SITE SUMMARY - Cosmic Evolution: An Interdisciplinar Approach

Welcome video, 5 minutes: Harvard Faculty Insight with Eric Chaisson

Link to Intro Movies, 15 minutes each: Arrow of Time and Cosmic Origins

When consciousness dawned among the ancestors of our civilization, men and women perceived two things. They noted themselves, and they noted their environment. They wondered who they were and whence they came. They longed for an understanding of the starry points of light in the nighttime sky, of the surrounding plants and animals, of the air, land, and sea. They contemplated their origin and their destiny.

Thousands of years ago, all these basic queries were treated as secondary, for the primary concern seemed well in hand: Earth was presumed to be the stable hub of the Universe. After all, the Sun, Moon, and stars all appear to revolve around our planet. It was natural to conclude, not knowing otherwise, that home and selves were special. This centrality led to a feeling of security or at least contentment—a belief that the origin, maintenance, and fate of the Universe were governed by something more than natural, something supernatural.

The ancients thought deeply and well, but not much more. Logic was paramount; empiricism less so. Their efforts nonetheless produced such notable endeavors as myth, religion, and philosophy.

Eventually, yet only a few hundred years ago, the idea of Earth’s centrality and the reliance on supernatural beings were shattered. During the Renaissance, humans began to inquire more critically about themselves and the Universe. They realized that thinking about Nature was no longer sufficient. Looking at it was also necessary. Experiments became a central part of the process of inquiry. To be effective, ideas had to be tested experimentally, either to refine them if data favored them or to reject them if they did not. The scientific method was born—probably the most powerful technique ever conceived for the advancement of factual information. Modern science had arrived.

Today, all natural scientists throughout the world employ the scientific method. Normally it works like this: First, gather some data by observing an object or event, then propose an idea to explain the data, and finally test the idea by experimenting with Nature. Those ideas that pass the tests are selected, accumulated, and conveyed, while those that don’t are discarded—a little like the evolutionary events described on this Web site. In that way, by means of a selective editing or pruning of ideas, scientists discriminate between sense and nonsense. We gain an ever-better approximation of reality. Not that science claims to reveal the truth—whatever that is—just to gain an increasingly accurate model of Nature.

Despite an emphasis on objectivity, some subjectivity does affect the modern scientific enterprise, for this is work done by human beings having strong emotions and personal values. Yet, with the test of time and repeated observations, objectivity eventually emerges and then dominates, enabling us to reach conclusions free of the biased viewpoint of any one scientist, institution, or culture. As a rational investigative approach used to formulate descriptions of natural phenomena, the scientific method is designed to yield a reasonably objective consensus on the nature, contents, and workings of the Universe.

People today still query along the same lines as did the ancients. We ask the same fundamental questions: Who are we? Where did we come from? What is the origin of all things? But our attempts to answer them are now aided by the intricate tools of modern technology: astronomical telescopes to improve our vision of the macroscopic realm of stars and galaxies; biological microscopes to display up close the minute world of cells and molecules; particle accelerators to probe the subatomic domain of nuclei and quarks; robotic spacecraft to gather facts unavailable from our vantage point on Earth; powerful computers to keep pace with the prodigious flow of new data, tentative ideas, and experimental tests.

We live in an age of technology—a time of rapid intellectual advancement unprecedented in history. And even though technology threatens to overwhelm us—perhaps even replace us—that same technology now provides us with a remarkable, yet still growing, understanding of ourselves and our richly endowed Universe.

The learning goals for the Site Summary are:

  • to understand the scientific method of inquiry, including its emphasis on critical thinking and repeated testing
  • to appreciate the size and scale of things in Nature, and the great durations of deep historical time
  • to recognize the grand, interdisciplinary connectivity among all the natural sciences that comprise cosmic evolution.


Of all the scientific achievements since Renaissance times, one discovery stands out most boldly: Our planet seems neither central nor special. Use of the scientific method has demonstrated that, as living creatures, we inhabit no unique place in the cosmos. Research, especially within the past few decades, strongly implies that we live on an ordinary rock called Earth, one planet orbiting an average star called the Sun, one Solar System in the suburbs of a much larger swarm of stars called the Milky Way, one galaxy among billions of others spread throughout an observable abyss called the Universe.

The Universe includes all the vast tracts of empty space and enormous stretches of time populated sparsely by ordered galaxies, stars, planets, and life forms strewn throughout the otherwise disordered darkness. More technically, the Universe is defined as the totality of all known or supposed objects and phenomena, formerly existing, now present, or to come, taken as a whole.

Consult Figures S.1 through S.3, and place some of these objects into perspective by studying Figure S.4.

