and notes similarities and differences between the human, geological, and cosmological scales. David Christian believes such "radical shifts in perspective" will yield "new insights into familiar historical problems, from the nature/nurture debate to environmental history to the fundamental nature of change itself." It shows how human existence has been changed by both human-made and natural factors: for example, according to natural processes which happened more than four billion years ago, iron emerged from the remains of an exploding star and, as a result, humans could use this hard metal to forge weapons for hunting and war. The discipline addresses such questions as "How did we get here?," "How do we decide what to believe?," "How did Earth form?," and "What is life?" According to Fred Spier it offers a "grand tour of all the major scientific paradigms" and helps students to become scientifically literate quickly. One interesting perspective that arises from Big History is that despite the vast temporal and spatial scales of the history of the Universe, it is actually very small pockets of the cosmos where most of the "history" is happening, due to the nature of complexity. Cosmic evolution Cosmic evolution, the scientific study of universal change, is closely related to Big History (as are the allied subjects of the epic of evolution and astrobiology); some researchers regard cosmic evolution as broader than Big History, since the latter mainly examines the specific historical trek from Big Bang → Milky Way → Sun → Earth → humanity. Cosmic evolution, while fully addressing all complex systems (and not merely those that led to humans) has been taught and researched for decades, mostly by astronomers and astrophysicists. This Big-Bang-to-humankind scenario well preceded the subject that some historians began calling Big History in the 1990s. Cosmic evolution is an intellectual framework that offers a grand synthesis of the many varied changes in the assembly and composition of radiation, matter, and life throughout the history of the universe. While engaging in issues of the origins of humanity, this interdisciplinary subject attempts to unify the sciences within the entirety of natural history—a single, inclusive scientific narrative of the origin and evolution of all material things over ~14 billion years, from the origin of the universe to the present day on Earth. The roots of the idea of cosmic evolution extend back millennia. Ancient Greek philosophers in the fifth century BCE, most notably Heraclitus,