The quest to identify the creature at the divergence of the human and ape lineages is a focus of paleoanthropology. This involves understanding the last common ancestor (LCA), the single species that evolved into two or more distinct species. A common misconception is that humans evolved from the chimpanzees we see today. In reality, both humans and chimpanzees share a common ancestor that was a unique species, neither human nor chimp. This ancestral species embarked on two separate evolutionary journeys, one leading to modern humans and the other to our closest living relatives.
Identifying Our Closest Relatives
To pinpoint our place within the primate family, scientists use genetics. By comparing the DNA of humans with that of other primates, genetic evidence confirms that chimpanzees (Pan troglodytes) and their close relatives, bonobos (Pan paniscus), are our nearest living kin. The similarity in our genetic blueprint is significant; studies show that the DNA sequences that can be directly compared between humans and chimpanzees are nearly 99 percent identical. This high degree of genetic overlap is a definitive marker of our shared ancestry, and the small percentage of differing genetic code accounts for the distinctions in anatomy, behavior, and intellect.
The Evolutionary Timeline and Split
With genetics identifying our closest relatives, the next question is when our evolutionary paths diverged. To answer this, scientists employ a method known as the “molecular clock.” This technique relies on the understanding that genetic mutations accumulate in DNA at a relatively predictable rate over long periods. By comparing the number of genetic differences between two species, researchers can estimate how much time has passed since they last shared a common ancestor.
Applying the molecular clock to the human and chimpanzee genomes places the split between our lineages in the late Miocene epoch. Most estimates from genetic data converge on a timeframe between 6 and 8 million years ago. Some studies have suggested a slightly wider range, from as recent as 5 million to as far back as 9.3 million years ago. This divergence was not a singular event but a process where populations of the ancestral species became separated and ceased to interbreed, setting them on distinct evolutionary trajectories.
Reconstructing the Last Common Ancestor
While no fossil of the chimpanzee-human last common ancestor (CHLCA) has been discovered, scientists can create a hypothetical reconstruction of this creature by combining genetic data with the fossil record. This ancestor was likely an ape adapted to a mosaic environment of forests and woodlands. Its diet would have consisted mainly of fruit, a conclusion drawn from the tooth morphology of fossils from that period. In terms of size, there is some debate, with some research suggesting an animal similar in size to a modern gibbon, while other evidence points to a creature closer in mass to a chimpanzee.
A significant area of scientific discussion revolves around how this ancestor moved. For years, it was assumed the LCA was a “knuckle-walker,” similar to modern chimpanzees and gorillas. However, the anatomy of early hominins like Ardipithecus challenges this view, as they lack the specialized wrist and hand features associated with knuckle-walking. This has led to the hypothesis that the LCA may have had a more generalized form of locomotion and that knuckle-walking in chimps and gorillas evolved independently. The goal is to understand the starting point from which the unique characteristics of both lineages, such as human bipedalism and chimpanzee knuckle-walking, evolved.
The Search for Fossil Candidates
The physical evidence for human evolution comes from the fossil record, with several finds from Africa. Although no specimen has been confirmed as the last common ancestor, a few fossil species are considered strong candidates because they date to the period of our split from the chimpanzee lineage. These fossils provide tangible clues that inform the reconstructions of what the LCA might have been like.
One of the most notable candidates is Sahelanthropus tchadensis, discovered in Chad and dated to between 6 and 7 million years ago. Known primarily from a cranium nicknamed “Toumai,” this species displays a mixture of features. It has a chimpanzee-sized braincase but also a flattened face and smaller canine teeth, which are more similar to later hominins. The position of the foramen magnum, the opening where the spinal cord connects to the skull, suggests it may have held its head upright, a possible precursor to bipedalism.
Another significant fossil is Orrorin tugenensis, found in Kenya and dated to approximately 6 million years ago. The available fossils of Orrorin include fragments of the jaw, teeth, and, importantly, the femur, or thigh bone. The shape of the femur shares similarities with later bipedal hominins, suggesting it may have walked on two legs. While these fossils are compelling, the record is incomplete, and it remains unclear whether these species are direct human ancestors, part of the chimpanzee lineage, or members of an extinct side branch.