There once was a time when our planet was different from what we see today. It had large lush green forests during the Eocene, and ice covered landscape in the Holocene. And, these climatic and environmental shifts helped to expand, diversify and create new species of organisms, including us. By observing trends in the geological timescale and comparing them to the hominid fossil record, an evolutionary checklist emerges, showing us exactly how and why certain traits were favored over others, and how we as a species came to be.
It is during the Miocene epoch (23.03 to 5.33MYA) that we see the emergence of apes and eventually the earliest hominids. Known as the age of apes, the Miocene shows a rapid radiation of primates across Africa and Eurasia. This expansive diversification of primates, especially apes, has been attributed to the abundance of wet, forested environments. Many of the early Miocene primates such as Victoriapithcus and Paranthropus show adaptations for a more forested environment (5). These traits included longer more curved phalanges, an opposable hallux, muscle attachments indicative of arboreality, and a more inferiorly placed foramen magnum. However, as time moves forward we see the emergence of early hominins (Sahelanthropus & Orrorin) who start to show adaptations to more open habitats, indicating a possible shift in environment. These traits include less curvature of phalanges, a more anteriorly placed foramen magnum, a robust femoral neck and head, and a medial angle positioning of the femur. Additionally, during this time period, the emergence of the African Rift Valley occurs, to which paleoanthropologists and geologists alike attribute climate change and the emergence of hominids.
The African Rift Valley is an extremely large rift or gorge that divides equatorial Africa into Western and Eastern/Southern tip portions. Prior to the full development of the rift valley in the late Miocene, the African continent presented a homogeneous landscape and climate that spanned from the Atlantic to the Indian Ocean. The climate and landscape at that time was warm, wet and overall temperate with forested and woodland regions interlaced with many rivers and lakes. However, at about 8 million years, a large tectonic crisis occurred on the African continent. As the tectonic plates collided with one another, one plate sank causing the valley portion of the African rift, while the rising plate caused the formation of the western mountain ridge surrounding the African rift we see today (3). This new formation disrupted normal air currents causing a gradual shift in climate on the Eastern/ Southern tip of the valley. In the west region, however, the maintenance of regular air currents was upheld. The western air currents were still able to push cooler, wetter air from the Atlantic Ocean, which then formed clouds when mixed with the warmer currents over the continent, resulting in little climatic change. However, due to the ridge formation on the Eastern portion of the continent, the air currents from the Atlantic was unable to form east of the rift. This resulted in the air and overall climate becoming more arid and seasonal, eventually developing into the Monsoon system we see today. With a gradual increase in aridity, surrounding fauna and flora evolved to match the changing environment. It is assumed that at the time this was all occurring, the common ancestor of both Hominidae and Panidae spanned across the entire continent. Once the African Rift Valley was established, our common ancestor was found on both sides of the valley resulting in the split between Hominidae and Panidae. This would explain why we are genetically similar but removed geographically from our chimp cousins.
Typically, paleoanthropologists will divide hominid specimens into four separate groups: the possible hominids, the archaic/transitional hominins, the hyper-specialized hominins and early homo. Divisions into these groups are determined by the geological timeline in which the hominid resides and by morphological adaptations exhibited. The first of this grouping, possible hominids, emerge during the Late Miocene and are the first of the hominids. In total, there are three different genera associated with this group: Sahelanthropus, Orrorin, and Ardipithecus. Like most early hominins all three genera exhibit both chimpanzee and hominin traits. However, these genera show the largest amount of chimpanzee traits, making researchers question the exact relationship they have with later hominins (21). Additionally, these Late Miocene fossil remains are, unfortunately, quite limited and fragmented which further complicates researchers’ ability to determine the exact lineage they represent. While these fossils might be few and disjointed, similarities do emerge suggesting to paleoanthropologists that these specimens might be possible humans. As a result, they are loosely attached to the hominin lineage and, thus, called possible hominids.
There are currently two speculations as to why these three genera would exhibit large amounts of primitive/chimp-like traits: 1) the environment during the late Miocene was still a highly riparian, mosaic landscape (rivers bordered by forests, woodlands, etc.), and 2) due to the theorized split of Hominidae and Pangidae species around this time, early hominids would likely exhibit a larger mixture of ancestral and modern traits (21). By using the African Rift Valley as a model, both speculations seem plausible. Throughout the Miocene, the paleolandscape was densely forested and wet (12). However, by the Late Miocene, a gradual shift to a more open and cooler climate occurs. This shift in environment coincides with the emergence of the possible hominins, suggesting a viable connection between ecology and morphology. And, even though the environment was shifting toward becoming more open, there was still a relatively large abundance of forests. This, in combination with how long adaptation occurs, would explain why the possible hominids would have had an abundance of chimp-like traits.