Along the timeline, colored numbers and arrows mark points of departure. These are moments in the history of science where the pathways of discovery associated with each of the 12 topical essays in the series might have begun to unfold.

Pathways of discovery are only smooth and linear when the nuanced and tortuous history of science is brutally edited to fit into a finite space. The items to the right represent the countless twists, turns, ironies, contradictions, tragedies, and other unkempt historical details that have synthesized into the far more complex and multitextured reality of the scientific adventure.

4th century B.C.: Aristotle rejects the atomism of Leucippus and Democritus in favor of the five-element material system -- earth, air, fire, water, and ether. Atomism doesn't surface seriously for 2000 years. Meanwhile, the four-element philosophy spurs alchemical thought and practice. The power of authority often still poses obstacles to new ideas.

9th to 15th centuries: The flow of science and technology is mostly into Europe from Islam and China. Among the innovations diffusing westward: paper, chemical technology, glassmaking, the compass, shipbuilding, and gunpowder. In the 15th century, the centers of science and technology begin shifting to the West.

1543: Copernicus helps begin the scientific revolution with his heliocentric theory of the universe. Yet he retains the perfect circularity of orbits demanded by Platonic idealism and Ptolemaic astronomy. His work embodies both a radicalism that unlocks new knowledge and a conservatism. These together account for much of the progressive nature of scientific knowledge.

1633: The Holy Inquisition threatens Galileo Galilei with torture unless he renounces heliocentrism. He acquiesces but is confined to house arrest for the rest of his life for previously defying Church authority by publicly promoting the theory. The Galileo affair has since represented -- in the most dramatic fashion -- the tension between scientific and religious authority.

17th century: Even as Isaac Newton ascends to become an icon of the scientific revolution, he devotes 1 million words and countless hours of his intellect to the practice of alchemy. The ability of the human mind to simultaneously entertain contrary belief systems is a perennial source of wonder.

c1900: Shortly after Röntgen discovered x-rays, the French physicist, René Blondlot, claimed discovery of what he called "N-rays." Other scientists confirmed his observations. Papers were published. Trouble was, N-rays didn't exist. This episode has become a classic case of "pathological science" in which self-deception, wishful thinking, and selective use and negligence of data can lead scientists astray.

1926: Alfred Wegener's theory of continental drift submerges due to the lack of support from his peers (to say the least). Forty years later, moving continents were established as scientific fact. The critical attitude always has been central to the scientific process, but it also can lead to the premature rejection of good ideas. This episode also is testimony to the faith among scientists that the truth will come out.

World War II: Often known as "the physicists' war," WWII demonstrated tight links between science, technology, and national security. Synthetic rubber, radar, proximity fuses, and the fission bomb all emerged from the most extensive government-supported R&D program in history. After the war, this new R&D infrastructure evolved into its present form.

1953: Without Rosalind Franklin's crystallographic data of DNA, James Watson and Francis Crick wouldn't have determined that molecule's structure when they did. The two of them and Franklin's senior co-worker, Maurice Wilkins, shared a Nobel Prize for the discovery. Franklin never received official credit for her contribution, and her role was written out of early historical accounts. Franklin represents the struggle for professional parity that women have waged, and continue to wage, in the scientific community.

Science has never been a value-free enterprise, and its double-edgedness expressed itself with a sobering regularity this century. Before there was "better living through chemistry," there were clouds of death-dealing chlorine on the Western Front of World War I. The next World War and the Cold War ushered in the awesome power of nuclear fission and fusion and a new kind of apocalyptic angst. Then came Rachel Carson. She alerted the world to global-scale ecological threats that can come with wide-scale environmental releases of synthetic chemicals. Her point was reiterated in the 1970s and 1980s when industrial activity and products were linked to planet-altering threats like ozone holes and global warming. Now, the growing ability to understand and manipulate our own genetic foundations opens up almost utopian medical possibilities. But that gleamy-eyed future only comes with the threat of unnerving compromises ranging from genetics-based job discrimination to the possibility of literally bumbling with the genetic constitutions of all generations that follow this one. Science has entangled itself within the most important values there are, and that brings with it unprecedented responsibilities for scientists of the next millennium.