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HUMAN EVOLUTION

HUMAN EVOLUTION Understandably, there is great interest in our own branch of the evolutionary tree, and the study of human evolution is a vibrant and exciting field. Our understanding of human phylogeny has historically been based primarily on the fossil record, but more recently, molecular biology and DNA analysis have played increasingly important roles. Even so, reconstruction of human phylogeny is often based on incomplete evidence. Consequently, our views of human phylogeny are frequently being revised as more evidence comes to light. Figure 8.1 shows a phylogeny of humans that includes the genera and species discussed in this chapter. Temporal ranges given for periods when human ancestors lived are, of necessity, rough estimates. Figure 8.1: A phylogeny of humans, focusing on groups discussed in this chapter. (Modified from http://www.amnh.org/education/resources/rfl/web/hhoguide/family-tree.html; Attribution: Shawn Meagher, Western Illinois University) Based on available evidence, H. sapiens (“wise man”) arose about 200,000 years ago in Africa. Homo sapiens is the most recent in a long lineage that may have begun around 7 MYA ago with Sahelanthropus tchadensis, possibly the common ancestor of humans and the genus Pan (chimpanzees and bonobos), although this is far from settled. In S. tchadensis the foramen magnum (the opening of the skull where the spinal cord exits) is located underneath the skull, indicative of bipedal locomotion, although this interpretation is under dispute. In apes, this opening is located more posteriorly, reflecting a more quadrupedal locomotion. fte Genus Homo is in only genus in the subtribe Hominina of the tribe Hominini, which, in addition, includes the subtribes Australopithicina (Australopithecus and closely related genera) and Panina (with the one genus, Pan). fte Hominini that arose after separation from the chimpanzees (i.e., the subtribes Hominina and Australopithicina) are collectively referred to as hominins, or the “human clade.” Orrorin tugenensis is a contender along with S. tchadensis for the title of earliest human. Orrorin tugenensis had microdont dentition (small teeth) reminiscent of humans, but with canines that are ape-like, but reduced. Specimens of this species have been dated from around 6 MYA. Orrorin tugenensis was probably a tree-climber, but also perhaps bipedal to some extent. If O. tugenensis is indeed an early part of the human lineage, then the Australopithecina, which occurred later, were side branches from this main human line. But again, this is under debate. Genera of Australopithecina include Ardipithecus (6.0 to 4.4 MYA), Australopithecus (4 to 2 MYA), and Paranthropus (2.7 to 1.2 MYA). fte best known species in the genus Ardipithecus, A. ramidus, was about 1.2 m tall. Based on pelvis and limb structure, Ardipithecus may have been facultatively bipedal, that is, bipedal when moving along the ground, but using all four limbs when moving in the trees. Its bipedal locomotion was primitive and inefficient compared to later hominins, and its brain size was only about 300–350 cm3, or about 25 to 30% that of modern H. sapiens. fte genus Australopithecus appeared later, around 4 MYA, and over the course of a couple of million years would be represented by several species. Australopithecus afarensis (Fig. 8.2) is probably the best known species, and is represented in the fossil record by the well-known partial skeleton named “Lucy.” Sexual dimorphism in Australopithecus was substantial; males were about 1.5 m tall, and females about a half meter shorter. Anatomical evidence and footprints preserved in volcanic ash in Tanzania indicate that Australopithecus were bipedal. Compared with A. afarensis, Australopithecus africanus appears to have pelvic structure better suited for efficient bipedalism. Australopithecus brain size was significantly greater than that of Ardipithecus, but still relatively small, measuring about 425 cm3, or about 35% that of modern humans. fte small brain coupled with bipedal locomotion has cast doubt on the long-held view that increased brain size preceded bipedalism in human evolution. Based on dental microwear and trace element studies, Australopithecus apparently ate primarily fruits, vegetables, and tubers, probably supplemented with meat. Fossils of butchered animal bones associated with Australopithecus sites also suggest meat-eating, as well as early tool use, dating about 3.4 MYA. Figure 8.2: A reconstruction of a female Australopithecus afarensis. (Wikipedia ‘Australopithecus afarensis;’ Source unknown) Download free eBooks at bookboon.com 141 Paranthropus are also known as “robust austalopithecines.” ftey probably descended from Australopithecus. Some paleontologists think that A. africanus evolved into Paranthropus, and Paranthropus species are sometimes included in the genus Australopithecus. Paranthropus were around 1.3 m tall, with males somewhat taller and larger than females. Paranthropus were generally stouter than Australopithecus, and