地球和人類是如何出現?

Modern understanding of human origins is derived largely from the findings of paleontology, anthropology, and genetics, and involves the process of natural selection. Although gaps in the fossil record due to differential preservation prevent the complete specification of the line of human descent, H. sapiens share clear anatomical, genetic, and historic relationships to other primates. Of all primates, humans bear particularly close affinity to other members of a group known as hominoids, or apes, which includes orangutans, gibbons, gorillas, chimpanzees, and humans. Humans and their immediate ancestors, known as hominids, are notable among hominoids for their bipedal locomotion, slow rate of maturation, large brain size, and, at least among the more recent hominids, the development of a relatively sophisticated capacity for language, tool use, and social activity.
Humans are mammals of the Primate order. The earliest primates evolved about 65 million yrs ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrews. Primates of the Eocene epoch (55 to 38 M yrs ago) were similar and ancestral to contemporary tarsiers, lemurs, and tree shrews, and are classified as lower primates or prosimians. During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental drift, diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys. The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 M yrs ago) or early Miocene (25 to 8 M years ago), a time period poorly represented in the fossil record. The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 M yrs ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 M yrs ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 M yrs ago and that chimpanzees and hominids diverged about 5 M yrs ago.
The earliest known hominids are members of the genus Australopithecus, the earliest of which date to more than 4 M yrs ago. Unlike other primates, but like all hominids, australopithecines were bipedal. Their crania, however, were small and apelike, with an average cranial capacity of about 450 cc in the gracile species and 600 cc in the robust forms. Australopithecines that have been considered ancestral in the lineage leading to the human genus Homo include A. afarensis (an important skeleton of which is popularly known as Lucy) and A. africanus. The exact position of these and other early species on the hominid family tree continues to be disputed.
The first member of the genus Homo, a small gracile species known as H. habilis, was present in east Africa at least 2 M yrs ago. H. habilis was the first hominid to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominid evolutionary history. By about 1.6 M yrs ago, H. habilis had evolved into a larger, more robust, and larger-brained species known as Homo erectus. Cranial capacities ranged from about 900 cc in early specimens to 1050 cc in later ones. H. erectus persisted for well over a million years and migrated off the African continent into Asia, Indonesia, and Europe .
Between 500k and 250k yrs ago, H. erectus evolved into H. sapiens. Transitional forms between H. erectus and H. sapiens are referred to as archaic H. sapiens. With the exception of H. sapiens neandertalensis (Neanderthal man), no additional subspecies are recognized. Indeed, some scientists consider Neanderthal a separate species. Archaic H. sapiens changed gradually, becoming somewhat larger, more gracile and larger-brained through time.
I hope this can help with your understanding. =)

來　自　石　頭