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
(Wikipedia)

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.
(Wikipedia)

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.

Emergent Phenomena: More Than the Sum of the Parts

I’ve been seeing the word emergence more and more in the last few years. But apart from the obvious sense that something arises, the meaning of the term—and what the excitement is all about—haven’t  been clear to me.

So a helpful source that I will summarize here has been “The Sacred Emergence of Nature” by Ursula Goodenough and Terrence Deacon (2008). As the title suggests, the authors not only describe emergence but also discuss its place in the perspective of religious naturalists.

The adage that “the whole is more than the sum of the parts” conveys a rough idea of the principle of emergence. Emergence occurs when a combination of entities has characteristics that are unlike the characteristics of its components. The common example is water: it combines hydrogen and oxygen but is like neither of those gases.

Goodenough and Deacon emphasize that emergence is the counterpart of reductionism, the process of breaking entities down into their parts. Though it tells us much about what a substance is made of, reductionism tells us little about how the parts came together in the first place and how properties emerged. In short, as the authors put it, reductionism is running the movie backwards. Emergence, in contrast, runs the movie forwards to show atoms forming compounds which then form  structures, and even how life may have begun and developed.

(azquotes.com)

(azquotes.com)

Two gasses merge to form a very different molecule of water. When, in turn, two or more water molecules come together, they again display characteristics as a solid, liquid, or gas that the single water molecule doesn’t possess.

In the same way, a sequence of atoms, molecules and complex compounds, merging and emerging one from the other, may have created life. The pivotal moment, according to Goodenough and Deacon, occurred when the sequence happened to create over again one of the first chemicals in its chain. At that point a cycle was created, the basis of the self-sustaining quality that is characteristic of life. Energy (food) would be needed, along with an internal recipe for the proper sequence (DNA), and a living things could emerge.

Goodenough and Deacon emphasize, interestingly, that it is not this coded recipe, the genome, that is driving the system. “Selfish genes” are not in control. “Genomes are in fact the handmaidens of emergent properties, not the other way around…. The whole point of life is to generate emergent properties that, if successfully executed, have the additional feature of permitting transmission of genomes.” It is the organism and its emergent properties that must survive and reproduce if the genome is to make  it through to the next generation.

As organic entities increase in number and complexity, examples of emergence abound. Molecules merge to form proteins, proteins merge to carry out organic functions, functional parts converge to form organs, neural cells form brains, brains merge to create mass behavior, language, ideas, cities, the Web.

Finally, Goodenough and Deacon describe the place of emergence in the view of nature and biology as sacred. Selected sentences from this rich discussion will have to suffice here. The theme is that emergent properties, by virtue of their originality, lie at the heart of what is wondrous and transcendent throughout nature.

On our place in nature: The understanding that human-specific traits are emergent—something else popping through from all that has gone on before and continues to surround us—is fully consonant with what we now know about the course of natural history, and a deeply satisfying way to think about who we are….Evolutionary theory asks us to situate the human in the natural world, and this can generate cognitive dissonance given that our mental capacities would seem to place us ‘above’ the natural world and our cultures ‘above’ the natural order. The emergentist perspective allows us to see ourselves not as ‘above’ but rather as remarkably ‘something else.’

On the magical and transcendent: The emergentist perspective opens countless opportunities to encounter and celebrate the magical while remaining mindful of the fully natural basis of each encounter. There is a way in which the universe is re-enchanted each time one takes in its continuous coming into being, and there is a way in which our lives are re-enchanted each time we realize that we too are continually transcending ourselves.

On morality: One’s moral framework is not some instinct that just bubbles up. It is something that each of us constructs, amplifying and reconfiguring primate social emotions in the context of cultural stimuli and teachings.

I understand emergence better now and appreciate it more. But I’m wondering about the relationship between emergence and evolution. Both name foundational aspects of how new things and characteristics come to exist. But I’m not sure whether they are most effectively viewed as two processes or as different aspects of a single process. Can mutations, the genetic glitches that open opportunities for biological evolution, be viewed as stages in emergence? Or are they “components” in the emergent process of an exceptional kind?

Any thoughts about this question or other aspects of emergence will be welcome.

Genesis for Non-Theists

Creation narratives are lively stories.  In the Bible, God creates the universe and earth in six days. In other traditions, creatures are dismembered, huge eggs hatch, birds create land. Even science’s own creation narrative starts with a Bang and once earth takes shape, the first organic molecules appear relatively quickly, within a billion years. 
 
