Fossils Human Evolution

Human Evolution, lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people evolved over a period of at least 6 million years.
A. From Ape To Man
Fossils from several different early australopith species that lived between 4 million and 2 million years ago clearly show a variety of adaptations that mark the transition from ape to human. The very early period of this transition, prior to 4 million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early australopiths, but
not everything about outward physical features such as the color and texture of skin and hair, or about certain behaviors, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes—particularly the African apes—to better understand how early australopiths might have looked and behaved, and how the transition from ape to human might have occurred.
For example, australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees. B. Australopethicus Afarensis

Australopithecus afarensis, one of the earliest human species, lived between 4 million and 3 million years ago.
This skull cast is a composite of bone fragments from different individuals of this species. The lighter portions represent bone fragments, and the missing pieces are filled in to reveal what an entire skull probably looked like.
One of the most complete specimens of A. afarensis found so far was announced in 2006. A team led by Ethiopian scientist Zeresenay Alemseged unearthed the partial skeleton of a three-year-old female at Dikika in the Afar region of Ethiopia. Nicknamed “Selam,” the Dikika child dates from around 3.3 million years ago. The well-preserved bones provide previously undocumented details of the skull and skeleton. Some features such as the shape of the shoulder blades, the long, curved fingers, and the semicircular ear canals involved in balance are more apelike, suggesting an adaptation for climbing trees. However, the leg bones and feet indicate an ability to walk upright even at an early age. The shape of the brain was preserved and its size indicates the species grew to adulthood more slowly than chimpanzees, a characteristic of later hominids, including modern humans. The hyoid bone that supports the tongue was found, as well. The bone is crucial to speech in modern humans but the shape in the Dikika child is like that found in modern great apes, and not
humans.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally.
Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centers on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of modern humans. This observation may indicate that early australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also indicates that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees.

Another controversy has to do with the scientific classification of the A. afarensis fossils. Compared with Lucy, who stood only 1.1 m (3.5 ft) tall, other fossils identified as A. afarensis from Hadar and Laetoli came from individuals who stood up to 1.5 m (5 ft) tall. This great difference in size leads some scientists to suggest that the entire set of fossils now classified as A. afarensis actually represents two species. Most scientists, however, believe the fossils represent one highly dimorphic species—that is, a species that has two distinct forms (in this case, two sizes). Supporters of this view note that both large (presumably male) and small (presumably female) adults occur together in one site at Hadar.

A third controversy arises from the claim that A. afarensis was the common ancestor of both later australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another australopith species—one from southern Africa, named Australopithecus africanus—makes it difficult to decide which of the two species gave rise to the genus Homo. C. Homo Sapiens
The origin of our own species, Homo sapiens, is one of the most hotly debated topics in paleoanthropology. This debate centers on whether or not modern humans have a direct relationship to H. erectus or to the Neandertals, a well-known, more modern group of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors.
Traditionally, paleoanthropologists classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus use the name archaic Homo sapiens to refer to a wide variety of fossil humans that predate anatomically modern H. sapiens. The term archaic denotes a set of physical features typical of Neandertals and other species of late Homo prior to modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincase (positioned somewhat behind, rather than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neandertals are sometimes classified as a subspecies of archaic H. sapiens—H. sapiens neanderthalensis. Other scientists think that the variation in archaic fossils actually falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neandertals belong to their own species, H. neanderthalensis.
D. Homo Erectus

Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugène Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or “erect ape-man.” Today paleoanthropologists refer to this species as Homo erectus
H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cu cm (50 to 80 cu in), an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the australopiths, it had a heavy and strong jaw.
Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus—such as those from the Solo River in Java—may have lived as recently as 53,000 to 27,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens.

E. Homo Habilis
Between 1960 and 1963, at Olduvai Gorge, Tanzania, a team led by Louis and Mary Leakey discovered the remains of an early human that seemed distinctly different from the australopiths. In 1964 Louis Leakey, South African paleoanthropologist Philip Tobias, and British primate researcher John Napier concluded that these remains represented a new species, which they named Homo habilis. The scientists placed the species in the genus Homo because its brain was estimated to be significantly larger than that of any known australopith. Other scientists questioned whether the amount of brain enlargement was sufficient for inclusion of the species in Homo, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team. According to recent estimates, H. habilis had a brain volume that ranged from 590 to 690 cu cm (36 to 42 cu in), well above the range for australopithecines.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths.
A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils—exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.

F. Homo Floresiensis
One of the most unusual branches of the human family tree was discovered on the Indonesian island of Flores in 2003 and first described in 2004. A research team digging in a cave, Liang Bua, uncovered the nearly complete skeleton of what appeared to be a miniature human that lived as recently as 18,000 years ago. The specimen, believed to be an adult female, was estimated to stand only about 1 m (3.3 ft) tall. Its brain, estimated at 380 cu cm (23 cu in), was as small as those of chimpanzees and the smallest australopiths. It had fairly large brow ridges, and its teeth were large relative to the rest of the skull. Despite being extremely small-brained, it apparently made simple stone tools. On the basis of these unique traits, the researchers assigned the skeleton to a new species, Homo floresiensis. The researchers concluded that H. floresiensis was probably descended from H. erectus, although this continues to be debated. The diminutive stature and small brain of H. floresiensis may have resulted from island dwarfism—an evolutionary process that results from long-term isolation on a small island with limited food resources and a lack of predators. Pygmy elephants on Flores, now extinct, showed the same adaptation.

AUSTRALOPETHICINES AFARENSIS

HOMO ERECTUS

Modern and Early Humans
Humans have undergone major anatomical changes over the course of evolution.
This illustration depicts Australopithecus afarensis (center), the earliest of the three species; Homo erectus (left), an intermediate species; and Homo sapiens (right), a modern human. H. erectus and modern humans are much taller than A. afarensis and have flatter faces and much larger brains. Modern humans have a larger brain than H. erectus and an almost flat face beneath the front of the braincase.

Neandertal Bones
A Neandertal skull, top right, and several bones were found at the La-Chapelle-aux-Saints rock shelter in southwestern France in 1908. Another Neandertal skull, bottom right, was found at the nearby La Ferrassie site in 1909. The remains found at La-Chapelle-aux-Saints, marked by arthritis and disease, did much to reinforce a conception of the Neandertal as a slouching, degenerate human form. Scientists now believe Neandertals were a strongly built and intelligent species that thrived in Europe for more than 150,000 years. HOMO HABILIS

Miniature Human Species
Australian researchers stand with a life-size illustration of a miniature human species that lived on the Indonesian island of Flores until at least 18,000 years ago. The species, Homo floresiensis, stood only about 1 m (3.3 ft) tall and had a tiny brain, yet it was intelligent enough to make stone tools. The illustration shows a male who has successfully hunted a giant
rodent.