The scientific consensus states that humans and modern apes, specifically chimpanzees and bonobos, share a common ancestor. This article explores the nature of this ancestor, its timeline, and the evidence supporting its existence.
Identifying the Last Common Ancestor
There is no single fossil specimen universally recognized as the last common ancestor (LCA) of humans and chimpanzees. The concept of a “missing link” is considered outdated and misleading in scientific circles. Instead, scientists envision a population of ancient apes that existed before the human and chimpanzee lineages diverged. This divergence is estimated to have occurred during the late Miocene epoch, approximately 6 to 8 million years ago.
Several fossil finds are considered potential close relatives or members of this ancestral population. Sahelanthropus tchadensis, discovered in Chad, lived around 7 to 6 million years ago and exhibits a mix of ape-like and human-like features in its cranium, including small canine teeth and an opening for the spinal cord positioned to suggest upright posture. Graecopithecus freybergi, found in Greece and Bulgaria and dated to about 7.2 million years ago, has also been proposed as a possible early hominin, though its classification remains debated due to limited and poorly preserved material.
A Portrait of Our Ancestor
Reconstructing the physical and behavioral characteristics of the last common ancestor relies on analyzing existing fossil evidence and comparing the anatomy of living apes and humans. This ancestor was likely a creature with a blend of features seen in both modern apes and early human relatives. Its size was probably similar to a modern chimpanzee, and it possessed a relatively small, ape-sized brain. Its diet likely consisted of fruits and leaves, similar to many modern primates.
The LCA was likely a quadruped, moving on all four limbs. Evidence suggests it was well-adapted for life in the trees, capable of skilled climbing and some suspensory behavior, where it could hang from branches using its arms. It may have also engaged in some knuckle-walking on the ground, similar to modern gorillas and chimpanzees. The environment it inhabited was probably African woodlands and forests, which were undergoing changes due to global climate shifts during the late Miocene.
The Great Divergence
Scientists use the “molecular clock” method to estimate when the human and chimpanzee lineages diverged. This method compares the rate of DNA mutations between species to calculate the time elapsed since their last common ancestor. Based on this genetic evidence, the split is estimated to have occurred approximately 5 to 8 million years ago, a range generally consistent with findings from the fossil record.
Major climate change in Africa during the late Miocene epoch is considered a significant environmental trigger for this divergence. This period saw a global cooling trend, leading to increased aridification and a reduction in dense forests, with an expansion of more open woodlands and savannas. These shifting environments created different selective pressures on the ancestral ape population. One group remained adapted to the dwindling forest habitats, eventually leading to modern chimpanzees and bonobos, while another lineage began adapting to the more open ground, favoring the evolution of bipedalism, or walking on two legs.
Our Shared Genetic Blueprint
The genetic evidence provides strong confirmation of the shared ancestry between humans and chimpanzees. Humans share approximately 98.8 percent of their DNA with chimpanzees. This similarity underscores the close evolutionary relationship between the two species.
A clear example of this shared ancestry is found in human chromosome 2. While humans have 23 pairs of chromosomes, chimpanzees, gorillas, and orangutans all have 24 pairs. Human chromosome 2 is the result of a fusion of two smaller ancestral chromosomes that remained separate in other primate lineages. Evidence for this fusion includes the presence of remnants of a second centromere and telomeric sequences in the middle of human chromosome 2, which are typically found only at the ends of chromosomes. This chromosomal rearrangement serves as direct genetic evidence of a common ancestor.