FIGURE S.1 FIGURE S.1— Earth is a planet, a mostly solid and molten object, though it has some liquid in its oceans and gas in its atmosphere. Discernible here in this global image of Earth taken in 2012 is mostly the north and central American continent and parts of the Pacific Ocean. (NASA)

FIGURE S.2 FIGURE S.2 — The Sun is a star, an extremely hot ball of gas, much bigger than Earth and held together by its own gravity. The dark blemishes are sunspots—disturbed areas on the Sun’s surface. (AURA)

FIGURE S.3 FIGURE S.3 — A galaxy is a collection of typically a hundred billion stars, each separated by huge regions of nearly empty space. This galaxy, called Andromeda, measures about a billion billion kilometers across. Our Sun is a rather undistinguished star near the edge of another such galaxy, called the Milky Way. (AURA)

FIGURE S.4 FIGURE S.4 — An artist's conception puts each of the previous figures into perspective. The bottom of this figure shows spacecraft (and astronauts) in Earth orbit, a view that widens progressively in each of the next five cubes, drawn from bottom to top—Earth, our planetary system, the local neighborhood of stars, the Milky Way Galaxy, and the closest cluster of galaxies. (Dana Berry)

Cosmic Evolution At the beginning of a whole new millennium, modern science is now helping us construct a truly big picture. We are coming to appreciate how all objects—from quarks to quasars, from microbes to minds—are interrelated. We are attempting to decipher the scenario of cosmic evolution: a grand synthesis of all the many varied changes in the assembly and composition of radiation, matter, and life throughout the history of the Universe. Broadly sketched in Figure S.5, these are the changes, operating across almost incomprehensible domains of space and nearly inconceivable durations of time, that have given rise to our galaxy, our star, our planet, and ourselves.

To be sure, change is ubiquitous in Nature. Some of that change is subtle, such as when our Sun shines daily or Earth’s continents drift slowly. Other change is more dramatic, such as when massive stars explode catastrophically as supernovae or when landmasses fault suddenly as quakes and volcanoes. Regardless of whether Nature is examined macroscopically with a telescope, microscopically with an accelerator, or mesoscopically with our own eyes, we see change. Thus, we give this process of universal change a more elegant name—cosmic evolution, which includes all aspects of evolution: particulate, galactic, stellar, planetary, chemical, biological, and cultural. These are the 7 major epochs along the arrow of time of this Web site. And, as a “gesture to the unknown,” an 8th epoch—future evolution—is included as speculation about the prospects for extraterrestrial life and the survivability of humankind.

FIGURE S.5 FIGURE S.5 — Some highlights of cosmic history are noted along this arrow of time, from the beginning of the Universe to the present, including several major epochs diagonally across the top. (Lola Chaisson)

Universal Change Now emerging is a unified worldview of the cosmos, including ourselves as sentient beings, based upon the time-honored concept of change. Change—to make different the form, nature, and content of something—has been the hallmark in the origin, evolution, and fate of all things, animate or inanimate. From galaxies to snowflakes, from stars and planets to life itself, we are beginning to identify an underlying pattern penetrating the fabric of all the natural sciences—a sweepingly encompassing view of the formation, structure, and function of all objects in our multitudinous Universe.

Heraclitus of ancient Greece had it correct: All flows, all fluxes; nothing permanent except change. It’s perhaps the best observation anybody ever made, minus the devilish details. Today, some 25 centuries later, scientific researchers are steadily discovering many of those details—and the results are both insightful and unifying, even awesome. We now have a reasonably good understanding, not only of how countless stars were born and have died to create the matter composing our world, but also how life has come to exist as a natural consequence of the evolution of matter. We can reliably trace a thread of knowledge linking the evolution of primal energy into elementary particles, the evolution of those particles into atoms, in turn of those atoms into stars and galaxies, the evolution of stars into heavy elements, and of those elements into the molecular building blocks of life, and furthermore the evolution of those molecules into life itself, of advanced life forms into intelligence, and of intelligent life into cultured and technological civilization.

To answer the fundamental questions of who we are and whence we came, we need to probe far back into the past—beyond our birth dates some tens of years ago, beyond Renaissance times centuries ago, beyond the onset of civilization ~10,000 years ago, beyond our ancestral hominids who emerged from the forests several million years ago, even beyond the time when multicellular life began to flourish on our planet about a billion years ago, some million millennia before now.

