When something is alive, it can die. That makes assigning life to some things easy—e.g., plants are alive, but computers are not, even though they have some characteristics that we think about when we reference living creatures, especially “intelligent” life.

The idea of the soul, or spirit, is one of the most fascinating aspects of being alive, at least for humans and probably for other animals. When we refer to dead bodies as “remains,” we are describing the absence of this characteristic of animate creatures. The unifying force that produces actions, personality traits, affective responses is gone; only a mass remains. What is this spirit? For animate creatures, this phenomenon reflects nervous system/ neurotransmitter/electrical activations that work in wonderful concert. This is especially so in humans, whose consciousness allows them to ask questions about being alive, but I don’t think this distinguishes “life” from “not life.” I can’t see how consciousness applies to plants or microorganisms such as bacteria, which I think of as being alive.

Although many people think they will be afraid of the body of a loved one after the conventional signs of life have ceased, in my experience they generally are not. It is comforting and comfortable to embrace loved ones at this time.

There is not a bright line between life and death: it is a continuum. In many situations, animals are considered to be dead if respiration and cardiac function cease, but resuscitation is sometimes effected, meaning that life has either returned or the person or animal never really died. This situation was memorably described by the character Miracle Max in The Princess Bride, when he was asked to treat the apparently dead hero brought in by friends. “It happens that your friend here is only mostly dead. There is a big difference between mostly dead and all dead. Mostly dead is slightly alive.” In these cases, I would argue that death had not actually occurred (at least death of the nervous system/brain). Furthermore, when people stop breathing and their hearts stop, there is a period before the body becomes cold and cannot be resuscitated that is like a twilight: facial expressions may seem appropriate and peaceful, there may be body movements, the body feels supple and warm to the touch. (Although many people think they will be afraid of the body of a loved one after the conventional signs of life have ceased, in my experience they generally are not. It is comforting and comfortable to embrace loved ones at this time in a way that is not true when warmth disappears and rigidity sets in.)

The alive-dead continuum raises other interesting conundrums. For example, zombies are mythical beings—humans who died, but through infection or some other means rise up and walk again, clearly with the reinvigoration of parts of the nervous system (brain stem). Generally, they do not act coherently. Although they may be thought of as being dead (and are often called the “walking dead”), they seem to breathe and they can be “killed.” When this occurs, does something that is “alive” die a second time, or are zombies really dead, but then destroyed when they are killed off? It’s also interesting to ask why such creatures fascinate us. Is it because of the life-death continuum that we carry around in our unconscious minds? There will be more issues like these as technology facilitates resuscitations, cloning, and other interventions that illuminate the continuum between life and death. For example, when people are cryogenically frozen after death—and eventually revived when and/or if the technology becomes available—were they dead while frozen or simply alive in some sort of suspended state? Similarly, it will be interesting to see if robots will eventually seem alive to us when they become sufficiently sophisticated (as in Blade Runner). And they will.

What is life? It seems pretty clear that life on Earth arose from a combination of natural chemical processes in the early oceans, augmented by cometary material falling from the sky. Everything in the universe is made of the same stuff—the 92 atomic elements on that periodic table in your high school chemistry lab.

If you ask, however, what things are made of, you find that life as we know it on Earth is special and distinct. If you were to put the entire universe in a blender and count its atoms, you would find that 93 percent of them are hydrogen—the simplest element of all—and 6 percent are helium, the next simplest. All of the other 90 atomic elements together comprise only one percent of the universe.

The blended planet Earth’s recipe, however, is quite different; its two most abundant atomic elements are oxygen (47 percent) and silicon (28 percent).

Living things are unlike anything else in the universe. Blended life is made up of 60 percent hydrogen, 25 percent oxygen, 10 percent carbon, and 2 percent nitrogen, with smaller amounts of calcium, phosphorus, and sulfur. If you are an animal, your recipe is seasoned with a tad of iron from your blood’s hemoglobin, and if you are a plant, your seasoning is magnesium from your chlorophyll. So life is very different from the nonliving universe—it has a distinctly different recipe.

