The term “simian,” or anthropoid, refers to a taxonomic grouping of primates that includes all monkeys and apes, with humans being a part of the ape lineage. Simians are classified under the infraorder Simiiformes, representing the “higher primates” within the order Primates. This group encompasses the vast majority of primate species, distinguishing them from the “lower primates,” such as lemurs and lorises. Simians diverged from the prosimian line approximately 60 million years ago, setting the stage for the development of the complex physical and cognitive traits seen today.
Defining Characteristics of Simians
Simians share advanced physical characteristics that distinguish them from other primates. They belong to the suborder Haplorhini, characterized by a “dry nose” and reduced reliance on the sense of smell compared to prosimian relatives. Simians rely heavily on vision, featuring forward-facing eyes with a fully enclosed bony socket, which allows for acute stereoscopic and color vision. This visual acuity is fundamental to their diurnal, or daytime, lifestyle.
Another defining trait is the relatively large brain size in proportion to their body mass. Their dental structure is derived, featuring a fused mandible and a general reduction in the number of teeth compared to ancestral primates. Female simians characteristically possess a single fused uterus, a condition rare among mammals.
The Major Divisions: Monkeys and Apes
The infraorder Simiiformes is divided into two major parvorders based on geographical distribution and physical traits: Platyrrhini and Catarrhini.
Platyrrhines, commonly known as New World monkeys, are found exclusively in the Americas. They possess a broad, flat nose with sideways-facing nostrils and generally have three premolars in each jaw quadrant. Many species, such as spider and howler monkeys, have prehensile tails that function as a fifth limb for grasping and climbing.
Catarrhines, which include Old World monkeys and apes, are native to Africa and Asia. They are characterized by a narrow nose with downward-facing nostrils and feature only two premolars per jaw quadrant. This group exhibits greater sexual dimorphism in body size and canine tooth size. The Catarrhini parvorder is split into Old World monkeys (Cercopithecoidea) and the apes (Hominoidea), which includes humans.
The primary difference between monkeys and apes lies in their skeletal structure and the presence of a tail. All true apes (gibbons, gorillas, and humans) lack a tail due to a mutation in the TBXT gene, while Old World monkeys possess non-prehensile tails. Apes also have a shallower, ball-and-socket shoulder joint and a fully extendable elbow, allowing for a greater range of motion necessary for suspension and brachiation. Monkeys, conversely, have a narrower shoulder socket and a more stable, L-shaped elbow joint suited for quadrupedal movement across branches.
Intelligence and Social Structure
Simians exhibit highly developed cognitive abilities and complex social organization. The social brain hypothesis suggests that navigating intricate social relationships, such as forming alliances and managing dominance hierarchies, drove the evolution of larger brains. This specialization is reflected in their capacity for rapid learning in social contexts, supported by enhanced perceptual and memory efficiency.
Advanced cognition is evidenced by documented instances of tool use across various species. Wild chimpanzees, for instance, use modified sticks to “fish” for termites and employ stones as hammers and anvils to crack nuts. Capuchin monkeys also use sticks to retrieve inaccessible food rewards. These learned behaviors are often passed down, forming local traditions that resemble cultural transmission.
Communication relies on a combination of vocalizations, gestures, and body postures. Apes, such as chimpanzees and orangutans, frequently utilize intentional gestural communication, sometimes incorporating objects to convey a message. Regular social grooming is a widespread behavior that reinforces social cohesion and reduces tension within the group.