A thousand (1000), a million (1,000,000), a billion (1,000,000,000), and even a trillion (1,000,000,000,000) can be spoken of easily in words. (Note that we use "billion" in the American sense to mean a "thousand million.") Not only are these huge numbers, but the differences among them are also large. For example, 1000 is familiar enough; at the rate of one number per second, we could count to 1000 in about 15 minutes. By contrast, to reach a million surprisingly requires more than 2 weeks of counting at the rate of one number per second, 16 hours per day (allowing 8 hours per day for sleep). And to count from 1 to a billion at the same rate of one number per second for much of each day would take some 50 years—the bulk of a human lifetime just to count to a billion!

Yet, in this Web site, we shall routinely consider time intervals spanning millions and even billions of not only seconds but also whole years. And we shall discuss objects housing trillions upon trillions of atoms, alas trillions of whole stars. Hence we must become accustomed to gargantuan numbers of things, enormous domains of space, and extremely long durations of time. Recognize especially that a million is much larger than a thousand, and a billion much, much larger still.

To appreciate our deep origins in a cosmic-evolutionary setting, we must broaden our horizons, expand our minds, and visualize what it was like long, long ago. Go back, for instance, 5 billion years, when there was no life on Earth, indeed no planet Earth itself. Nor were there a Sun, a Moon, or a Solar System. These objects were only then forming out of a giant, swirling gas cloud near one edge of a vast galaxy of older stars that had already existed in one form or another for a long time before that.

Modern science now combines a wide variety of curricula—physics, astronomy, geology, chemistry, biology, anthropology, among others—in an interdisciplinary attempt to address the two most fundamental issues of all: the origin of matter and the origin of life. If we can decipher the scenario of cosmic evolution, then perhaps we can determine precisely who we are, specifically how life originated on this planet, and, incredibly enough, how living organisms managed to invade the land, generate language, create culture, devise science, explore space, and even study themselves.

As intelligent beings, we humans now reflect back on the matter of the Universe that gave us life. And what we find is a natural history, a universal history, a rich and abiding story of our origins that is nothing less than an epic of creation as understood by modern science—a coherent worldview, or weltgeschichte, that people of all cultures can adopt as currently true as truth can be.

Think Big This Web site concerns all these things: space and time, matter and life, and the energy exchanges that infuse them. We herein explore our cosmos, our planet, ourselves. We summarize where science stands today regarding answers to some of the time-honored philosophical questions: Who really are we? Where and when did we come from? How did everything around us—the air, the land, the sea, the stars—originate? What is the source of order, form, and structure characterizing all material things? How do we, as intelligent beings, relate to the rest of the Universe? In short, what are our origin and our destiny? What are the origins and destinies of the Earth, the Sun, the Universe?

Built for eclectic individuals having a broad interest in Nature, this Web site explains valid contemporary science in a mostly non-technical manner. Accuracy has not been sacrificed, however, and a feeling for the frontiers of science has been included. Even so, readers must recognize that answers to some of the most basic queries are not yet entirely clear. Even among colleagues, scientists are often unable to provide precise and complete solutions for great and profound questions. Only within the past few decades have we gained the technological expertise needed to transfer these issues from the realm of philosophy to that of science.

Researchers now sense that the cutting edge of knowledge resembles a thinning haze rather than a sharp boundary. The research front resembles the “fog of war,” meaning that scientific work is hardly ever crystal clear in real time, while the work is underway. Rather, the intellectual landscape is often revealed only later, after the subjective confusion has abated and a certain objective reality has emerged. That’s because the enterprise of science is now advancing rapidly, acquiring new information at a phenomenal rate and requiring novel interdisciplinary ventures to put it straight. Less than a hundred years ago we didn't understand how stars shine, how heredity works, that the Universe is filled with galaxies, or even that it had a definite beginning. Furthermore, much of science as a "work in progress" involves the human condition, which ensures many false starts and occasional botched logic among the many valid, proven ideas. As a fair assessment, we might say that a pencil sketch of the answers to some of the most basic questions is now at hand, but that many specifics are yet wanting.

Think Interdisciplinary In a descriptive and illustrative way, then, we probe here the essential nature of the cosmos. These pages render the prevailing scientific view that the atoms in our bodies relate to the Universe in general. We elucidate the modern paradigm of cosmic evolution—an astrobiology, a cosmogenesis, a whole new scientific philosophy—whereby changes, both gradual or episodic and generational or developmental, in the composition and structure of matter have given rise to galaxies, stars, planets, and life. We attempt to synthesize the essential ingredients of astrophysics and biochemistry, for these two subjects more than any others are greatly impacting our philosophical conceptions of ourselves as human beings and of our place in the Universe.

In short, this Web site presents the broadest view of the biggest picture. It analyzes, using the best science available, some of the most fundamental questions of all—neither the most relevant nor the most practical questions, perhaps, for 21st-century society, but deeply fundamental ones. We develop an appreciation for our rich universal heritage, for an expansive perspective like no other. We seek to know the nature and behavior of radiation, matter, and life on the grandest scale of all. And in deciphering the fabric of Nature, we discover that technological humans now reside at the dawn of a whole new era.


