Is mortality necessary?

Living things age and die. But there is quite a range of life spans. There is a general correlation between the size of the creature and its longevity, but this is not a strict rule. Human beings find themselves at the larger and longer-lived end of the scale, wondering why—or even whether—death is necessary, how to prolong their lives, and how to avoid the degenerative effects of aging. Whether those are desirable goals is a separate question, both for the individual and the collective. Here, my question is to what extent it is feasible to indefinitely extend human life and healthful functioning. Does anything in nature render it theoretically impossible for the human organism to avoid aging and mortality? (Note that failure to find such an obstacle is not proof against it.) Under what conditions could an immortal conscious form of life evolve naturally or be engineered artificially? Could human intervention in natural biology (e.g. genetically) circumvent aging and death for the human organism—and with what consequences for the individual, the species, and the biosphere? I don’t propose to answer these questions, only to unpack them a little.

The prospect to defeat natural senescence—for example, through gene manipulation—might hinge on a clear understanding of why senescence occurs naturally in the larger evolutionary scheme. Since that includes the whole history of life on earth, this would involve modeling in detail the entire functioning of the biosphere from a “systems” point of view. (Such an understanding would be useful in astrobiology as well, to anticipate possible alien forms of life.) If possible at all, such a project could begin either at the highest or the lowest level. “Synthetic biology” is an approach at the bottom level, attempting to create an artificial analog of DNA that can replicate, self-maintain, and evolve. Even there, success in the simulation does not guarantee success in the real world. To engineer useful biological products is trivial compared to the challenge of a complete theory of all possible biospheres.

Genetic “engineering” usually means specific interventions in natural biology as we know it—not building an organism artificially from scratch, following a “blueprint,” which may not be possible even in principle, let alone in practice. The simple reason is that our ideas about any natural thing can never be perfect or complete. We can only have such exhaustive knowledge about things we make ourselves—that is, about things that follow from given definitions, such as machines and conceptual models. Those have well defined parts that are supposed to be fixed and stable, related to each other in ways specified by their designers. The parts of organisms as proposed by human observers may not correspond to reality. They are ill-defined, squishy rather than hard, and may interrelate in multiple ways that elude human grasp. We cannot properly define what the human organism is, let alone replicate it. (Does it include, for example, the microorganisms that live within it and on its surface, do not share its DNA, yet far outnumber the cells that do? That biome is known to profoundly influence human perception and behavior. It is integral to the health of the body and may be integral to its definition as well.) This fact alone may preclude reverse-engineering the human form, whether to renew it or build it from scratch.

Conversely, a being engineered from an original defining blueprint would be an artifact, not a natural organism, and thus would technically not be human. That is not to say that an artificial person is impossible, whether or not one calls it human. But at the least, an artificial person would first have to be an artificial organism, which is a self-defining system, not merely a product of human definitions and design. To predict whether an artificial organism could be conscious hinges on a thorough understanding of consciousness in natural organisms. Even the extent to which it would behave as a natural human being would depend on details of its structure that may be no more accessible than such details of any natural thing.

Then, what about mortality and senescence as natural phenomena, and the hope of circumventing them through genetic manipulation as currently understood? Single-celled organisms can self-reproduce simply by dividing (mitosis). If this process of self-cloning can go on indefinitely without error, then the original cell is “immortal” in the sense that an identical copy exists any number of generations down the line. But already this is a different notion than the continuing existence of the original cell. It raises the question of how to keep track of an individual cell in a culture of rapidly dividing cells or an individual hydra or fruit fly in a rapidly multiplying population. In any case, one must distinguish between the mortality of the creature as a whole and that of the units composing it. The longevity of a coral colony, for example, should be distinguished from that of an individual polyp. We want to know how the longevity of cells relates to that of the organism they compose.

