Stem Cells: How To Build and Maintain Bodies, Including Plants

Until recently, I didn’t know much about stem cells except that they produced other kinds of cells and that the medical research on them was controversial. In the context of the history of life, it turns out, their importance is as fundamental as you can get.

It took more than a billion years for the first cell with a nucleus to come together. Since then, the only reliable source for a new cell has been another cell. Every cell is an offspring. True for plants as well as animals.

An embryonic stem cell (Wikipedia)

An embryonic stem cell

But while cells are specialized for one task or another, they are not always very good at dividing and reproducing. Muscle cells, blood cells, and nerve cells don’t reproduce at all. Other cells in the body divide only under some circumstances or only a limited number of times.

But reproduction is the stem cell’s specialty. When it divides, it produce another stem cell, ready for the next round, along with a muscle cell or blood cell or nerve cell or a cell of another organ. It looks the part for such flexibility—blob-like, unstructured, not committed until needed.

Stem cells are stationed throughout the body, small groups of them in each organ, like local hospitals on call to repair the sick and damaged. They are a profound piece of bodily engineering, a design for the long-term, like a futuristic car that carries little 3-D printers throughout the engine and chassis to create new parts and replace the old parts automatically.

In human embryos, in contrast to adults, stem cells literally build the body. When an embryo is only a few days old, its stem cells begin to form all—all—of the specialized cells needed in a body, some 200 of them.

In this root tip, the number 1 marks the relatively unstructured stem cells in the meristem. (Wikipedia)

In this root tip, the number 1 marks the relatively unstructured stem cells in the meristem.

Plants have stem cells too. Located near the tips of the roots and stems in a layer called the meristem, plant stem cells divide into both specialized cells for the plant and additional stem cells. Stem cells are, in other words, the place where a plant grows.

One of the wonders of any living thing is the sheer variety of its parts, the inventory of its tubes, organs, fluids, surfaces, protrusions, electric circuits and rigid pieces. As we pause to appreciate this profusion, sing the praises of the smudgy cell that creates and repairs them all.

Steven Pinker on Emotions and Genes

Steven Pinker’s How the Mind Works might well be subtitled “And the emotions too.” It’s one terrific book. It offers a barrage of insights and connections about humans and evolution that can feel intoxicating. It stirs up the nature-nurture controversy with a blender. It does not see you as you almost certainly see yourself. And it is often very funny.

Despite the book’s title, Pinker talks more about emotions than about the mind itself. He sees them working together. The mind, he says, is computational: it processes information. Much of this information comes from the body’s biological systems. Emotions are units, modules, that use this bodily information to take direct steps—fear, anger, hunger, lust, egotism, empathy—that will promote survival and reproduction.

Humans, Pinker writes, are not, as we often believe, divided into thoughts and feelings that work against each other.

The emotions are adaptations, well-engineered software modules that work in harmony with the intellect and are indispensable to the functioning of the whole mind. The problem with the emotions is not that they are untamed forces or vestiges of our animal past; it is that they were designed to propagate copies of the genes that built them rather than to promote happiness, wisdom, or moral values. We often call an act ‘emotional’ when it is harmful to the social group, damaging to the actor’s happiness in the long run, uncontrollable and impervious to persuasion, or a product of self-delusion. Sad to say, these outcomes are not malfunctions but precisely what we would expect from well-engineered emotions. (Kindle location 7688)

So the good news is that our seemingly perverse emotional moments do not mean that something is wrong with us. But the not-so-good news is that our emotional acts are more deeply engrained in us than our well-meaning searches for happiness, wisdom, and virtue.

So are we doomed by the genes that build these emotional responses, responses that often mean we get  carried away just when we want to stay cool and collected? Pinker addresses this issue often, here in a discussion of love:

The confusion comes from thinking of people’s genes as their true self, and the motives of their genes as their deepest, truest, unconscious motives. From there it’s easy to draw the cynical and incorrect moral that all love is hypocritical. That confuses the real motives of the person with the metaphorical motives of the genes. Genes are not puppetmasters; they acted as the recipe for making the brain and body and then got out of the way. (8342)

I like the recipe metaphor. As I take it, genes are like the list of the ingredients and the steps for making a cake, but the flavor and texture of the cake itself is quite different from that sheet of instructions.

A human

A human “cake” and his genetic “recipe”

But if the genes have built emotions to keep us alive, doesn’t that mean those emotions are quite inflexible? Yes and no. Our own emotional core might not change much in our life time, but in species-time, the story is different.

Might the software for the emotions be burned so deeply into the brain that organisms are condemned to feel as their remote ancestors did? The evidence says no; the emotions are easy to reprogram. Emotional repertoires vary wildly among animals depending on their species, sex, and age. Within the mammals we find the lion and lamb. Even within dogs (a single species) a few millennia of selective breeding have given us pit bulls and Saint Bernards. (7721)

Pinker, in conclusion, tells us about ourselves in ways we may have difficulty recognizing. Modules and systems fine-tuned to an ancient past may seem non-human and even anti-human. But it’s not so difficult to absorb how science depicts the machinery of our emotions at the same time that we are inquiring thoughtfully about the meanings of our lives. Or, to put it another way, we can come to understand our recipe while we ponder what it is like to be the cake.

For more on the man, the book, and the debate, here is a lively and helpful article.

The Most Amazing Thing About Life

“The most impressive aspect of the living world is its diversity. No two individuals in sexually reproducing populations are the same, nor are any two populations, species, or higher taxa [categories of organisms]. Wherever one looks in nature, one finds uniqueness.” So wrote Ernst Mayr in This is Biology, published in 1997.

Grains of sand under an electron microscope (wikipedia)

Grains of sand 

This was, to an extent, a new idea to me. Clearly each species is different from the next. But I had not fully absorbed the notion that every single organism, if it reproduces in pairs, is different from every other in its species. Every individual grass plant, every tree, every insect, every ant is as different from the one next to it as two human beings are. Why? As Mayr explains, diversity ensures that some individuals will fit the environment; as the environment weeds out some versions, others will survive.

But what about the diversity in the non-biological, inanimate world? “Nature” includes not only living things but also rocks, water, air, light and other forces and materials. Aren’t they unique in their own ways? Snowflakes are famously singular. Clouds are constantly changing. So is the surface of the ocean. Air flows and spins. I’m pretty sure I’ve never seen two rocks that are identical. It’s a good bet that every asteroid, planet and star is different from the others in some ways. Looking out over the dessert, the ocean, or the skies, we certainly see diversity in shape, motion, color and light.

Diversity and fertility in grass (


Still, I think the diversity of living things does “impress” us, as Mayr wrote, in a distinct way. The variety of the organisms in a species is more individualistic, unique, than the motions, colors and contours of the elements around us. And it captures our attention partly because of the sheer power of fertility. New life is always thrusting itself in front of us—in a sister’s baby, in a new puppy, among the trees at the back of the backyard, in the horde of ants and bees and birds of spring. In Origin of Species, Darwin wrote, “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.”

Diversity is the music, but fertility is the amplifier. It’s the combination that gets our attention. And we are not remote observers. We are both notes in the music and members of the orchestra.