had more robust cranial anatomy. fte brain of Paranthropus was larger than that of Australopithecus. ftere is evidence that Paranthropus used tools, perhaps to a greater extent than did Australopithecus. Like Australopithecus, Paranthropus was bipedal. fte diet of Paranthropus boisei appears to have been based to a great extent on C4 plants such as grasses and sedges, which would have been plentiful in the open savanna habitats in which P. boisei lived. Paranthropus overlapped with the earliest members of the genus Homo, Homo habilis, and may have coexisted with other Homo species as well. But Paranthropus went extinct about 1.2 MYA, whereas Homo survived and flourished. Perhaps the more specialized diet and less advanced tool use of Paranthropus led to their demise. fte genus Homo first appeared in Africa about 2.1 MYA, with Homo habilis (Fig. 8.3) as its earliest representative. Among species in the genus Homo, H. habilis is the least similar to modern humans, with some ape-like features such as relatively long arms and forward-jutting jaws. Homo habilis were small compared to later Homo, with a height of about 1.25 m. Some paleontologists have suggested that H. habilis is more appropriately included in the genus Australopithecus. However, H. habilis had a brain roughly 50% larger than Australopithecus, though still less than half that of modern humans. Homo habilis used stone flake tools, probably for cutting meat from carrion rather than for active hunting or defense. Fossil evidence indicates that H. habilis frequently fell victim to large predators such as the saber-toothed cat, Dinofelis. Homo habilis was probably not the first human tool user; evidence suggests that Australopithecus had used tools previously, and Paranthropus used tools as well. But H. habilis may have been the first tool makers. Figure 8.3: A reconstruction of Homo habilis. (Wikipedia ‘Homo habilis;’ Attribution: Photographed at Westfälisches Museum für Archäologie, Herne, by Lillyundfreya) Homo habilis persisted until about 1.5 MYA, and thus coexisted with another human species, Homo erectus (Fig. 8.4), which first appeared about 1.9 MYA. fte relationship between these two species is still being debated. Some paleontologists consider the two species to be separate lines from a common ancestor, but others suggest that H. erectus was derived from an isolated population of H. habilis, and the remainder of the latter species coexisted with H. erectus for a few hundred thousand years. With an average height of about 1.7 m, H. erectus was substantially taller than H. habilis, although there was a great deal of height and size variation in H. erectus. Figure 8.4: A reconstruction of a Homo erectus skeleton from Tautavel, France. (Wikipedia ‘Homo erectus;’ Attribution: Gerbil) Compared to H. habilis, H. erectus had greater brain capacity as well, about ¾ the capacity of modern humans. fte larger body size and brains of H. erectus required greater energy inputs, and meat, being more easily digestible, played a greater role in the diet of H. erectus. Tool making and use became more sophisticated than that found in H. habilis. However, some paleontologists have recently suggested that several species of Homo that existed around 1.8 MYA, including H. habilis, Homo rudolfensis, Homo ergaster, and H. erectus, in reality represent variation within a single species, best referred to as H. erectus. Homo erectus has the distinction of being the longest surviving species of Homo, existing for over a million years before becoming extinct. In comparison, H. sapiens have existed for roughly 200,000 years. During their time, H. erectus became quite widespread; fossils have been found in Africa, Eurasia, Indonesia, China, and Southeast Asia. Homo heidelbergensis (Fig. 8.5) is a relatively recent human species, first appearing about 600,000 years ago. Homo heidelbergensis were larger in both body size (about 1.75 m and 1.57 m in height for males and females, respectively) and brain case volume (1100 to 1400 cm3) than previous Homo species. ftese measurements substantially overlapped the ranges of modern humans in both respects. Homo heidelbergensis were the first humans to inhabit cold climates in substantial numbers, and their compact body forms were an adaptation that increased heat retention due to reduction in surface area-to-volume ratio. Homo heidelbergensis were innovative in many ways. ftey were the first humans to use controlled fire, and also used wooden spears to hunt large game. ftey were also the first to build shelters, using wood and rocks. Figure 8.5: A reconstruction of Homo heidelbergensis. (Wikipedia ‘Homo heidelbergensis;’ Attribution: Jose Luis Martinez Alvarez from Asturias, España) It is thought that separate branches of the H. heidelbergensis lineage gave rise to H. sapiens as well as “Neanderthal man.” Some paleontologists consider Neanderthals to be a subspecies of H. sapiens, Homo sapiens neanderthalensis. However, most consider them to be a separate species, Homo neanderthalensis (Fig. 8.6). ftere was substantial overlap between