and human and fourteen shared by mouse, rat and rabbit, indicating a basal divergence between Euarchonta and Glires. But the very limited genomic information available for colugos and lagomorphs handicapped this study 23, such that relationships between colugos, primates and tree-shrews were left unresolved. Some reports 2829 based on mitochondrial DNA sequences challenged the monophyly of primates, linking colugos to higher primates (Anthropoidea) to the exclusion of prosimians (lemurs, lorisiforms and tarsiers). Schmitz et al. 30 replicated this aberrant finding, and conducted a test using SINEs. They identified a substantial set of transposable elements present in all major groups of extant primates but lacking in colugo, thus clearly supporting primate monophyly.

Within the Archonta, colugos have sometimes been linked most closely to tree-shrews and sometimes to primates. Support for the former association has now been provided by Nie et al. 8, who generated a G-banded karyotype for the Malayan colugo G. variegatus and used reciprocal chromosome painting with human and G. variegatus chromosome-specific probes to establish the first genome-wide comparative map matching Galeopterus to human. This enabled them to define 44 segments in the G. variegatus genome homologous to segments in humans. Comparisons across similar published maps from other species within Euarchontoglires revealed that Galeopterus and a tree-shrew (Tupaia belangeri) share a unique derived association between two human syntenic segments, an association confirmed by Nie et al. 8 by reverse painting of human chromosomes by T. belangeri and G. variegatus probes. Moreover, this association is borne on a large autosomal chromosome that is seemingly identical in both. Nie et al. 8 thus provide more evidence for the hypothesis that Scandentia and Dermoptera have a closer phylogenetic relationship to each other than either of them has to Primates. This is confirmation of previous studies indicating that colugos and tree-shrews constitute a monophyletic group 31. Such a group, labeled Sundatheria, was, for example, indicated by cladistic analysis of dental features 32, and several authors have reported molecular evidence linking colugos to tree-shrews 2933343536.

But phylogenetic studies are never simple, and another recent study has interpreted DNA sequence and genomic data as showing a closer association of colugos to primates. Janecka et al. 9 combined two independent molecular approaches to explore relationships within Euarchonta: screening of almost 200,000 protein-coding exons to identify rare deletions, and generation of a phylogenetic tree using a 14-kb DNA sequence dataset from nuclear genes. The monophyly of Euarchonta was supported by three specific deletions. No specific deletions linked colugos to tree-shrews. However, seven deletions were common to colugos and primates, whereas tree-shrews and primates shared only a single deletion. A relationship between colugos and primates was also indicated by the phylogenetic tree generated from DNA sequences. Overall, the results thus indicate that colugos are closer to primates than to tree-shrews.

So the relationships among colugos, tree-shrews and primates still await resolution (see Figure 2). It is evident, however, that any eventual solution will require convergence at the molecular level, because of the mosaic distribution of shared derived features identified in different studies. Some apparent conflicts may be attributable to polymorphism in common ancestors followed by differential lineage sorting. Despite remaining problems, some provisional conclusions are permissible. First, the superorder Euarchontoglires is uniformly supported by molecular studies, even though no strong morphological evidence favored inclusion of colugos. The prevalent interpretation among morphologists was that bats and colugos are sister groups 611, a conclusion resoundingly rejected by all molecular studies. Second, subdivision of Euarchontoglires into two monophyletic sister groups, Euarchonta and Glires, has generally received most support, but there have been several divergent findings. Tree-shrews have sometimes emerged as a basal offshoot in Euarchontoglires or have even been linked specifically to lagomorphs, thus disrupting the monophyly of Glires. Third, a link between colugos and primates within Euarchontoglires has frequently emerged from molecular studies 20, whereas inferred relationships between tree-shrews and colugos or primates have been less consistent and far more variable.

A general drawback in many studies has been inclusion of only a single colugo (usually the Philippine rather than the Malayan species). Taking a single representative for an isolated mammalian group can generate misleading results because of long-branch attraction. The study by Janecka et al. 9 laudably included both Cynocephalus andGaleopterus. One immediate benefit of this was demonstration of a surprisingly deep divergence between the two colugos, indicated at approximately 20 million years ago. This not only bolstered the validity of generic separation but also alleviated the effects of long-branch attraction.

Whatever the eventual outcome, colugos must clearly be considered in future discussions of primate evolution. Morphological comparisons between colugos, tree-shrews and primates, rare in the past, are now mandatory. Consideration of colugos will doubtless throw new light on key issues. To take just one example, the brain of colugos is unusually small relative to body size and morphologically very primitive 17. If colugos are close relatives of primates and/or tree-shrews, this means that any advanced features in tree-shrews and primates are almost certainly convergent, confirming one past interpretation 17. As both Nie et al. 8 and Janecka et al. 9 noted, proper understanding of morphological and genomic evolution of primates requires identification of the sister group, and colugos (with or without tree-shrews) are definite candidates. For this reason, determination of a draft genome sequence for colugo should certainly be a high priority 37.