Treeshrew is a kind of small mammals native to the tropical regions of Southeast Asia and South Asia (Figure 1). Since trees hrews and squirrels share many common morphological features and behavioral characteristics, Tupaiia, the genus name for tree shrew, is derived from tupai which is a Malay word used by the Malaysian for both tree shrews and squirrels (Martin, 1990). All trees hrews have some common features including relatively small body mass, generally omnivorous (eating arthropods and fruits), and a skeleton which has an unspecialized placental mammalian pattern, all digits have claws, and the hands and feet are not prehensile (Martin, 1990). For nearly a century (since 1922), the taxonomic status of tree shrews has been unsolved (Young, 2011).

1 Do tree shrews belong to insectivore?
Some scholars believe that tree shrews should be classified into the Insectivora order according to characteristics of tree shrew’s morphology (longer snout, digits (toe) being claw, the number of teeth) and features of feeding habits similar to insectivors. The order Insectivora is made up of elephant-shrews, shrews, hedgehogs, tenrecs, moles, and possibly tree-shrews. Also it was suggested that the order Insectivora should be divided into the suborders Menotyphla (elephant-shrews and tree-shrews) and Lipotyphla (shrews, hedgehogs, tenrecs, and moles) (Haeckel, 1866; Weber, 1928; Martin, 1990). It has been noted that, compared to other members of the Insectivores, tree shrews have very primitive characteristics of placental mammal (Carlsson, 1922; cited in Martin, 1990), so tree shrews were recognized as a senior insectivores in rank (Martin, 1990). Insectivora was later found to be a multiple phylogenetic clade, Its members have become independent or been placed in other orders one after another, and eventually the order Insectivora was canceled (Hutterer et al., 2005; Young, 2011).

2 Do tree shrews belong to primate?
Some scholars suggested that tree shrews should be classified into suborder Prosimii under Primates based on skull characteristics, such as the well-developed brain, smaller area of the olfactory nerve in the brain, the orbital bone bridge and formation of bone orbit, as well as some similar characteristics shared by primitive prosimians, such as the middle ear structure similar to lemurs, thumb (toe) separated from other digits (toe and teeth having premolars).

3 The unresolved controversy: Phylogenetic status
There has been a very big controversy regarding whether tree shrews should be classified as Insectivora, Primates, or a standalone order Scandentia in between (Young, 2011). In fact, tree shrews are the primitive type of placental mammals, which share common characteristics with primates (Martin, 1990). Tree shrews and primates have many shared features and characteristics listed in Table1, thus, tree shrews were classified into Primata. However, tree shrews lack some common characteristics shared in primates (Martin, 1990), and some characteristics of the primate animals derived from the Primate; some features are unique to tree shrews, which are not found in any other mammals (Martin, 1990) (Table 2). Thus, it can’t be said with certainty that tree shrews are primate animals (Martin, 1990).

Tree shrews were moved from Insectivora to the Primates order, because of certain internal similarities to the latter (for example, similarities in the brain anatomy, highlighted by Sir Wilfred Le Gros Clark), and classified as a primitive prosimian.

However, mitochondrial genome analysis showed that tree shrews had a closer relationship to Lagomorpha than to Dermoptera or Primates (Xu et al., 2012). Molecular cytogenetic data supported a Scandentia-Dermoptera sister clade (Nie et al., 2008; Hallstrom and Janke, 2010). A recent study by Hallstrom and Janke (2010) grouped tree shrews into Glires (including rodents and lagomorphs) based on the 3 000 gene phylogenetic analysis, suggesting that the tree shrews had a closer genetic relationship with mouse or rabbit. However, the evidence with several nuclear genes showed that treeshrews had a closer genetic relationship with Primates including human (Janečka et al., 2007, Lindblad-Toh et al., 2011). These latest molecular phylogenetic studies mentioned above strongly suggested that the tree shrews should be given the same order status as the primates, and should be placed in clade Euarchonta together with primates and flying lemurs class (colugos). According to this classification, Euarchonta are sister to the Glires (Lagomorphs and rodents), which together make up of the clade Euarchontoglires (Bartolomucci et al., 2002).

4 Tree shrews are much closer in relation to primates
The completion of the genome sequencing of the northern treeshrew (Tupaia belangeri chinensis) provided evidence at the molecular level for settling the long-standing disputes on the classification of tree shrews. Based on whole genome data of Chinese tree shrew and comparison with 14 species including 6 primate species, Fan et al (2013) analyzed the 2117 single-copy genes to demonstrate the tree shrews was first clustered with primate species. All phylogenetic classification signals were supported with high bootstrap, including coding sequences with all codon positions and peptide sequence (Figure 2) (Fan et al., 2013).
 

Furthermore, Fan et al (2013) estimated the divergence time among these 15 mammalian genomes (Figure 2; Fan et al., 2013). The results indicated that the tree shrews might have diverged from the clade encompassing the six primate species dating back about 90.9 million years ago, whereas the rodent clade around 96.4 million years ago. The close genetic relationship of tree shrews to non-human primates was mostly demonstrated by the clustering pattern in the phylogenetic tree and relatively shorter divergence time (Fan et al., 2013).

5 Conclusions
The evidence of whole genomic sequencing data basically quelled an ongoing controversy regarding the phylogenetic placement of tree shrews. The tree shrews are relative primitive small mammals in the Order Scandentia in Euarchontoglires, with a close genetic relationship to primates.

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