yielded unprecedented results; smallpox now only existed in labs. Machines came to be used in all areas of production, feeding increasingly intricate supply chains that allowed mankind for the first time to be constrained not by how much it could produce, but by peoples' willingness to consume. Trade improvements greatly expanded upon the limited set of food-producing techniques used since the Neolithic period, multiplying the diversity of foods available and boosting the quality of human nutrition. Until the early 19th century, life expectancy from birth was about thirty in most populations; global lifespan-averages exceeded 40 years for the first time in history, with over half achieving 70 or more years (three decades longer than a century earlier). Overview Chronological history The 20th (twentieth) century began on January 1, 1901, and ended on December 31, 2000. It was the tenth and final century of the 2nd millennium. Unlike most century years, the year 2000 was a leap year, and the second century leap year in the Gregorian calendar after 1600. The century had the first global-scale total wars between world powers across continents and oceans in World War I and World War II. Nationalism became a major political issue in the world in the 20th century, acknowledged in international law along with the right of nations to self-determination, official decolonization in the mid-century, and related regional conflicts. The century saw a major shift in the way that many people lived, with changes in politics, ideology, economics, society, culture, science, technology, and medicine. The 20th century may have seen more technological and scientific progress than all the other centuries combined since the dawn of civilization. Terms like nationalism, globalism, environmentalism, ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, profoundly changed the foundational models of physical science, forcing scientists to realize that the universe was more complex than previously believed, and dashing the hopes (or fears) at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. It was a century that started with horses, simple automobiles, and freighters but ended with high-speed rail, cruise ships, global commercial air travel and the Space Shuttle. Horses and other pack animals, every society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within a few decades.