Having encountered many of the astronomical and physical (or “astrophysical”) concepts needed to appreciate the origin and evolution of matter, we now consider some of the biological and chemical (or “biochemical”) ideas central to the origin and evolution of life. It is the synthesis of these two vibrant interdisciplines that comprise the essence of cosmic evolution.

We are moving across the threshold connecting matter and life. And although life itself is closer to us in space and time—indeed, we are life—that doesn’t necessarily mean that we understand it any better than matter. The reason is that living systems are so much more complex than any inanimate objects; a potted plant is more complicated than the most splendid galaxy, a simple amoeba more intricate than the most internally layered star in the sky. Much as some missing links hamper our current knowledge of distant stars and galaxies, gaps also plague our understanding of the history of life right here on Earth.

Yet, each day brings new discoveries, tests, and refinement of our modern ideas of chemical, biological, and cultural evolution. And with these advances comes greater objectivity, and progress too, in our search to know reality. In this, the fifth, CHEMICAL EPOCH, we explore the ways and means that the building blocks of life eventually became life.

Properties of Life Let’s first ask, what is life? And immediately we are stumped. By contrast, physicists throughout the world, regardless of country or creed, agree on a definition of matter—namely, anything that has mass and occupies space. Matter is among the basic stuff of the Universe and we have a reasonably good idea how it (at least detectable, normal matter) behaves from quark to quasar. But biologists are hard-pressed to offer a clear, concise, standard definition of life. At issue, again, is life’s complexity. Life is so intricate, it’s hard to describe even though we ourselves are examples of it! Frankly, the biological community has been unable to reach a uniform consensus about life’s true character.

Usually, biologists attempt to define life operationally by appealing to its practical properties. By noting a few of life’s attributes, we can begin to know its important features, especially those that distinguish life from matter. We might, for example, suspect that living systems differ from nonliving systems because the whole is greater than the sum of the parts of which it’s made. An individual cell dies when removed from a living organism of which it was a part, since the interactions of that cell with other parts of the whole living organism are vital to the cell’s health. On the other hand, should a single cell be nourished in a comfortable laboratory environment having optimum temperature and density—a so-called culture medium—such a cell could once again flourish outside its original living organism.

At first glance, then, we might take the italicized property above to be a peculiarity of life. But on second thought, this property is not at all restricted to life, for it’s also a property of matter. To see this, imagine removing a small part of a star normally fusing hydrogen into helium. The extracted chunk of matter would no longer release nuclear energy, since it would immediately disperse into space and grow cold. Yet if that chunk were surrounded by additional matter having an appropriate temperature and density, it would once again shine as brightly as before.

These statements are not meant to suggest that stars are somehow “living.” Quite the contrary. It is precisely because we can be sure that incredibly hot stars cannot possibly be alive that this comparison demonstrates how tough it is to define life. Thus we cannot claim that the “whole being greater than the sum of its parts” is a property solely of living systems. This property applies equally well to many objects that are not living, as in a watch, for instance, which is surely more than the sum of the gears and springs (or nowadays silicon chips and integrated circuits) of which it’s made. A watch’s structure is made of atoms, but its function tells the time!

Biologists often claim that the ability to heal itself is a peculiar property of a living system. A shallow cut on a finger, for example, usually heals quickly and the system goes on living. On the other hand, the aforementioned star from which a small chunk of matter was extracted would also eventually “heal” itself. The star would adjust a bit, eventually attaining a new balance between the inward pull of gravity and the outward pressure of heat. Having resumed its original spherical shape, the star would then go about its business of shining as a perfectly normal, though slightly smaller star.

We might say that living systems have a special property that allows them to react to unforeseen circumstances. But a star wouldn’t expect to have a small part hypothetically removed, yet it would react quite adequately to this unexpected occurrence. Stars can react, and adapt, to new states too.

The ability to reproduce is clearly a special property of living systems. Still, we could imagine a contracting protostar which, because of faster and faster rotation, divides into two separate protostars. In this way, angular momentum is sometimes judged an agent of replication, or at least subdivision. Admittedly, this example probably occurs rarely, yet it has undoubtedly happened many times in the billions upon billions of years since the start of the Universe. Some of the binary stars in our Milky Way Galaxy may well have been formed in just this way. A better example of “replication among the stars” might be the process of sequential star formation proffered toward the end of the third, STELLAR EPOCH, whereby the concussive deaths of some stars naturally lead to the birth of other stars. Furthermore, mules don’t reproduce and neither do sterile men—in fact neither do any individual humans replicate, rather two are needed—so perhaps reproduction isn’t such a definitive, unique quality of life.

