Scientists have thought for a long time that tree shrews are the closest living relatives of primates. More recently, use of DNA data together with morphological comparisons suggested that colugos are also very closely related to apes, monkeys (& us). These so-called 'flying' lemurs use extensive flaps of loose skin, stretched between their outspread front and back legs, to glide from tree to tree. A paper recently published in Science has attempted to sort out the relationship between these three groups.
Jan Janecka & his colleagues (2007) compared data on the genomes of colugos, treeshrews, and primates. They looked at 'indels' (insertions & deletions) and found that colugos and primates had 7 indels in common, while only one linked primates and treeshrews. This finding was supported by a phylogenetic tree using nuclear gene sequences. They were also able to use the data to estimate the time when the three groups diverged. It seems that colugos, treeshrews and primates shared a common ancestor around 88 million years ago, with the primates-colugo group diverging from the treeshrew lineage about 2 million years later. What's needed now is more fossils, so that researchers can look at the morphological changes that accompanied this divergence.
J. Janecka et al. (2007) Molecular and genomic data identify the closest living relative of primates. Science 318: 792-794