Our Milky Way system is a genuine galaxy—an enormous collection of ~100 billion stars held loosely together by gravity. This is more stars than people who have ever lived on Earth. Beyond the Milky Way lie billions of other galaxies—each a gargantuan assemblage of matter likewise having hundreds of billions of stars. Light radiation travels outward in all directions from those galaxies, carrying the message of their existence. A minute portion of that light is intercepted at Earth, to be captured occasionally in photographs like those of Figure S.6 .

FIGURE S.6 FIGURE S.6 — Very distant galaxies can be observed through powerful telescopes. Mind-boggling at first sight, many of the objects in this photograph are vast galaxies unto themselves. Each such galaxy resembles the Andromeda Galaxy or our own Milky Way Galaxy, and each contains ~100 billion stars held together in a swarm by the pull of gravity. Radiation from distant galaxies yields information about the earlier Universe when particulate evolution was underway, combining deep-space astronomy with elementary-particle physics. (STScI)

Light doesn’t travel infinitely fast; it moves at a finite speed—the velocity of light. Consequently, even light needs time to travel through the almost unimaginably vast regions that separate objects in the Universe. The farther an object is from Earth, the longer it takes for its light to reach us. By studying that radiation, we can learn what conditions were like long ago, when distant objects emitted their light. Our perception of the Universe is therefore delayed. We see the Universe as it was, not as it is.

Looking out from Earth, then, is equivalent to looking back into time. Telescopes are time machines, and astronomers are historians. Much like archaeologists who dig for information buried in decayed bones and ancient artifacts, astronomers sift through "old" radiation just arriving at planet Earth. Yet astronomers study more than the origins of men and women. By looking out far enough into space—with the best telescopes—astronomers can potentially address the origins of matter itself.

Radiation from distant galaxies implies that our Universe began in a cataclysmic event—a "big bang"—approximately 14 billion years ago. Unimaginably hot at first, the fireball of this cosmic “bomb” gradually cooled as it thinned. We don't know for sure if the Universe will continue to expand forever, or if it will someday stop and eventually contract back to a single point much like that from which it began. What we do know for sure is that the Universe is not static; it's changing with time—it's evolving.

This initial event seems to be the ultimate origin of all things, as we shall see in the first, PARTICLE EPOCH. In the earliest moments of the fireball, conditions were apparently right for the construction of the subatomic elementary particles from which all matter is made. However, energy was severe at the time, completely overwhelming matter, and breaking it apart as soon as it assembled into anything more ordered. Intense radiation prohibited even the simplest elementary particles from combining into the type of matter that we now call atoms.

Sometime later, the energy weakened and the matter cooled enough to allow some of the elementary particles to combine—a kind of particle evolution—thus forming the simplest and most abundant element, hydrogen. At about the same time, the next heaviest element, helium, materialized when hydrogen nuclei collided with one another. But once the Universe had expanded and cooled a little more, conditions were no longer suitable for the formation of elements heavier than helium. The elements composing the screen (or page) you are now reading, the air we breathe, and the coins in our pockets were not created in the aftermath of the initial expansion. There simply wasn't enough time; events at the start of the Universe happened very rapidly.


Astronomers don’t yet understand how the galaxies formed, yet all the data we currently have imply that the bulk of all galaxies originated during the first few billion years after the big bang. We surmise this partly because observations show that most galaxies are old and partly because we’ve never clearly seen galaxies emerging at the present time. Some may still be growing and evolving, but none seem to have originated in the past many billions of years. Evidently, the great turbulence of the cooling matter (including its dark component) in the aftermath of the early fireball—physical conditions favorable for the formation of galaxies—have changed substantially now, more than 10 billion years after the bang.

Yet galaxies exist. We see them in great abundance virtually everywhere in the Universe. Somehow they got there, and this Web site explores some of the plausible ways they may have done so. But our inability to explain precisely how is perhaps the biggest missing link in the entire scenario of cosmic evolution. How galaxies change or evolve over the course of time is another major problem that astronomers have not yet solved. All these issues we shall tackle in the second, GALACTIC EPOCH.

Figure S.7 shows evidence for two galaxies in the act of colliding, or at least experiencing a close encounter. Such episodes occur over the course of millions of years, but typical results are seen elsewhere among many other galaxies—galactic evolution that not only occurs intrinsically within galaxies, but also via mergers and acquisitions externally among neighboring galaxies. The methods by which they do so are only now being discovered at leading observatories around the world.

