Primate evolution traces back to small mammals that lived alongside the dinosaurs. Over millions of years, these early forms diversified, leading to the many species of primates alive today. Understanding this history requires focusing on the earliest members of major groups, which provide a window into the origins of modern forms. Basal anthropoids represent an important ancestral group, revealing the initial steps that separated our direct lineage from other primates. They form the evolutionary foundation for all living monkeys, apes, and humans.
Defining the Anthropoid Lineage
The term “anthropoid” refers to the infraorder Simiiformes, the group that includes all monkeys, apes, and humans, often called the “higher primates.” The word “basal” describes the earliest, most primitive, or stem members of this lineage. Basal anthropoids are fossils that mark the evolutionary split, distinguishing Simiiformes from earlier primates like the strepsirrhines (lemurs and lorises) and tarsiers.
This group represents the common ancestor of all later anthropoids. Key fossil examples include the tiny Eosimias from Asia and Biretia from Africa, both known mainly from dental and jaw fragments. The Eosimiiformes, a group containing Eosimias, are considered the most basal clade of Anthropoidea currently known.
Key Physical Characteristics
Basal anthropoids exhibited a mosaic of ancestral and newly evolved features. A significant difference is seen in the dentition, particularly the absence of the specialized “tooth comb” found in strepsirrhine primates. Their lower incisors were implanted nearly vertically, a feature consistent with later anthropoid forms.
The structure of the eyes and skull also began to show changes that foreshadowed later anthropoid anatomy. These primates displayed increased orbital convergence, meaning their eyes were more forward-facing, which improved binocular vision and depth perception. While crown anthropoids possess a complete bony wall behind the eye (a postorbital septum), the earliest basal forms likely had a bony ring, or postorbital bar. The trend toward a more enclosed orbit was already underway, protecting the eyes and supporting the visual system.
Postcranial elements, or bones below the head, in species like Eosimias show derived anatomical traits otherwise seen only in living and fossil anthropoids. These traits suggest an arboreal locomotion style. However, the small body size of early anthropoids, estimated around that of a mouse-lemur (approximately 75 grams for Eosimias), meant they retained many primitive features.
The Geographic and Temporal Context
Basal anthropoids appear in the fossil record during the Eocene epoch (roughly 56 to 34 million years ago). Undisputed fossil evidence for this group dates back to about 45 million years ago in Asia. The question of whether the anthropoid lineage first arose in Asia or Africa has been a long-standing debate in paleoprimatology.
The earliest fossils, such as Eosimias, were discovered in China, suggesting an Asian origin for the group. Slightly later, but still very early anthropoids, like Biretia and finds from the Fayum Depression in Egypt, are known from Africa. This dual presence led to the hypothesis of an Asian origin followed by a dispersal event into Africa during the Middle Eocene.
Fossils like Afrasia from Myanmar, which is similar to the later African genus Afrotarsius, provide strong evidence for this cross-continental migration. This dispersal was likely facilitated by land bridges or “rafting” across the narrow Tethys Seaway. This colonization of Africa by Asian anthropoids likely involved multiple waves of migration, bringing the stock that would later diversify across the African continent.
The Evolutionary Significance
Basal anthropoids represent the common ancestor of the entire Simiiformes infraorder. Studying the features of these basal forms provides a baseline for understanding which traits were present at the beginning of the lineage and how they were modified later.
The colonization of Africa by these early anthropoids set the stage for an evolutionary radiation. Once established, the lineage split into the two major groups of modern anthropoids: the Platyrrhines and the Catarrhines.
The Platyrrhines, or New World monkeys, later crossed the Atlantic to South America, while the Catarrhines remained in the Old World. The Catarrhine lineage further diversified into the Old World monkeys and the apes (Hominoidea), which includes humans. The evolution of traits like a larger brain and a complete postorbital septum can be traced back to the initial anatomical shifts seen in the basal anthropoids.