But at that point the scientific story of life slows way down. Life remains at the stage of single cells for the next two billion years. What was happening to our smallest, oldest ancestors all that time? Why did it take so long to move beyond the stage of one-only? Was evolution on hold?
timeline

From “Oldest bacteria fossils” to “Multi-cellular eukaryotes” 2 billion years later, life on earth was single-celled.
(vector-clip-art.com)

What took so long was the creation of the building blocks for being alive. It’s a story with parallels to the first chapters of Genesis. The biblical sequence: plant life emerges on the third day, including “fruit trees bearing fruit in which is their seed,” followed over the next three days by creatures of the water, air, and land, including man and woman. A few verses later we read about the Garden of Eden and, symbolically, the beginnings of sex and death.

Here briefly is science’s version: life evolved from the simplest cells to cells with a nucleus that enclosed the protected “seed” of DNA. This change set in motion the end of one kind of immortality, the beginnings of sex and death, and the emergence of a new immortality.

The process was slow because the changes were huge.

LUCA
Like the Bible, science has a name for our first ancestor. LUCA, our “last universal common ancestor,” was a single-celled organism, a kind of bacterium, from which all life on earth is descended. Inside LUCA was a floating coil of DNA, sections of which have been passed down to every living thing.
Prokaryote

Our common ancestor, a cell with DNA but no nucleus
(shmoop.com)

LUCA reproduced simply by dividing, with one set of genes in each new cell. The new cells were identical, a long line of Adam clones without an Eve.

LUCA’s membrane enclosed only watery liquid and the genes. Gradually LUCA’s descendants “ate” and absorbed other bacteria. Some of these bacteria turned into the nucleus of the cell that absorbed them. They became the container for the cell’s genes. Such cells advanced from  prokaryotes (before a nucleus) to eukaryotes (a true nucleus, and pronounced “you carry oats”). The nucleus was a seed, a seed that provided the DNA with a chemical environment of its own and helped grow more complex DNA and much larger cells.
Sex, Death…
cell

Cells get a nucleus–and more.
(biogeonerd.blogspot.com)

Early cells were, in their own way, immortal. The genes in both prokaryotes and early eukaryotes would reproduce and then the cell would split into two identical cells, as bacteria still do. Did such cells die? Eventually, but only from accident or the environment. In this Eden, cells did not get older. They became their own offspring and could theoretically live forever.

Eukaryotes, however, found a new way to reproduce. One would rub up against another eukaryote and portions of their DNA sets would be inserted into the other—the original sex act. With this exchange of DNA, genetic variation sped up, at last. So did natural selection.
 
In the next step, sex became specialized. As some early organisms became multi-celled, such as algae, they reproduced not by division of the whole parent organism but, as with us, by means of specialized germ cells (not the disease kind of germ but the creative kind, as in the “germ of an idea”).
No longer was the parent reincarnated in a clone, as in bacteria. It was left behind, and it aged and died. As in Genesis, the co-mingling of different living things brought sex and death. Cellular life moved beyond Eden.
 
…and Immortality
So we have lost the immortality that the prokaryotes enjoyed. But we have found it in another, more complex form. Our immortality runs through the genetic line of our children and other blood  relatives. It turns out that it is not the body, the soma, that is the crucial package. It is the germ cells that carry the DNA forward. 
 
But is this an adequate and satisfying idea for us humans who dream of living forever? Is the continuity of DNA a meaningful form of immortality? Here is one answer from Harvard biology professor George Wald, in his 1970 lecture on “The Origins of Death.”
 
We already have immortality, but in the wrong place. We have it in the germ plasm; we want it in the soma, in the body. We have fallen in love with the body. That’s that thing that looks back at us from the mirror. That’s the repository of that lovely identity that you keep chasing all your life. And as for that potentially immortal germ plasm, where that is one hundred years, one thousand years, ten thousand years hence, hardly interests us.
 
I used to think that way, too, but I don’t any longer. You see, every creature alive on the earth today represents an unbroken line of life that stretches back to the first primitive organisms to appear on this planet; that is about three billion years. That really is immortality. For if the line of life had ever been broken, how could we be here? All that time, our germ plasm has been living the life of those single celled creatures, the protozoa, reproducing by simple division, and occasionally going through the process of syngamy — the fusion of two cells to form one — in the act of sexual reproduction. All that time, that germ plasm has been making bodies and casting them off in the act of dying. If the germ plasm wants to swim in the ocean, it makes itself a fish; if the germ plasm wants to fly in the air, it makes itself a bird. If it wants to go to Harvard, it makes itself a man. The strangest thing of all is that the germ plasm that we carry around within us has done all those things. …
I, too, used to think that we had our immortality in the wrong place, but I don’t think so any longer. I think it’s in the right place. I think that is the only kind of immortality worth having — and we have it.