The Primates are an ancient and diverse eutherian group, with
around 233 living species placed in 13 families. Most dwell in
tropical forests. The smallest living primate is the pygmy mouse
lemur, which weighs around 30 g. The largest is the gorilla,
weighing up to around 175 kg. Primates radiated in arboreal
habitats, and many of the characteristics by which we recognize
them today ( shortened rostrum and forwardly directed orbits,
associated with stereoscopic vision; relatively large braincase;
opposable hallux and pollex; unfused and highly mobile radius
and ulna in the forelimb and tibia and fibula in the hind)
probably arose as adaptations for life in the trees or are
primitive traits that were retained for the same reason. Several
species, including our own, have left the trees for life on the
ground; nevertheless, we retain many of these features. Primates
are usually recognized based on a suite of primitive
characteristics of the skull, teeth, and limbs. Some of these
are listed above, including the s