A cell can divide into two equal cells or into a “mother” and “daughter” cell, with different properties. Stem cells have the capacity to divide into one cell that remains “totipotent” and one that is differentiated to become a specific tissue. (Stem cells of hydra, for example, replicate every three days; those of small rodents every 4 weeks; of cats, every 10 weeks; of human beings, every 50 weeks.) Plants, hydra, and some other simple animals can regenerate a whole new organism from a severed part (in the case of plants, from a single stem cell). Many non-vertebrates can regenerate limbs and other parts, but regeneration is limited in large mammals. Whatever the cause, there are trade-offs between the size and complexity of the organism and the potentials of its various cells.

Even aside from top-down production from design, if a human individual could duplicate itself in the way that a single cell can duplicate itself, there would be serious questions of identity (see the film The Sixth Day). Would the original or the copy be the bona fide person? Presumably they would have different experience (as identical twins do), at least by virtue of occupying different places. If the point of cloning oneself in this way is for “me” to continue living healthfully, then the duplicate version would have to be free from the defects of the original that accrue through aging, disease and accident. In other words, it would not be a copy of the original but a fresh example cast from the original mold, so to speak, but using fresh materials. (Compare the difference between manufacturing two items from the same design and copying a manufactured item already in use, with its imperfections and wear.) To continue to be “me,” the copy would also have to duplicate the psychological identity and all the stored memories of the original. In any case, nature has not opted to clone a complex multi-celled organism in the way a single cell can clone itself.

The mortality of a multi-celled organism is usually associated with sexual reproduction, which separates cells into two types: the “immortal” gene line and the somatic line which forms most of the organism’s tissues. Senescence appears to be “the inevitable fate of all multicellular organisms with germ-soma separation” [Wikipedia: senescence]. Yet, the question is whether senescence and mortality really are inevitable. To put it another way, why are the somatic cells generally not immortal? Since disease causes cell damage, and cell damage is a marker of senescence, we want to know how “built-in” senescence (the Hayflick limit) differs from the effects of disease.

Aging is not merely the passage of time. It has been defined as “a progressive deterioration of physiological function, an intrinsic age-related process of loss of viability and increase in vulnerability” [Wiki]. It might be compared to entropy in physical science: the inevitable increase of disorder. However, life has long been recognized as a process that goes against entropy insofar as it can create local order. The question is whether or how an individual organism could maintain its internal order indefinitely. To a human engineer or designer, it might seem an incongruous design flaw that nature would go to all the trouble to develop a complex multi-celled creature and not provide the ability to permanently maintain itself. However, products of human design often have built-in obsolescence. The designer’s naive goal to produce something perfect and lasting is only part of the bigger picture. The product on the drafting board must serve the corporation’s goals in the marketplace. Just so, the individual specimen must fit in with the system of life as a whole.

Natural selection is a slow and haphazard process. Basically, what exists at a given time is co-determined by the presence of other life forms and their needs, indeed by the entire history of such forms as they have built on each other. The evolutionary tree of life is a game tree of branching and narrowing possibilities. Evolution does not easily go backwards or start entirely new branches. There might not be time for life to begin more than once on a given planet. And once begun, there is no ecological space for a radical alternative to emerge. Rather, evolution cobbles variations onto existing variations, incrementally, with changes only manifest in the succeeding generation. Sometimes they may seem to regress, but always they are new adaptations.

One can imagine (as Lamarck did) an alternative system in which a change in the phenome could result in a change in its genome. The changes in a body that could alter itself adaptively would pass directly into the next generation. But such an organism wouldn’t need a next generation in order to adapt through natural selection! It would reproduce only to maintain and expand the colony. In any case, nature does not work that way, at least for organisms in which genome and phenome exist separately. Perhaps that separation is one of the irreversible choices that life has made while crawling onto its present limb. We can conceive an alternative system in which changes are induced deliberately by human beings (as in genetic engineering or even traditional breeding). But even such imagined possibilities have to fit in with the existing system of nature. We can hardly presume to redesign the whole biosphere—at least not yet, for this planet, without first wiping clean the slate of all existing life, which would of course include us!