FIGURE S.7 FIGURE S.7– This image captures two spiral galaxies apparently passing each other like majestic ships in the night. Strong tidal forces inevitably rearrange their stars, gas, and dust, causing galaxies to change, or experience galactic evolution. This subject mostly combines astronomy with computer science, which helps simulate the inferred changes over timescales much longer than can be observed directly. (STScI)


Although current conditions seem unfavorable for the formation of galaxies, within most galaxies the conditions are ripe for the formation of stars. At numerous places within our own Milky Way, giant parcels of gas are known to be now assembling into stars. Astronomers have direct observational evidence that huge quantities of matter are needed for gravity to hold together a pocket of stellar gas. How much matter? No less than a thousand billion billion billion billion billion billion (or 1057) atoms. That's obviously a large number, in fact much larger than all the grains of sand on all the beaches of the world or all the atoms in the entire Earth. It's larger than anything with which we're familiar because there's simply nothing on Earth comparable to a star.

The lifetime of a star, once formed, depends on its mass. Our Sun is a star of medium mass and has been emitting energy for almost 5 billion years. Based on our knowledge of stellar evolution, the Sun should endure for roughly another 5 billion years, continuing to provide that valued source of heat and light needed to maintain future generations of life on Earth. Because we know it so well, the Sun will be the “model” star against which we compare all others in the third, STELLAR EPOCH.

Toward the end of its duration as a star, our Sun will become unstable and expand in one last gasp, extending its "life" perhaps several million years. Some of the interior planets, including Mercury, probably Venus, and perhaps even Earth, will be engulfed in the Sun itself as it evolves toward this swollen phase close to "death." Thus we can be sure that our planet is eventually doomed. It cannot last forever, and will finally become a dark, dead clinker in space. Much like humans and other forms of life, stars and planets are born, mature, and die. Fortunately, our Sun will not enter such death throes for another several billion years—time enough, if we can survive as a civilization, to undertake galactic engineering projects literally out of this world.

During the final stages of stellar evolution, elements heavier than hydrogen and helium are created in stars’ cores. The heavy elements are fused together at extremely high temperatures within the hearts of stars. In small or medium stars such as the Sun, these “heavies” remain trapped in the stellar interior. But stars that are more massive than our Sun don’t perish so simply. Bigger stars die catastrophically like that shown in Figure S.8, heaving their newly created elements into the surrounding regions of space. In this way, galaxies are regularly enriched by exploding stars that eject heavy elements needed in the formation of later-generation stars, planets, and other interesting things—such as life and intelligence.

FIGURE S.8 FIGURE S.8 — This remarkable photograph shows the remnants of a massive star that exploded long ago, spilling its newly created elements into space. Called the Crab Nebula since it resembles that kind of marine creature, it resides far beyond our Sun yet is still within the Milky Way. Stellar evolution includes the study of the origin of Nature's elements, a subject that combines the study of astronomy and nuclear physics. (ESA/NASA)


Given the orderly arrangement of objects in our Solar System today, the planets didn’t likely originate by a random collision between stars or by some other such rare accident. Instead, the birth of planets is thought to be a natural and frequent by-product of star formation. Although astronomers don’t yet agree on the specific details, they do concur on the following broad outline, developed at length in the fourth, PLANETARY EPOCH.

A huge ball of gas and dust must have become flattened by rotation, after which it fragmented into boulder-sized chunks orbiting at various distances from the centrally forming Sun. Far from the young Sun, where the temperature was low, chemistry favored the production of ices and gases that later accumulated into giant gassy balls like Jupiter; closer to the young Sun, conditions were too warm to form ices but just right to form rocks which then collided, stuck, and became the small rocky planets like Earth. It all happened ~5 billion years ago.

Geologists realize that the earliest stages on Earth have been eroded away by eons of weather as well as by erupting volcanoes and bombarding meteorites. The surface of early Earth was surely barren, with shallow, lifeless seas washing upon grassless, treeless continents (Figure S.9). The atmosphere probably contained chemicals poisonous to most present-day life; oxygen and nitrogen were hardly as abundant as they are now. And the Sun's piercing radiation blazed strongly to the ground; the protective atmospheric layers had not yet fully formed. Although no life as we know it could have existed on primitive Earth, these harsh conditions were apparently among the steps of planetary evolution needed to originate life.

FIGURE S.9 FIGURE S.9 — The environment on our early planet probably resembled a hell-on-Earth. With time—in fact, billions of years of time—our home became more conducive to life in the water, land, and air. Planetary evolution mainly combines the study of astronomy, geology, and chemistry. (Smithsonian)


Experiments conducted in laboratories during the past few decades have demonstrated that energy causes simple chemicals to become more complex. A mixture of ammonia, methane, carbon dioxide, and water "cooked" for about a week, produces some of the building blocks of life. The result of this chemical evolution is not life itself but the basic ingredients needed to form all life, from simple cells to complex humans.

