It is interesting to note that more is known about the origin of distant stars than about the origin of our own solar system, which consists of the Sun and the planets, asteroids, comets, etc. that orbit it. This is because the solar system formed long ago, and we have no other examples of young solar systems to observe at close range. A current approach to studying the formation of the solar system is to study the formation of new stars. It is hoped that this data can be used to create a model that explains the formation of solar systems. Scientists are able to train their telescopes on multitudes of large stars in various stages of development; unfortunately, many of these stars are too far away to allow observation of how planets may be forming in association with them.

At one time, it was widely believed that planets in our solar system formed when a star passed close to the Sun, tearing away matter that then condensed into planetary bodies. Subsequent calculations have shown that matter removed in this way would either fall back to the Sun or fly into outer space. Today, most experts favor some variation on the solar nebula theory, which holds that the solar system formed from a cloud of particulate matter called cosmic dust. The details of this theory, however, are widely debated.

Even the details regarding the formation of the Sun, the star in our solar system, are not agreed upon. The Sun may have formed from material remaining from a star that imploded, or collapsed violently, to form heavy elements. After this collapse, a supernova—or an exploding star that casts off matter of low density—was formed. The supernova remnants formed a dense cloud or nebula that then formed the Sun. There is also a possibility that it formed alone from a nebula made of various types of dust and gas not necessarily remaining from a supernova and then captured a cloud of cosmic dust that then formed the planets. Alternatively, the Sun and the planets may have formed from the same dust cloud.

Amidst all of the uncertainties regarding the origin of the solar system, one thing seems certain: The planets formed either during or soon after the birth of the Sun. In fact, the Sun and the planets appear to have originated during a remarkablyshort interval of time possibly no longer than about 100 million years.

Choose the one word or phrase that best completes the sentence. Click on the letter (A), (B), (C), or D in the answer space provided.

Identify the one underlined word or phrase that must be changed in order for the sentence to be correct.

We human beings have developed special technologies, such as air conditioning and oil burners, to protect ourselves from discomfort and death by extremes of heat or cold. Our bodies have also evolved strategies for maintaining a fairly constant body temperature despite large fluctuations in the environmental temperature. One such strategy involves the unconscious brain-based rhythms that time when we sleep and wake. The timing of sleep is controlled by a brain clock designed for twin strategies: survival and energy conservation.

In the mid-nineteenth century, German physiologists recognized the link between sleep and body temperature. They established the clinically important fact that human beings normally experience daily fluctuations in core body temperature of about 1.5 degrees Fahrenheit, with a peak in the late morning or early afternoon and a trough at night—coinciding with sleep. It has since been well documented that this rhythmic change is not a reflex response to the more extreme daily fluctuations of ambient light and temperature. It is, rather, a rhythm generated by the brain, which anticipates the ambient variability and fits behavior to it.

Why does body temperature vary? And why do people sleep when body temperature is low? The answers lie in the connection between body temperature and metabolism. Our bodies constantly generate heat as well as obtain biochemical building blocks for our tissues. To sustain metabolic activity, we must obtain energy by consuming food.

Within the same organism, a high level of metabolic activity will coincide with a high body temperature. Because sleep is a state of lower metabolic activity, with a lower body temperature, it is energy conservative. Even if we cannot seek food during sleep, we do not use up much energy, because our muscles become inactive. And interestingly, we do not normally become hungry while asleep. Removing individuals temporarily from the effort to find nourishment is one of the main contributions sleep makes to the survival of mammalian species. By decreasing the amount of food that needs to be consumed, sleep makes it possible for animals to compete more successfully for limited food supplies.

The invention of writing was a gradual process. Scholars have identified several stages, not only in the evolution of writing systems but also in the diffusion of literacy (the ability to read and write) to different groups in society. Mnemonic symbols that assist memory evolved into systems of words separated into syllables among early peoples. It happened first among the Sumerians and later the Babylonians and Egyptians as commerce necessitated more abstract symbols that could record different products and individuals. From these word-syllabic systems evolved more complex syllabic systems and ultimately, through the Phoenicians, the alphabetic writing of the Greeks.

Beyond these stages of writing development, the early history of literacy may be divided into periods according to the level at which literacy spread among the population. For some centuries after the introduction of alphabetic writing, literacy was restricted to a small elite and limited to a few functions, chiefly religious, accounting, and sometimes political administration. Historians have called these centuries a time of restricted literacy. As writing gained a foothold in more areas of activity, it prompted more general literacy among the elite. At the same time, the diffusion of writing tended to consolidate and standardize regional language.

