Human Evolution

D

Late Homo

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.

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1

Neandertals

The Neandertals lived in areas ranging from western Europe through central Asia from about 200,000 to about 28,000 years ago. The name Neandertal (sometimes spelled Neanderthal) comes from fossils found in 1856 in the Feldhofer Cave of the Neander Valley in Germany (tal—a modern form of thal—means “valley” in German). Scientists realized several years later that prior discoveries—at Engis, Belgium, in 1829 and at Forbes Quarry, Gibraltar, in 1848—also represented Neandertals. These two earlier discoveries were the first early human fossils ever found.

In the past, scientists claimed that Neandertals differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neandertal skeleton that showed it with bent knees and a slouching gait. This reconstruction gave the common but mistaken impression that Neandertals were dim-witted brutes who lived a crude lifestyle.

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On the contrary, Neandertals, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at about 1,500 cu cm (about 90 cu in), slightly larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neandertals as compared with modern humans, which usually correlates with a larger brain size.)

Compared with earlier humans, Neandertals had a high degree of cultural sophistication. They appear to have performed symbolic rituals, such as the burial of their dead. Neandertal fossils—including a number of fairly complete skeletons—are quite common compared to those of earlier forms of Homo, in part because of the Neandertal practice of intentional burial. Neandertals also produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made.

Along with many physical similarities, Neandertals differed from modern humans in several ways. The typical Neandertal skull had a low forehead, a large nasal area (suggesting a large nose), a forward-projecting nasal and cheek region, a prominent brow ridge with a bony arch over each eye, a nonprojecting chin, and an obvious space behind the third molar (in front of the upward turn of the lower jaw).

Neandertals also had a more heavily built and large-boned skeleton than do modern humans. Other Neandertal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones relative to the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature—males averaged 1.7 m (5 ft 5 in) tall and 84 kg (185 lb), and females averaged 1.5 m (5 ft) tall and 80 kg (176 lb).

The short, stocky build of Neandertals conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neandertal fossils come from western Europe and date from approximately 28,000 years ago.

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Other Late Homo Populations

At the same time as Neandertal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neandertals, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a somewhat higher forehead and flatter midface. Fossils of humans from East African sites older than 100,000 years—such as Ngaloba in Tanzania and Eliye Springs in Kenya—also seem to show a mixture of archaic and modern traits.

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.

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Anatomically Modern Homo sapiens

The oldest known fossils that possess skeletal features typical of modern humans date from 195,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only slightly projecting face of reduced size, located under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neandertals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu cm (60 to 120 cu in), with the average being about 1,350 cu cm (80 cu in).

Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa; and Skhūl Cave, Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 195,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.

E

Theories of Modern Human Origins and Diversity

Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction.

Those who think modern humans originated only in Africa and then spread around the world support what is known as the out of Africa hypothesis. Those who think modern humans evolved over a large region of Eurasia and Africa support the so-called multiregional hypothesis.

Researchers have conducted many genetic studies and carefully assessed fossils to determine which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The question raised is whether the physical differences among modern humans evolved deep in the past or relatively recently.

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