More recent experiments have probed closer to the origin of life. Chemists now know that additional heat causes organic (carbon-rich) molecules to group into oily compounds resembling biological cells (Figure S.10). Curiously, though they are not alive, these organic droplets do seem to simulate matter within the fuzzy domain between the living and nonliving. What’s more, they also resemble the fossilized remains of some of Earth's oldest living organisms found in the sedimentary rocks of our planet. These fossils, radioactively dated to be possibly as old as 3.3 billion years, appear to have structures similar to those of modern blue-green algae (Figure S.11).

FIGURE S.10 FIGURE S.10 — When heated, simple chemicals that must have existed on primitive Earth tend to cluster into carbon-rich droplets. Resembling simple cells a few microns across, such droplets are seen here in this photograph made with a high-powered microscope. Chemical evolution is the process whereby simple chemicals change into complex chemicals, and includes study of the environmental conditions that gave rise to the origin of life. This subject involves some astronomy and biology, but mostly physics and chemistry. (S. Fox)

FIGURE S.11 FIGURE S.11 — Fossils are the dead remains of organisms that once lived. Among the oldest fossils, those (at left) photographed through a microscope are dated to be at least 3 billion years old. They show striking resemblances to the carbon-rich droplets (Figure S.10) produced in laboratory simulations of primitive Earth, and to modern blue-green algae photographed (at right) near today’s backyard streams. (E. Barghoorn)

Scientists now have reliable information suggesting that life is hardly more than a combination of simple chemicals operating in complex ways. Furthermore, we theorize that the origin of life is a natural (though not necessarily inevitable) result of the evolution of that matter. If correct, then the general picture of chemical evolution is in place. Only the details remain to be unraveled, as noted in the fifth, CHEMICAL EPOCH.

Information about advanced life forms is even more complete, partly because we have younger, better preserved fossils. The fossil record does demonstrate how life on Earth became widespread and diversified. Biologists no longer have any reasonable doubt that biological evolution has occurred and is continuing to occur. We now recognize that accidental changes in the genes enable some organisms to be selected for the best available niche within gradually changing environments. The result is that some species perish and become extinct, while others thrive, multiply, and change yet more, as suggested in Figure S.12.

FIGURE S.12 FIGURE S.12 — Changes in the basic structure of some DNA chemicals (whose functional units are genes) enable species to come and go, as well as change, all the while life diversifies. Fossils clearly show evidence of change over long durations of time, as exemplified by this relatively recent (and highly simplified) line of ascent toward humans. Biological evolution studies the changes, from generation to generation, experienced by life throughout the history of Earth. This subject involves a combination of biology and chemistry, and a little geology. (Smithsonian)

Studies of the fossils of dead organisms, as well as of the behavior of those alive, grant perspective regarding the major events of biological evolution. Briefly, as told much greater in the sixth, BIOLOGICAL EPOCH, the facts show the following: Simple, one-celled life began on Earth possibly as long ago as ~3.5 billion years, but didn't advance to more complex forms until ~2 billion years ago. Single cells combined into multicells only ~1 billion years ago, after which life rapidly diversified into a wide variety of increasingly complex organisms—plants, flowers, birds, reptiles, mammals. Surprisingly, humans have existed for only the past few million years—modern humankind in particular for only the past ~150,000 years—a very short time compared to the entire scenario of cosmic evolution.

Useful Analogy Some people have trouble comprehending such lengthy durations. As noted above, millions and billions are such big numbers they often lose their meaning. To better understand Earth's history, consider a helpful analogy. Imagine the entire lifetime of Earth to be 50 years rather than 5 billion years. This time scale is then comparable to a human life span making various highlights of Earth's history more understandable.

With this analogy in mind, we can say that no record exists whatever for the first decade of Earth's existence. Rocks hardened and life arose quickly thereafter. Life might have originated as long ago as 35 years, when Earth was only ~15 years old in our analogy. Our planet's middle age is largely a mystery, though we can be sure that life continued to evolve, and that mountain chains and oceanic trenches steadily built up and eroded down. Not until ~6 years ago, in our 50-year analogy, did abundant life flourish throughout Earth's oceans. Life came ashore ~4 years ago, plants and animals mastered the land ~2 years ago, and dinosaurs reached their peak ~1 year ago, only to die suddenly ~8 months ago. Human-like apes changed into ape-like humans only last week, and the latest major ice age occurred only yesterday. Modern Homo sapiens—our species—did not emerge in this 50-year analogy of Earth's history until ~5 hours ago. In fact, the invention of agriculture is only ~1 hour old, the Renaissance a mere 3 minutes old!

