12 The History of Life

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Walking UprightMany hypotheses have been proposed to explain the evolutionary success of thehominids. Enlarged brain size and the ability to make and use tools were formany years among the most accepted ideas. However, fossil discoveries haverevealed that another trait came beforetool use and the large brains—walkingupright on two legs. Upright postureand two-legged walking requiredchanges in skeletal anatomy. Thesechanges can be found in intermediatefossils between hominoids that walkedonly on all fours and early hominidsthat walked on two legs. Animals thatVISUAL VOCABBipedal is an adjectivethat describes twoleggedor uprightwalking. Bi- means “two,”and ped means “foot.”can walk on two legs are called bipedal (BY-PEHD-l). This trait has importantadaptive advantages. It allows higher reach into tree branches while foraging,and perhaps most importantly, it frees the hands for foraging, carrying infantsand food, and using tools.Connect What is another common animal that is bipedal?QUICK LABMODELINGGeologic ClockOne way to understand the relative length of time in Earth’s history is tocompare its age to a clock face. Precambrian time goes from 12 noon toabout 10:30 P.M. The time span from early human ancestors—more than5 million years ago—to Homo sapiens covers less than a second on our12-hour clock!PROBLEM How do different geologic time periods compare?PROCEDURE1. Draw a large circle and mark the 12, 3, 6, and 9 positions of a clock face. Usethe scale 1 hour = 400 million years ago, and label the four positions with theappropriate number of years, starting with 12 o’clock = 4800 million years ago.(Example: the three o’clock position = 3600 million years ago.)2. Using the geological time scale, label Precambrian time and the threeeras on your clock, along with the approximate time frames in whichthey occurred.3. Label the following events on your clock in the appropriate positions,also filling in the approximate time frames they occurred: formation ofEarth, oldest rocks, first stromatolites, first aerobic prokaryotes, firsteukaryotes, first fishes, first flowering plants, first dinosaurs, first birds,and earliest hominids.ANALYZE AND CONCLUDE1. Synthesize How are eras and periods related? Where would theperiods fit in this diagram?2. Calculate Using your scale of 1 hour = 400 million years, how manymillions of years in Earth’s history would 1 minute represent?9?124800 MYA?6? 3MATERIALS• paper• pencilChapter 12: The History of Life 381
MAIN IDEAThere are many fossils of extinct hominids.FIGURE 12.19 Computer technologyallowed scientists to piecetogether 7-million-year-old skullfragments found in Africa. Thethree-dimensional reconstructionsuggests that this may be the oldestknown hominid ancestor ofHomo sapiens; it has been namedSahelanthropus tchadensis.Most hominid species are classified into two groups: the genus Australopithecus(aw-STRAY-loh-PIHTH-ih-kuhs) and the genus Homo. The australopithecineswere a long-lived and successful genus. Australopithecus afarensis (AF-uh-REHN-sihs), who lived 3 to 4 million years ago in Africa, is one of the betterknown species of early humans. Although its brain was much smaller thanthat of a modern human—about the size of a modern-day chimpanzee’sbrain—A. afarensis had very humanlike limbs.The earliest member of the genus Homo was Homo habilis. Nicknamed“handy man” because of the crude stone tools associated with its skeletons,H. habilis lived 2.4–1.5 million years ago in what are now Kenya and Tanzania.This species may have lived alongside the australopithecine species for about1 million years. H. habilis is the earliest known hominid to make stone tools.The brain of H. habilis was much larger than that of any of the australopithecines,and it more closely resembled the modern human brain in shape.Another hominid species was H. neanderthalensis, commonly calledNeanderthals for the Neander Valley in Germany, where their fossils were firstfound. This group lived from 200,000 to 30,000 years ago in Europe and theMiddle East. Some evidence suggests that H. neanderthalensis coexisted withmodern Homo sapiens. Did the two species live side by side? Or did they engagein a fierce competition for resources, causing the extinction of the Neanderthalsby the better-adapted H. sapiens? This puzzle has not yet been solved.Observations from the fossil record demonstrate a trend toward increasedbrain size in the human lineage. Although brain size can only be looselyrelated to intelligence, the combination of modern-day humans’ physical andcultural adaptations has no doubt contributed to our success as a species.Hypothesize What type of evidence could indicate that H. sapiens and H.neanderthalensis coexisted?MAIN IDEAModern humans arose about 100,000 years ago.Fossil evidence reveals that the first appearance of Homo sapiens, the speciesname of modern humans, dates to roughly 100,000 years ago in what is nowEthiopia. Many of their features are different than those of humans today.After becoming a distinct species, H. sapiens clearly did not stop evolving.The Role of CultureHuman evolution is influenced by culture. Tools are among key markers ofculture in human evolution, although they are used by some other animals aswell. A comparison of tools from their first appearance some 2.5 million yearsago, through their association with later Homo fossil sites, shows a steadytrend of increasing sophistication and usefulness.382 Unit 4: Evolution
Page 1 and 2: CHAPTER12The Historyof LifeKEY CONC
Page 3 and 4: 12.1 The Fossil RecordKEY CONCEPT F
Page 5 and 6: Connecting CONCEPTSChemistry of Lif
Page 7 and 8: CHAPTER 12MATERIALS• 10 pennies
Page 11: 12.3Origin of LifeKEY CONCEPT The o
Page 14 and 15: FIGURE 12.11 RNA AND DNARNA, in the
Page 16 and 17: MAIN IDEAEukaryotic cells may have
Page 18 and 19: DATA ANALYSISGeneration Times of Ba
Page 20 and 21: The middle of the Paleozoic era was
Page 22 and 23: 12.6Primate EvolutionKEY CONCEPT Hu
Page 26 and 27: FIGURE 12.20 Examples of Hominid Sk
Page 28 and 29: INVESTIGATIONUnderstanding Geologic
Page 30 and 31: Chapter AssessmentChapter Vocabular
Page 32 and 33: For more test practice,go to ClassZ
Page 34 and 35: Staph InfectionsThese athletes were

12 The History of Life

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