In the 150 years since Darwin described the origin of species, scientists have gathered extensive fossil evidence in support of his theory of evolution by natural selection. This evidence helps them understand how humans have evolved from a common ancestry with apes.

Many scientists focus on locomotion, diet, and cranial capacity as key characteristics to understand human evolution. For example, bipedalism, or walking on two feet, is used to separate the earliest human ancestors from the ancestors of apes. In bipeds, the foramen magnum—the large opening through which the spinal cord enters the skull—is located near the middle of the skull. In quadrupeds, or animals that walk on four feet, it is located toward the back of the skull. Likewise, the shape and position of the pelvis (hip), femur (leg), and tibia (shinbone) distinguish bipeds from quadrupeds, as do the relative lengths of their lower limbs and upper limbs.

Comparing fossilized skull characteristics provides clues about diet that help anthropologists classify species of hominins, the term used for humans and human ancestors. For instance, the larger crests, longer faces, and narrowing of the skull behind the eye sockets of modern apes and early hominins help support the powerful jaw muscles required to consume hard diets. Scientists assert that the reduction in these features that came with softer diets may have allowed for the development of larger brains in more modern hominins.

Scientists often interpret such facts by classifying similar fossils into groups with similar characteristics, thus defining genera and species that represent key stages in the evolution of human ancestors. By doing this, they give structure and meaning to fossils. However, species classification can be difficult. Many gaps in the fossil record remain and new fossil discoveries challenge scientists to revise their classifications. To complicate the process further, while dates of fossils reveal when these human ancestors existed, it is not possible to determine when they first appeared and when they went extinct or evolved into other species. For example, there is no clear date to define the transition from Homo erectus to Homo sapiens, or whether H. erectus even gave rise to H. sapiens.

Therefore, while defining species is essential for understanding evolutionary trends, classifying fossils involves constant reorganizing of data as new techniques and findings emerge. Many questions about our human evolutionary tree remain unanswered. Although facts associated with each fossil may be clear and general facts of human evolution are well established, the process of classifying fossils based on those facts is one of scientific interpretation, leaving the specifics of human evolution up for continued debate.

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