Apparently, it takes time—lots and lots of time—to construct life, intelligence, and civilization.


Cosmic evolution has guided matter from simplicity to complexity, from inorganic to organic. Advancing with time, evolutionary change has fashioned intelligent life on at least one planet—Earth—and possibly on others as well. Since earliest times, evolution has consistently favored those organisms able to gather energy (and information) efficiently. This is apparently true whether the organism was a primitive microbe swimming in the early primordial sea, or a semi-cultured ape-man roaming the ancient forests of our planet.

Fossils document the increase in brain size of our ancestors over tens of millions of years. Some 30 million years ago, our cat-sized ancestors who came down out of the trees—the so-called Aegyptopithecus prosimians—had relatively small brains. These then gave way to the moderately sized brains of the best candidate for the "missing link," the so-called Australopithecus ape-men of several million years ago. Finally, we have the larger brains of our streetwise friends, namely Homo sapiens sapiens, or the wise guys of modern times.

Competition for survival apparently favored monkey-like creatures having an ability, for example, to judge accurately the distance to a banana, as well as to grasp branches and reach out to get that food. Those who successfully secured the food were able to survive and reproduce; those who could not, either fell off the tree and died or stayed in the tree and starved. Only those creatures having traits adaptable to changing environments were naturally selected for survival. Likewise, our more recent ancestors were also granted advantageous traits while inventing fire, tools, language, and agriculture, as well as developing the qualities of foresight, memory, and curiosity. Many of these advances in cultural evolution, one of which is sketched in Figure S.13, had an effect on the brain: It got bigger.

FIGURE S.13 FIGURE S.13 — Controlled use of fire by humans, as well as their invention of tools, language, and agriculture, were among the most important cultural and social advances that helped mold technological intelligent life. Cultural evolution is the study of the changes in the ways, means, actions, and ideas of society, including their transmission from one generation to another. This subject brings together a wide variety of disciplines, including anthropology, geology, biology, and sociology. (Smithsonian)

The maze of cells within our skull—the brain—is among the most complex clumps of matter known anywhere in the Universe. As best we can tell, the human brain is an extraordinary example of the marvelous extent to which matter has evolved over eons of time. The brain is a living machine enabling us to acquire information, to store it as memory, and to pass it on to succeeding generations. It is, in fact, the human brain that allows us to order civilization, to appreciate the arts, to invent technology, and to unlock secrets of the Universe—crowning and ongoing achievements of the seventh, CULTURAL EPOCH. Above all, our brain permits us to study the cosmic contents from which we arose—which is, by the way, a central goal of the cosmic-evolutionary narrative.


The most advanced energy-using, information-gathering animal—that's us—has come to dominate life on planet Earth. The process has been an evolutionary one, wherein the concept of change has played a key role. But now, another basic change is underway—a most significant change. After billions of years of Nature's evolution, the dominant species on Earth is beginning to tinker with evolution. Whereas previously, environments and genes had governed evolution, humans are suddenly gaining control of each of these factors. We are now tampering with matter, thereby causing the environment to change. And we stand on the threshold of altering life, potentially changing the genetic makeup of human beings.

For sure, stellar evolution continues unabated in the hearts of stars everywhere. Chemical evolution still occurs at remote sites such as galactic clouds and exotic moons. Biological evolution persists for most Earth species. And cultural evolution no doubt endures in many corners of our world. But for modern men and women, the essence of evolution itself seems to be evolving.

What’s the cause of this dramatic change—or, we should say, this dramatic change in the way things change? The answer, for the most part, is technology. We've become very intelligent—a lot smarter than any other known species in the Universe. But our technological youth and inexperience are causing problems unlike any faced by previous societies on Earth. Intelligence aside, just how wise are we?

Doomsayers regard technological intelligence as the end of Nature's evolution on our planet. They argue that many of today's global problems threaten to end Earth civilization and perhaps all of Earth life itself, and that eventually one such problem will terminate us. Optimists, on the other hand, do not view technological intelligence as the peak of material development. Instead, they regard technology as a natural stepping stone—an intermediate stage within the grand cosmic evolutionary scenario—enabling us to attain greater heights of consciousness heretofore unimagined.

We cannot know which path will be taken; evolution is not predictable, a point well stressed in this Web site, including in the eighth, FUTURE EPOCH. What we do know is that the cosmic-evolutionary narrative takes us on an intellectual journey far and wide—and perhaps better prepares us for the path eventually taken. Life now contemplates life. It probes matter and energy. It explores the planetary system we call home. It quests for new knowledge.

