请将第一单元正文中第2自然段(P1)逐句翻译。注:一句英文对应一句中文,手写后拍照上传。 The earliest humans have access to only a very limited number of materials. those that occur natural stone, wood, clay, skins, and so on. With time they discovery techniques producing materials that had properties superior to those of the natural ones: these new materials included pottery and various metals. Furthermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. At this point, materials utilization was totally a selection process that involved deciding from a given, rather limited set of materials, the one best suited for an application by virtue of its characteristic. It was not until relatively recent times that scientists came to understand the relationship between the structural elements of materials and their properties. This knowledge acquired over approximately the past 100 years has empowered them to fashion, to a large degree, the characteristics of materials Thus, tens of thousands of different materials have evolved with rather specialized characteristic that meet the needs of our modern and complex society, including metals, plastics, glass, and fibers.