Euprimates are the first true primates to appear in the fossil record, representing the foundational group from which all living primates, including humans, descend. They are often referred to as “primates of modern aspect” because they possess the defining anatomical features that characterize the entire primate order today. The emergence of these early ancestors marks a significant evolutionary leap, occurring around the beginning of the Eocene epoch approximately 55 million years ago. Euprimates replaced the earlier, more primitive primate-like mammals, the Plesiadapiforms, establishing the body plan that would dominate primate evolution.
Distinguishing Features of Euprimates
The defining characteristic that sets Euprimates apart from their ancestors is a suite of traits related to enhanced vision and arboreal locomotion. A skeletal feature known as the postorbital bar, a bony ring encircling the eye socket, provides protection and rigidity to the orbit. This bony structure stabilized the eye during chewing and muscle contraction, ensuring high visual acuity.
The eyes were positioned forward on the face, a condition known as orbital convergence, which allowed for overlapping visual fields. This arrangement resulted in stereoscopic vision, providing the depth perception necessary for navigating a three-dimensional environment like a forest canopy. This reliance on sight corresponded with a reduction in the size of the snout and the brain regions associated with the sense of smell.
Euprimates also exhibited specialized hands and feet adapted for grasping branches. Their digits possessed flattened nails instead of the claws found on most other mammals. Furthermore, they developed an opposable big toe (hallux) and sometimes an opposable thumb (pollex), allowing for a powerful, controlled grip.
The Paleogene Origin and Context
The appearance of Euprimates coincided with the Paleocene-Eocene Thermal Maximum (PETM), a profound climatic event occurring about 55.8 million years ago. This rapid event saw a massive influx of carbon, causing the global average temperature to rise by approximately 5 to 8 degrees Celsius and transforming the planet into a greenhouse world.
This intense warming fueled the expansion of tropical forests deep into the northern latitudes, creating vast, interconnected arboreal habitats across North America, Europe, and Asia. This newly available environment favored the small, agile, visually-oriented Euprimates, leading to their rapid diversification and dispersal. Euprimates were highly successful during this warm, humid Eocene epoch, which provided an ideal niche for their arboreal lifestyle.
The Adapiform and Omomyiform Lineages
The Euprimates quickly diversified into two major, distinct groups: the Adapiforms and the Omomyiforms, which dominated early primate faunas. These two lineages represent the earliest manifestations of the diversity seen in modern primates, each possessing a unique combination of traits.
Adapiforms were generally larger primates, with some genera like Leptadapis weighing up to 9 kilograms, making them among the largest Eocene primates. Their relatively small eye sockets suggest they were primarily diurnal, or active during the day. Skeletal evidence from genera such as Notharctus, found in North America, indicates they had lemur-like body proportions and were capable of vertical clinging and leaping. Most Adapiforms had teeth suggesting a diet focused on leaves and fruit, and many are considered morphologically similar to modern Strepsirhines, such as lemurs and lorises.
In contrast, the Omomyiforms were typically much smaller, with some species being diminutive in size. They often possessed large eye orbits, which suggests they were primarily nocturnal, or active at night. Their teeth were generally suited for a diet of insects and softer fruits.
One of the oldest and most widespread Omomyiform genera is Teilhardina, found almost simultaneously across North America, Europe, and Asia at the beginning of the Eocene. Omomyiforms are generally viewed as having morphological similarities to modern Tarsiers.
Evolutionary Significance for Modern Primates
The legacy of the Euprimates is that they established the fundamental body plan for the entire primate order. Although both Adapiforms and Omomyiforms eventually went extinct, they represent the common ancestral stock for the two major subdivisions of living primates.
The Adapiforms are linked to the evolutionary origin of the Strepsirhine lineage (lemurs and lorises). The Omomyiforms are thought to be ancestral to the Haplorhine lineage, which includes tarsiers, monkeys, apes, and ultimately, humans.