Cosmic evolution has brought us forth, and now, having done so, enables us to study that very same scenario of cosmic evolution. In a very real sense, we have gained such intellectual prowess as to be able to reflect back upon the material contents that gave life to us.

Provided that we realize our position in the cosmic scheme of things, that we embrace wisdom as well as intelligence as a guiding principle, and above all that we remain curious, thinking beings, then perhaps we can better appreciate and analyze life’s options as our planet and our civilization move forward toward an uncertain future.

In this Web site, we attempt to develop an appreciation for our rich universal heritage. We seek to decipher the nature and behavior of matter on the grandest scale of all. We try, literally, to put life into perspective—a cosmic perspective. And we strive to explain where science now stands in our understanding of the truly big picture—a powerful worldview for the 21st century.


The scenario of cosmic evolution is a human invention. It’s a long and spectacular story, an evolutionary epic that includes the storyteller. Despite its 7 major epochs, this grand narrative was not handed to us on a stone tablet atop some mountain. The scientific community has gradually deciphered the story, is now telling it forthrightly, and continues to refine it as we learn more.

Nor is the idea that we are children of the Universe a new one. That notion may be as old as the earliest Homo sapiens to contemplate existence. Nor is the underlying concept of change especially novel, persistent as it was through the ages, to be sure throughout all the epochs arrayed along the arrow of time. The idea of ceaseless, everlasting change, somehow causing, forcing, or allowing beings to become, was philosophically embraced thousands of years ago.

While entering the new millennium, we can now begin to identify scientifically some of the major astrophysical and biochemical events, indeed changes, that demonstrate the cosmos as the origin and source of our reality. The intellectual approach is decidedly interdisciplinary, interweaving knowledge from virtually every subject universities offer. And—of great import—many parts of the lengthy and ongoing scenario sketched here have recently been confirmed with experimental and observational evidence.

Cosmic evolution is an inclusive working hypothesis that strives to integrate the big and the small, the near and the far, the past and the present, into a unified whole. Though many details remain outstanding, the overall conceptual framework of existence, including the rise of complexity from radiation to matter to life, seems reasonable and comprehensible. To understand, or at least appreciate, all this—with breadth, depth, and a sense of unification among the natural sciences—is the task before us.


This Web site, version 7, derives from an earlier textbook, Universe: An Evolutionary Approach to Astronomy by Eric Chaisson, originally published in 1988 by Prentice Hall and now revised, updated, expanded, and rewritten for this presentation in cyberspace, and from Chaisson's trade book, Epic of Evolution: Seven Ages of the Cosmos, 2006, Columbia University Press. The Advanced Track on the navigation bar at left, which includes separate PDF files for each epoch of this Web site, is based partly on his more advanced book on the same topic, Cosmic Evolution: The Rise of Complexity in Nature, 2001, Harvard University Press; these PDF Advanced Tracks are updated every few months. The contents of this Web site comprise the distilled essence of a course taught mostly at Harvard University for the past 35 years.

© 2013, Eric J. Chaisson. All rights reserved.


Altschuler, D., Children of the Stars, 2002, Cambridge Univ. Press, Cambridge.

Bryson. B., A Short History of Nearly Everything, 2003, Random House, New York.

Chaisson, E., Cosmic Dawn: The Origins of Matter and Life, 1981, Atlantic Monthly Press, Boston; reprinted 1987, W.W. Norton, New York.

Chaisson, E., Epic of Evolution: Seven Ages of the Cosmos, 2006, Columbia Univ. Press, New York (a revised, updated, and expanded version of Cosmic Dawn above).

Christian, D., Brown, C.S. and Benjamin, C., Big History: Between Nothing and Everything, 2014, McGraw-Hill, New York.

Dick, S. and Lupisella, M.L. (eds.), Cosmos & Culture: Cultural Evolution in a Cosmic Context, 2009, NASA Press SP-2009 4802, Washington.

Field, G.B., Verschuur, G.L., and Ponnamperuma, C., Cosmic Evolution: An Introduction to Astronomy, 1978, Houghton Mifflin, Boston.

Lineweaver, C., Davies, P., and Ruse, M. (eds.), Complexity and the Arrow of Time, 2013, Cambridge Univ. Press, Cambridge.

Rees, M., Our Cosmic Habitat, 2001, Princeton Univ. Press, Princeton.

Reeves, H., Atoms of Silence, 1984, MIT Press, Cambridge.

Sagan, C., Cosmos, 1980, Random House, New York.

Swimme, B. and Berry, T., The Universe Story, 1992, Harper San Francisco.


Cosmic Evolution Site (for latest updates):

Multi-Scale Times:

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