molecules. This hidden machinery enables a cell to persist and maintain itself, to take in substances and energy from the environment and use them to carry out the metabolic reactions without which there is only death.4
At the turn of the century, the substructure and organization on which the animation of cells depends was largely beyond the resolving power of the microscopes. But it was clear enough that not all cells are alike in their composition and structure. Bacteria and protists have rather little in the way of visible internal organization. They belong to a class of micro-organisms called prokaryotes, and they are typically round or elongated and sausage-shaped. The language of biological classification is always a little presumptuous, but it takes nothing away from bacteria to say that their cells are structurally relatively “simple”. They lack a nucleus – hence the label “prokaryotes”, meaning “pre-nucleus”. (More presumption – as though bacteria just haven’t yet discovered the wisdom of having a nucleus but will wake up to it one day. In fact, bacteria have existed for longer than eukaryotes; they and other prokaryotes dominate much of the planet’s ecology, and evidently have no need of “greater sophistication” in order to thrive.)
Human cells, along with those of other animals, plants, fungi and yeast, are said to be eukaryotes: a term that simply connotes that their cells have a nucleus. Eukaryotic organisms may be multicelled, like us, or single-celled, like yeast. The latter is an example of a “lower” eukaryote: more presumption, of course, but meaning that the degree of organization in the cell is less than that evident in the higher eukaryotes like peas, fruit flies and whales.
For now we can set prokaryotes aside. There is, mercifully, no need either to look in detail at what all the complex structure of the human cell is about, other than to say that it can be usefully regarded as a compartmentalization of the processes of existence. Membrane-wrapped substructures of the cell are called organelles, and each can be somewhat crudely considered to carry out a specific task. Mitochondria are the regions where a eukaryotic cell produces its energy, in the form of small molecules that release stored chemical energy when transformed by enzymes. The Golgi apparatus functions as a kind of cellular post office, processing proteins and dispatching them to where they are needed. The nucleus is where the chromosomes are kept: the material encoding the genes that are passed on when a cell divides or an organism reproduces. What we do need to hear more about, very shortly, are those chromosomes, because they are an important part of what defines you as an individual, and absolutely vital for orchestrating the life processes that enabled you to grow and which sustain you daily.
The human cell.
By the early twentieth century, it was clear that what sets living matter apart from the inanimate is not merely a question of composition: of what life is made of. Neither is it just a question of structure. Organisms and cells clearly did have a hierarchy of significant, specific yet hard-to-interpret structures reaching down to the microscopic and beyond. And that mattered. But the real reason living matter is not equivalent to some other state of matter such as liquids and gases is that it is dynamic: always changing, always in the process of doing something, never reaching a steady equilibrium. Staying alive is not a matter of luxuriating in the state of aliveness but is a relentless task of keeping balls in the air.
Researchers today might rightly point out that this dynamic, out-of-equilibrium character is not unique to life. Our planet’s climate system is like that too: a constant channelling of the energies of the sun and of the hot planetary interior into orchestrated cycling movements of the oceans, atmosphere and sluggish rocky mantle, accompanied by flows of chemical elements and heat between the different components of the planetary system. The system is responsive and adaptive. But this is precisely the point: there are parallels between a living organism and the planet itself, which is why the independent scientist James Lovelock pushed the point from analogy to the verge of genuine equivalence in his Gaia hypothesis. Arguments about whether the planet can be truly considered “alive” are moot, because the living systems – rainforests, ocean microfauna, every creature that takes in chemicals and turns them into something else plus heat – are in any case a crucial, active part of the planet’s “physiology”.
This activity of the planetary biosphere commenced close to four billion years ago and has not ceased since. Virchow’s omnia cellula e cellula has a significance barely any lesser than that of Darwinian evolution, which ultimately depends on it (ironically, given Virchow’s views on Darwin). It establishes a basis for what Aristotle imagined as a Great Chain of Being, in which the fundamental unit is no longer the reproducing organism but the dividing cell. All cells are, in evolutionary terms, related to one another, and the question of origin reduces to that of how the first cell came into being. Since that obscure primeval event, to the best of our knowledge no new cell has appeared de novo.
At the same time, Virchow’s slogan is a description, not an explanation. Why is a cell not content to remain as it is, happily metabolizing until its time runs out? One answer just begs the question: if that is all cells did, they would not exist, because their de novo formation from a chemical chaos is far too improbable. Then we risk falling back again on anthropomorphism – cells intrinsically want to reproduce by division – or on tautology, saying that the basic biological function of a cell is to make more cells (“the dream of every cell is to become two cells”, said the Nobel laureate biologist François Jacob). Biological discourse seldom does much better than this. Cell and molecular biologists and geneticists have a phenomenal understanding of how cells propagate themselves. But explaining why they do so is a very subtle affair, and it’s fair to say that most biologists don’t even think about it. Yet that “impulse” is the engine of Darwinian evolution and consequently at the root of all that matters in biology.
There is not a goal to this process of life, towards which all the machinery of the cell somehow strives. We can’t help thinking of it that way, of course, because we are natural storytellers (and because we do have goals, and can meaningfully ascribe them to other animals too). So we persuade ourselves that life aims to make babies, to build organisms, to evolve towards perfection (or at least self-improvement), to perpetuate genes. These are all stories, and they can be lovely as well as cognitively useful. But they do not sum up what life is about. It is a thing that, once begun, is astonishingly hard to stop; actually we do not know how that could be accomplished short of destroying the planet itself.
* * *
Life’s unit is the cell. Nothing less than the complete cell has a claim to be called genuinely alive.5 It’s common to see our body’s cells referred to as “building blocks” of tissues, much like assemblies of bricks that constitute a house. To look at the cells in a slice of plant tissue, such as Wilson’s drawing of the onion earlier, you can understand why. But that image fails to convey the dynamic aspect of cells. They move, they respond to their environment, and they have life cycles: a birth and a death. They receive and process information. As Virchow suggested, cells are to some degree autonomous agents: little living entities, making their way in the world.
Anything less than a cell, then, has at best a questionable claim to be alive; from cells, you can make every organism on Earth. We have known about the fundamental status of the cell for about two centuries but have not always acknowledged it. For much of the late twentieth century, the cell was relegated before the supremacy of the gene: the biological “unit of information” inherited between generations. Now the tide has turned again. “The cell is making a particular kind of reappearance as a central actor in today’s biomedical, biological, and biotechnological settings,” writes sociologist of biology Hannah Landecker. “At the beginning of the 21st century, the cell has emerged as a central unit of biological thought and practice … the cell has deposed the gene as the candidate for the role of life itself.”
Cells do
