Nearly everything on Earth is made of elements heavier than hydrogen and helium. We need not be clever chemists to realize that the air, land, and sea are all partially made of matter cooked in the hearts of stars. The heavies are an essential prerequisite for the continued evolution of complexity in the Universe. Although it sounds poetic to claim that much of everything around us—and within us—is “starstuff,” it happens to be true.

The bluish oceans of liquid water consist partly of a heavy element, for water is after all not just two parts hydrogen but also one part oxygen. Only a fortuitous combination of temperature and density, unlike that on any other known planet or moon, allows large quantities of water, enough to cover nearly three-quarters of our planet’s surface, to remain in the liquid phase. Tidally pulled this way and that, and occasionally evaporated into the air only to condense back down as rain, water is an integral part of our changing terrestrial environment.

The air we breathe and the land we walk are also rich in heavy elements. Composed mainly of nitrogen and oxygen, Earth’s atmosphere is constantly yet unevenly subjected to solar heating and variable weather, causing billowy clouds to dominate our planet from afar. And the brown and gray tracts of mineral-rich soil and silica-rich rock, looking immutable from space and feeling stable beneath our feet, drift imperceptibly across our globe. Surprisingly, iron is the most abundant element (in terms of mass) in all of Earth, most of it having sunk to the core long ago.

Oceanographic change, meteorological change, geological change. All these changes are up and running in our home in the Universe. Chemical change functions no less, for eventually the heavy elements begin to interact, react, and complexify.

Life Arising The most remarkable heavy-element assemblage on Earth is life. Plants, animals, and bacteria are widespread in our biosphere—on the land, in the sea, and throughout the air—though only within the past 10% of our planet’s history have they become bigger, some of them mobile, a few of them sentient. Of particular note and telling perspective, the heavy-element concoction known as men and women have existed for well less than one-tenth of 1% of Earth’s history.

To connect with previous epochs in time, not only our planet but also our bodies are peppered with the heavy elements fused in ancient supernovae, and all of them as well with the ancestral element hydrogen of the early Universe. The principal epochs of cosmic evolution are overlapping; our story is becoming more interdisciplinary. The old adage that life is mainly made of stardust is never more apt than while noting the celestial roots of our chemical makeup. Literally, many stars have died so that we might live.

Life everywhere now seems biologically adapted to planet Earth, but adaptation is a never-ending effort. Change is inevitable. Nothing is immutable, nothing at all. The climate alters. The Alps build. The Atlantic widens. Even the rock-solid aspects of our sturdy planet quake and drift, evolving over timescales immense compared to human life spans. What we can’t see is tough to believe, but we are witness to so brief a time interval. Even the 10,000-year duration of human civilization is a mere wink in the grand pageant of eternal change.

It’s that time along the arrow of time (on Earth anyway) to survey the salient changes of the CHEMICAL EPOCH that originally led to life long ago. Here, the pace of change ramps up, with novel ordered systems coming forth in greater numbers, greater diversity, and greater spectacle. Additional changes, either natural or legislated, among today’s life forms could either yield smarter, perhaps even wiser, creatures or someday conceivably render planet Earth uninhabitable. Our fate mixes again chance with necessity.

The learning goals for this epoch are:

• to appreciate the lack of clarity separating that which is living from that which is not
• to realize that life is more complex than any non-living piece of matter
• to know the central ideas of modern biochemistry
• to understand cells and the building blocks of life that comprise them
• to appreciate some of the laboratory experiments that simulate life's origin on Earth
• to recognize the role of energy in the origin and evolution of life and the organization of all things
• to know how thermodynamics helps explain the rise of complexity in Nature.