Human Evolution

Abstract

The study of human evolution involves (1) understanding the evolutionary context and the circumstances surrounding the origin of the branch of the Tree of Life, technically referred to as a clade, whose only extant taxon is modern humans; (2) recognizing the extinct species that are more closely related to modern humans than to the closest living apes (i.e., chimpanzees and bonobos); (3) reconstructing the morphology and behavior of those species; (4) determining how they are related to each other and to modern humans; (5) investigating the factors and influences that shaped their evolution; and (6) reconstructing the origin(s) of modern human anatomy and behavior. The study of the fossil evidence for human evolution is traditionally referred to as hominid paleontology, which reflected the then-prevailing conventional wisdom that the differences between modern humans and the great apes were profound enough to merit being recognized at the level of the family, with modern humans in the family Hominidae (a.k.a. hominids), and the great apes in the family Pongidae (a.k.a. pongids). But the molecular evidence is consistent with a particularly close relationship between Homo sapiens (the formal Linnaean name for modern humans) and two of the great apes, chimpanzees and bonobos. In the light of this compelling evidence, many researchers use the tribe (the taxonomic category below the level of the family and above the level of the genus) Hominini to accommodate the species and genera more closely related to modern humans than to chimpanzees and bonobos. So, in the new terminology the study of the human fossil record should be referred to as hominin paleontology. The study of the artifacts (e.g., stone and bone tools, drawn and carved images, early structures, evidence of decoration, etc.) made in prehistoric times is called prehistoric archaeology. In the United States of America, the combined study of hominin paleontology and prehistoric archaeology is called paleoanthropology, human prehistory, or just prehistory—this article focuses on hominin paleontology. The data available for reconstructing human evolutionary history come from molecules (from DNA and proteins), the phenotype, from true fossils (i.e., bones and teeth), and from trace fossils (i.e., footprint trails and natural endocasts). Genetic data include information about modern-human genetic variation that allows researchers to reconstruct the relatively recent migration of modern humans, plus ancient DNA that so far has been recovered from modern humans, Neanderthals, Denisovans, and the fossils from a 430-thousand-year-old site in Spain called the Sima de los Huesos. Phenotypic evidence, which is divided into macroscopic, microscopic, and molecular, can be gathered from the surface of true fossils (i.e., bones and teeth) as well as from their internal structure and their proteome. The internal structure of true fossils can be accessed nondestructively by using imaging techniques or destructively by making sections of bones and teeth. The trace fossils that are most relevant for human evolution are footprints such as the c. 3.6-million-year-old hominin footprint trails from Laetoli in Tanzania, and natural endocasts that provide information about the size and shape of the brain.