Surely, some property must be associated with life and only life. Bioscientists often raise the possibility that living systems can learn from experience. Most living organisms do have a memory of sorts. Yet some nonliving systems can also remember, and even learn from experience, such as chess-playing computers. When a well-programmed computer makes a mistake, it doesn’t forget it. These so-called neural networks can store mistakes in their hardware memory, never to be made again under the same circumstances. Accordingly, few humans can beat our best computers at chess and no one can beat them at checkers or blitz-chess (when the timescale for moves is much shortened). So some of our more advanced machines, which are still merely clusters of matter, can seemingly learn from experience, much like living systems.

Finally, life is often operationally defined as having an entire hierarchy of functions. Much of the activity of living systems is controlled by chemical hormones; hormones in turn are controlled by secreting organs called glands; glands by brain cells, and so on. Such hierarchies characterize all living systems from simple amoebas through advanced humans. Similarly, though, we can regard nonliving matter as being controlled by a hierarchy of functions: The motion of the Moon is dictated by Earth; Earth’s motion in turn is directed by the Sun; the Sun by the Galaxy, and so on through the galaxy superclusters. Many material systems have governing hierarchies that resemble those of living systems.

The point worth stressing is that we cannot easily specify any property applicable to life, and only life. Apparently, under some circumstances, common properties of life can also apply to matter. In short, there seems to be no clear dividing line between what’s alive and what’s not—no obvious distinction between matter and life.

All this back-and-forth discussion reinforces the notion that life is surprisingly difficult to define, even operationally. The old saw, popular even among biologists, that “I know life when I see it,” is cute, but not useful in a scientific context. An idiosyncratic definition of life will be offered toward the end of this fifth, CHEMICAL EPOCH.

Degree of Difference Living and nonliving systems, then, do not seem to differ in kind. Their basic properties cannot be readily distinguished. However, living and nonliving systems do differ in degree. It is generally agreed that all forms of life are more complex than any form of nonliving matter.

As a result, we could reasonably postulate that life is merely an extension of the complexities of matter. If correct, then everything around us—galaxies, stars, planets, and life—might well comprise a grand interconnected spectrum or ranked order of all known objects in the material Universe, including ourselves. This is the crux, the very heart and soul, of the interdisciplinary subject of cosmic evolution.

Is Life Inevitable? A central issue for the CHEMICAL EPOCH is Nature’s path from simplicity to complexity: Does it always lead from matter to life? In other words, is life’s origin merely a natural event, or is it perhaps inevitable? Given the laws of physics and chemistry, as well as the proper ingredients and much time, the subject of biology would seem to arise naturally. The path from atoms to molecules to life seems straightforward enough, based on what is known of modern science today. But it’s not a certainty.

An important, though as-yet unanswered, question concerns the direction and nature of the path from matter to life. Is there only one way that complex matter eventually becomes life? Or, can molecules cluster in many ways to create life? Both of these choices, sketched diagrammatically in Figure 5.1, are consistent with the basic ideas of cosmic evolution, yet the chances for life elsewhere in the Universe depend critically on which of these two cases pertain.

FIGURE 5.1 — The complexification of matter might lead directly and inevitably to life, given the right conditions and enough time (a), or there could be numerous paths toward complexity, only one (or few) of which lead to life (b).

One case (Figure 5.1a) depicts a single path from matter to life. Provided that the environmental conditions are not adverse and time is abundant, matter becomes increasingly complex until eventually some system resembling a single cell originates. We have no way of knowing how long this process of chemical evolution usually takes. The time scale probably depends mostly on the surrounding physical and chemical resources. Temperature, density, energy, and raw materials all play key roles in the origin of life. Furthermore, many false starts are likely wherein life almost forms (or even does so temporarily) only to be destroyed quickly thereafter.

If life definitely emerges once a certain complexity of matter is reached—a critical threshold—we can be reasonably sure that life is not only a natural consequence of the evolution of matter but an inevitable one as well. A rather direct evolutionary path from matter to life greatly increases the chances that life resides elsewhere in the Universe.

On the other hand, if matter can become complex in many ways, only one (or few) of which leads eventually to life, we cannot justifiably claim that life is inevitably produced from matter. Life would indeed be a natural consequence of the evolution of matter, but not an inevitable one. This second case (Figure 5.1b) implies that vastly complex clusters of matter might form without ever crossing a threshold to become a system rightfully judged “living.” If so, the prospects for extraterrestrial life are poor.

Reality might also lie between these two extreme cases, with the likelihood for life’s origin being moderate and the prospects for extraterrestrial life neither good nor bad. To repeat a theme often implied throughout this Web site: Nature is often neither black nor white, but more like shades of gray everywhere.