Some qualifications to the concept of stages in literacy are necessary. First, not all societies move through all stages. For example, nonliterate societies sometimes adopted alphabetic writing from abroad, skipping the earlier stages. Second, although the stages are generally sequential, their order is sometimes reversed. For example, in Europe the literate population narrowed during the medieval period, and in Greece, between the twelfth and ninth centuries B.C., writing utterly vanished. Third, the stages are not mutually exclusive; they express gradual and overlapping developments.

These early developments took place in the context of mass illiteracy and pervasive oral culture. Oral culture did not decrease in contact with written culture; rather, the written word modified and extended communication networks. Not only has the great majority of Earth's population been illiterate through history, the bulk of communication in literate societies is still oral. Nonetheless, writing is a technology, and it allows new modes of communication, administration, and record keeping, as well as innovations in economic, political, and cultural activities.

Next to its sheer size, the profound isolation of its many small islands is the most distinctive feature of the Pacific Ocean. Over 25,000 islands are scattered across the surface of the Pacific, more than in all the other oceans combined, but their land area adds up to little more than 125,000 square kilometers, about the size of New York State, and their inhabitants total less than two million people, about a quarter of the number that live in New York City. The oceanic islands of the Pacific are some of the most isolated places on Earth. Many are uninhabitable, by virtue of their small size and particular characteristics, but even the most favored are very isolated fragments of land, strictly circumscribed by the ocean, strictly limited in terms of the numbers of people they can support. This basic fact of environmental circumstance has been the most pervasive influence in determining the social arrangements and cultural practices of the people that settled in thePacific Islands.

The peopling of the Pacific Islands has been described as the greatest feat of maritime colonization in human history. Contrary to the conclusions of Thor Heyerdahl's Kon-Tiki expedition of 1946, the evidence of plant dispersal, archaeology, linguistics, and genetics now shows quite conclusively that the Pacific Islands were not populated from the east by South Americans who drifted on balsa-wood rafts and the prevailing wind and current, but from the west, by groups from mainland Asia who gradually spread from island to island out into the Pacific. The process began over 40,000 years ago and reached Easter Island—the most isolated place on Earth—about 1,500 years ago. It ended about 1,000 years ago, when people first settled in Hawaii and New Zealand.

Simply surviving those ocean crossings of indeterminate length, in open canoes, to arrive on the shores of uninhabited and hitherto unknown islands, was a formidable achievement. But having found an oasis of land in a watery wilderness, crossed its reef, and landed on its shores, the survivors then faced a series of pressing problems for which solutions had to be found quickly if the small group was to become a vigorous, self-sustaining island population.

In the United States, many social reformers in the late nineteenth century demonstrated a concern for improved housing conditions for workers. George Pullman (1831-1897), the wealthy industrialist who introduced luxury railway cars with beds, built his model city called Pullman in 1880 to address housing problems caused by Chicago's industrialization. In constructing the town, Pullman hoped to produce an ideal environment that would help attract workers of a superior type to the railway car industry and retain them. Pullman inhabitants were expected to embody values of thrift, industry, and morality. They were taught to develop propriety and good manners, cleanliness and neatness of appearance, diligence, and self-improvement through education and savings. Like the brick clock tower that dominated the town center, Pullman kept a regulatory eye on his workers.

In its first five years, this new experiment in industrial life received little criticism, except from radical political groups. Crediting the town of Pullman with producing a new type of dependable and ambitious worker in a rationally ordered environment, reformers, at first, praised it as a successful model for modern industrial life. However, after 1885, with the high gloss of the experiment dulled, it became clear that the residents of Pullman had honest grievances about the overcharging of rent and other services.

In 1893, the World's Columbian Exposition, an exhibition that aimed to promote American cultural, economic, and technological development, and in which George Pullman was a major investor, was held in Chicago. The town of Pullman became a popular tourist stop, attracting more than its share of curious travelers. There were 10,000 foreign visitors alone during the exposition year. In fact, the first Baedeker Travel Guide to the United States advised visitors to tour Pullman. Frequent trains and trolley cars connected the fairgrounds of the exposition with the town, and on several occasions, George Pullman himself guided the tours. Constructing a fantastic environment for the benefit of tourists, he made sure that any real tensions between his office and the working inhabitants of the town were rendered invisible to the tourist gaze.