H. neanderthalensis and H. sapiens temporally, and the two species coexisted in Europe for about 5,000 years after the arrival of H. sapiens there. In terms of body size, H. neanderthalensis was similar to H. heidelbergensis, but somewhat shorter and stouter, again reflecting adaption for heat conservation in cold climates. Homo neanderthalensis had large brains; as large as or larger than that of modern H. sapiens. H. neanderthalensis took the behaviors found in H. heidelbergensis to new levels of sophistication. Homo neanderthalensis made and used a variety of complex tools; they constructed shelters, made clothing, and engaged in symbolic behaviors such as burying their dead and marking graves with objects such as flowers. Homo neanderthalensis were accomplished hunters and, like H. heidelbergensis, incorporated meat into their diet, particularly during the winter when plant foods were scarce. fte diet of H. neanderthalensis often included large, dangerous game; frequent bone fractures in H. neanderthalensis attest to this. Figure 8.6: Skeleton and reconstruction of a Neanderthal man. (Wikipedia ‘Neanderthal;’ Attribution: Photaro) Homo neanderthalensis disappeared about 30,000 years ago, soon after the arrival of H. sapiens in Europe. What caused the demise of H. neanderthalensis? Several hypotheses have been proposed, some of which invoke direct or indirect interactions with H. sapiens. One suggestion is that H. neanderthalensis was unable to compete for food and resources effectively with H. sapiens, perhaps because their short, stout limbs did not allow H. neanderthalensis to move as effectively as H. sapiens. UCLA biologist Jared Diamond has proposed that anatomical factors limited the ability of H. neanderthalensis to produce complex speech and communicate effectively in comparison to H. sapiens. Diamond has also suggested that direct conflict between the two species could have resulted in violence and genocide of H. neanderthalensis. Some have also suggested that H. sapiens may have hunted and eaten H. neanderthalensis. Homo sapiens may have also contributed to the extinction of H. neanderthalensis by transmitting pathogens or parasites which were new to H. neanderthalensis and to which H. neanderthalensis therefore had little immunity. It is also possible that H. neanderthalensis was poorly equipped to survive climate change that was occurring at the time. Certainly these potential causes are not mutually exclusive, and it is quite possible that a combination of these or other factors may have led to the extinction of H. neanderthalensis. However, there is some genetic evidence of interbreeding between H. neanderthalensis and H. sapiens, meaning that H. neanderthalensis genes may still persist in modern humans. Homo neanderthalensis and H. heidelbergensis, along with another species, Homo rhodesiensis, are generally referred to as “archaic humans,” anatomically distinct from H. sapiens. fte origins of modern H. sapiens have been debated for decades, but recent evidence supports the “recent single-origin hypothesis” (otherwise known as the “out of Africa” hypothesis). In this view, supported by genetic and fossil evidence, H. sapiens evolved from archaic humans solely in Africa between 200,000 and 60,000 years ago. One group of H. sapiens left Africa between 125,000 and 60,000 years ago, and over time displaced other species in the genus Homo in other geographic locations. A competing hypothesis, the “multiregional hypothesis,” asserts that one continuous human species, which includes archaic humans as well as H. sapiens, evolved throughout the world into the various geographic populations of H. sapiens. ftis hypothesis currently has much less support in the paleoanthropological community than does the recent single-origin hypothesis. By 30,000 years ago, Homo sapiens had spread from Africa to Europe, Asia, and the Australasian region. Approximately 15,000 years ago, during the late Wisconsinan Glaciation, low sea levels exposed the Bering Land Bridge, allowing colonization of the Americas from Asia. About 13,000–12,000 years ago, the North American megafauna underwent large-scale extinctions, a pattern that had occurred during earlier time periods in other parts of the world. ftese extinctions may have had multiple causes. Climate change may have been an important factor. However, evidence suggests that, in several locations, these extinctions correlate closely with the arrival of humans, and the animals that went extinct were large herbivores or flightless birds that would likely have been the focus of human hunting. fte idea that humans were responsible for megafaunal extinctions is known as the Pleistocene overkill hypothesis. In addition to those species discussed in this section, the human clade has included numerous others that have entered and exited the human evolutionary stage over the last 7 million years. See the works listed at the end of the chapter for more information on these.

Kategoria: Moje artykuły | Dodał: kolo (2019-03-11)
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