Here on the surface of the Earth, if you make organic chemical systems complex enough, you get living systems, and we can agree that two important things these living systems do is eat and reproduce.

What is a human being? A walking bag of saltwater with organic molecules on the inside. Some of those complex molecules, proteins, are thousands upon thousands of atoms long, and they do very specialized things. And it’s hard to see how they could have assembled from simple molecules in primordial oceans. It might have been accelerated by more complex molecules delivered by meteors.

The universe will also create complex organic molecules in space, and we see them in the gas between the stars. Here on the surface of the Earth, if you make organic chemical systems complex enough, you get living systems, and we can agree that two important things these living systems do is eat and reproduce. There are something like a half dozen chemical ways that living things extract and store the energy they need to be alive.

And we all know the role that chemical DNA plays in reproduction. The biochemistry of Earth’s living systems is almost certainly not a unique solution to the challenge of becoming (and staying) alive, but it is compelling evidence to me that, at least when they start, organisms are going to be chemical-based and will be the product of organic chemistry.

I would be astonished if extraterrestrial life had DNA that was the same as ours using exactly a three-base sequence for codons and things like that. In fact, if a life-form passed its genetic message down in ways that were different from ours, it would be proof that that life-form was extraterrestrial.

Imagine planet Earth before life: water, rocks, volcanoes, and lifeless landscapes. Now, imagine it millions of years later, increasingly filled with tiny globules capable of growing, self-replicating, and evolving—the first cells. How did this transition occur? How did the laws of physics and chemistry—the same laws that govern the world today—transform the inanimate matter of early Earth into our unicellular ancestors? I think of this as one of the most fascinating open questions in science—at the crossroads of physics, chemistry, and biology. Multiple bits of evidence suggest that the first living cells emerged on our planet around 3.8 billion years ago. To put this in perspective, if the whole history of life, from those early stages, were rescaled to one year, the rise of modern humans would occupy just the last half hour.

The precise details of how life started may be lost forever in the meanders of history. This does not mean that we cannot understand what aspects of chemistry and physics make life possible. A lot of research on the origin of life focuses on understanding the mechanisms that could have given rise to the basic molecular players essential for life as we know it today (such as amino acids, which combine to form proteins, nucleotides that make up genomic DNA, as well as DNA’s versatile cousin, RNA). This is very important. However, even if we put a lot of these molecules together in a test tube, life will not emerge from it. In addition, it is not obvious that the molecules that compose present-day life are a prerequisite for the emergence of life in general. Whether this is the case can significantly affect the way we search for life elsewhere in the universe. To make an analogy at the human scale, imagine that we want to learn about the history of flight. If we focus solely on the materials that make modern airplanes, we would completely miss the very early stages of that endeavor.

For life to evolve and build increasingly complex and sophisticated strategies for survival and reproduction, natural selection has to ensue, and this requires inheritance and variability.

Hence, I think that in order to understand the transition from an early chemical mixture to the first living cell, we need to gain a much better understanding of the fundamental physico-chemical processes that make life possible. Living systems have to be far from thermodynamic equilibrium—i.e., they require a flow of energy to stay organized. Lack of an energy input results in disorder (very much like what happens in a typical household!). One of the key properties made possible by this energy flow is the capacity of living systems to self-reproduce. A subtle and often debated aspect of self-reproduction is that no single molecule ever has to self-replicate in order for a cell to reproduce, or for life to begin. Rather, one can think of cellular life as a collective phenomenon, in which many different types of molecules help one another replicate in an intricate, but well-organized, network of chemical reactions. Inheritance and variability, essential for establishing an evolutionary process, could be a natural outcome of the dynamics of these complex networks, even prior to the advent of genomes.

Understanding all these processes and how they relate to one another will constitute a key component of our path toward understanding life and its origin. It will require the cooperation of scientists from different disciplines and the combination of theory and experiment. One should keep in mind that the early appearance of living cells is just the first step in a long series of fascinating transitions in the history of life. For example, we still know very little about the conditions and processes that gave rise to complex multicellular life, without which we would not be here, searching and contemplating.