Cоnsider hоw these аdаptаtiоns may have influenced the development of conditions and injuries affecting the musculoskeletal system, and the role of physical therapy in addressing them. Explore the idea of evolutionary mismatches—instances where our modern lifestyle may differ from the conditions our ancestors evolved under. How might these mismatches contribute to a health issue and what role can physical therapy play in addressing or preventing it? Include researched evidence and examples in your answer. Your answer should include internal citations in APA format (see the last question in the assignment concerning submitting references).
DESCRIPTION: Pаleоntоlоgists hаve studied the fossil record of humаn evolution just as they have done for that of other major transitions—including the transition from fish to tetrapods and dinosaurs to birds. The short film Great Transitions: The Origin of Humans highlights the most important hominid fossil discoveries of the past 50 years and the insights they provide into human evolution, focusing on three key traits: bipedality, tool use, and larger brains. KEY CONCEPTS The fossil record details the history of life on Earth, including transitions from one major group of animals to another. DNA evidence indicates that the human lineage split from that of our closest relative, thechimpanzee, about 7 million years ago—a timing that is consistent with fossil evidence. Traits that distinguish modern humans from modern chimpanzees include bipedality (theability to walk upright on two legs), extensive tool use, and larger brains. Fossil evidenceprovides information about when and where each of these traits evolved. Tracing the evolution of distinct traits shows that, like other major transitions, the evolution ofhumans from quadrupedal apes occurred in distinct phases. Fossil evidence reveals that bipedalism arose over 4 million years ago and predates tool useand the evolution of larger brains by at least a million years. Fossils also provide evidence about the environment in which a species lived. For example,bipedalism evolved when our human ancestors were still living in forests and climbing trees. Finding and identifying fossils is difficult and time-consuming work. Almost every individualthat lived on Earth left no fossil evidence of its existence. BACKGROUND: It wasn’t until the publication of The Descent of Man in 1871, 12 years after On the Origin of Species, that Charles Darwin wrote about the evolution of humans. In The Descent of Man, he hypothesized that humans and modern apes share a common ancestor that lived in Africa. He predicted that fossil evidence would one day be found in Africa to support his hypothesis—and he was right. The film Great Transitions: The Origin of Humans explores several key fossil discoveries from Africa and what they reveal about human evolution. What Are Humans?Humans, along with familiar species such as lemurs, gibbons, and chimpanzees, are primates (Figure 1). Primates are hundreds of species that form a mammalian order, characterized by many traits, including forward-facing eyes, generalized teeth, collarbones, and nails instead of claws on their fingers and toes. In the film, Dr. Sean Carroll explains that members of our own species, Homo sapiens, are distinguished from other primates by three primary traits: large brains, bipedalism, and tool use. These traits must have evolved after the lineage leading to modern humans split from the one that would eventually split into modern chimpanzees and bonobos, our closest living relatives. Figure 1. The Primate Family Tree. Humans belong to the primate order, and Darwin had observed that humans shared many similarities with the African great apes. DNA studies have since shown that chimpanzees and bonobos are humans’ closest living relatives; we shared a common ancestor with them about 7 million years ago. Scientists have examined the fossil record to understand when and where these three distinctly human characteristics evolved. The film describes fossils of species that were either side branches of our immediate family tree or ancestors on the evolutionary lineage that led to modern humans (Figure 2). In the film, these species are referred to as hominids (the nontechnical form of the zoological family name Hominidae). As used in the film, “hominid” is synonymous with the alternative term “hominin,” increasingly used in the scientific literature and many textbooks. The Earliest Fossil Finds“On July 17, at Olduvai Gorge in Tanganyika Territory, at site FLK, my wife found a fossil hominid skull, at a depth of approximately 22 ft. below the upper limit of Bed I.” This is the first line in a now-famous paper written by Dr. Louis Leakey and published in the journal Nature in 1959 announcing the discovery of the 1.76-million-year-old Zinjanthropus boisei (today classified as Australopithecus boisei or Paranthropus boisei). For three decades, the Leakeys had been searching for evidence of human evolution in the Olduvai Gorge in Tanzania, but they found only stone tools. Finally, they thought they had the toolmaker. In the Nature paper, Dr. Leakey proposed that this species was “the oldest yet discovered maker of stone tools,” but later finds would soon cause him to rethink this inference. The Leakeys concluded that Zinjanthropus was the toolmaker because the skull was found with stone tools in the same layer of sediment. Further analysis of the skull, however, indicated that its large back teeth and sagittal crest for the attachment of large jaw muscles put it on a specialized and now-extinct “side branch” of the human family tree. Two years after the Zinjanthropus discovery, the Leakeys’ son, Jonathan, discovered a new, slightly older (1.8 million years old) fossil, dubbed “Olduvai Hominid Number 7,” or “OH7.” The new fossil, with jaw and skull pieces more similar to those of modern humans, led the Leakeys to conclude that this was the real “toolmaker,” and they named it Homo habilis, or “handy man.” How can we know for sure that Homo habilis was the toolmaker? The answer is we cannot. The fact that Homo habilis appeared to be more closely related to modern humans and had a larger brain than Zinjanthropus made it the more likely candidate for the toolmaker. However, even today we cannot exclude the possibility that Zinjanthropus may also have been making and/or using tools. Figure 2. Major Fossil Finds. The film describes three key hominid fossils: early Homo (Homo habilis), Lucy(Australopithecus afarensis), and Ardi (Ardipithecus ramidus), shown here along a geological timeline. These threespecies each belong to a group, or genus, that also contains other species not shown in this figure. Note: “mya” means “millions of years ago.” The Importance of Stone ToolsThe stone tools found by Louis and Mary Leakey were simple ones. There is evidence that earlyhumans shaped these tools to have sharp edges with which they could cut the meat off animalcarcasses and then smash the bones to get at the fat-rich marrow. Although other animals are knownto make and use tools, none of them modify stone in this fashion. The oldest firmly dated anddiagnostically identifiable stone tools made by hominids date back to 2.6 million years ago. LucyOn November 24, 1974, paleoanthropologist Dr. Donald Johanson discovered almost half of thefossilized skeleton of a hominid that lived nearly 3.2 million years ago. He nicknamed the fossil “Lucy”and later included it in a new species, Australopithecus afarensis.Bones of this species indicated that A. afarensis was bipedal. The shape of Lucy’s pelvis and kneesallowed her to balance on one leg at a time, a requirement for efficient upright walking on two legs.The discovery of two trails of 3.6-million-year-old fossil footprints (called the Laetoli footprints) foundon the edge of the Serengeti Plain in Tanzania lent more evidence to the conclusion that A. afarensiswas a biped. As for brain size, Dr. Johanson and his colleagues analyzed the skull fragments of Lucy and other A.afarensis fossils and determined their brain sizes to be 380 to 550 cubic centimeters (cc or cm3)—larger than the average modern chimpanzee (about 400 cubic centimeters), but much smaller thanaverage for modern humans (about 1,300 cubic centimeters). ArdiAn international team discovered the partial skeleton of 4.4-million-year-old Ardipithecus ramidus(nicknamed “Ardi”) in 1994. In the film, Dr. White described the species as “neither a chimp, nor …human.” What does he mean by that? Ardi had a rigid outer part of the foot, likely used as a lever for pushing off during upright walking. Incontrast, the bones of chimpanzee feet are more flexible, allowing the feet to be used more like hands.But surprisingly, Ardi’s feet also had an opposable large toe that could be used for grasping whilemoving about in trees. The upper part of Ardi’s pelvis (the ilium) was flared out to each side, whichsupported muscles necessary for an upright walker to maintain balance and forward motion.However, the lower part of the pelvis (the ischium) was more like that of a chimpanzee than that of amodern human, anchoring muscles important in climbing. Ardi’s skull was small, with a brain volumeof 300 to 350 cubic centimeters, similar to that of a chimpanzee. Taken together, skeletal evidence suggests that Ardi was capable of bipedalism, but that she hadclimbing abilities superior to those of Australopithecus. Dr. White and his team had not only pushedback the origin of bipedalism to 4.4 million years ago, but they had also discovered that it evolvedwhile our ancestors were still spending time in the trees. Other Important TraitsSome traits that were not discussed in the film but that also differ between humans and other greatapes include the shapes and sizes of teeth—and in particular the canines. Male chimpanzees haveprominent, pointed canines that they display to compete with other males for social status and forfemales. In comparison, humans have stubby canines that are similar in size between males and females. Ardi’s and Lucy’s species had canines that were blunter than the chimpanzee’s but more prominent than those of modern humans. Savanna or Woodland?Beginning as early as the 1800s, scientists reasoned that bipedality evolved in hominids adapting tothe open grasslands of eastern and southern Africa. Standing upright would have also allowed earlyhominids to both see potential predators as well as defend themselves with handheld weapons—advantages in an exposed, open area such as a savanna. As early as 1981, however, anthropologist Dr. C. Owen Lovejoy suggested that, given the risk ofpredation in the savanna ecosystem and the variety of habitats available to early hominids, the mostlikely habitat for the evolution of upright walking was forest habitat or a combination of forest andgrassland, not grassland alone. The tens of thousands of plant and animal fossils recovered from theancient environment in which Ardi lived provided convincing evidence that her species was adaptedto a woodland existence. You will watch the film, Great Transitions: The Origin of Humans (transcript). The film focuses on three key fossils because they illustrate distinct phases in human evolution and also because these fossils were remarkable in being nearly complete. Many times, scientists only find parts of a fossil. When did Charles Darwin address the question of human evolution?
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