How Do Women Make Living People Out of Dead Stuff?

…the answer has cosmic implications

Spring semester is winding down. As usual, I’m teaching Introductory Biology and a neuroscience course about brain function. This one's called “Medical Mysteries of the Mind.”

Because all of my courses deal with living organisms, each starts with the same question: “How did life first emerge on the Earth some 3.5 billion years ago?”

I begin with that question because you can’t really understand biology unless you understand what life is. And, I don’t think you can understand what life is unless you understand how it started.

Most instructors don’t bother with this topic because it’s one of those ‘hard’ questions, hard to explain and difficult to teach. I guess I’m not clever enough to avoid it.

It’s not just me who thinks this is a ‘hard’ question. I’ve just finished my third reading of the book Life as No One Knows It: The Physics of Life's Emergence by Sara Imari Walker. (I think I need to read it one more time.) Sara is an intellectually creative theoretical physicist and astrobiologist interested in discovering the laws of physics that could explain how and why life emerged on Earth, and how we could recognize alien life if we found it on another planet. She and her collaborators think it’s a pretty hard question, too.

However, it seems to me that if we set aside the physics and mathematics for a moment — those are the really hard parts — and just think about the processes, the question becomes tractable, and the answer approachable.

The interesting thing about those life-generating processes is that we all go through them every day without realizing it. But, let’s be honest, women do them best.

When I first pose the emergence-of-life question to my students, they have a difficult time imagining how living things could come into existence on an ancient earth composed only of dead stuff like water, elemental atoms, rocks, and gases. “It makes no sense,” they’ll say. “How can dead stuff come alive?”

I understand their skepticism. After all, they’ve been taught repeatedly that the spontaneous generation of life — at least as proposed by the ancients — is ‘dead’ wrong.

So, to nudge them toward an answer, I tell them a story…

Building a Living Person from Dead Stuff

Imagine you just found out you’re pregnant. This will be easier for some of you. The rest of you, just do the best you can.

Now, imagine you walk into a grocery store and start looking around. Here’s what you might be thinking: “Wow! Look at all this dead stuff. Dead animal parts, dead fish, dead plants, dead fruit, and a bunch of mixed-up dead stuff in boxes and cans.”

You smile. A warm feeling comes over you, and you say to yourself, “These are the raw ingredients I’m going to use to make my baby! I’m going to buy a bunch of this dead stuff, dissolve it into its fundamental ingredients, and use those dead ingredients to build a living human being.”

So, you fill your cart — thank goodness you didn’t get the one with the wobbly wheel again — pay for your dead stuff, and head home.

That evening you start to make dinner. Because you’ve read some articles online, you know that there might be some unwanted living stuff on your dead stuff, like the bacteria and fungi that cover all the surfaces in your home (including you). So, you diligently wash your dead plants and carefully cook your fish until it reaches a temperature that should kill any living stuff on the dead stuff.

Having done your best to remove all forms of life from your meal, you sit down to eat. (“Wow, I wish I could have a glass of Chardonnay,” you think.)

As you eat, your body’s going to take a few final precautions to make sure there’s no residual living stuff in your food. About half a dozen antibacterial agents in your saliva and a solution of hydrochloric acid in your stomach should finish the job.

Once your dinner passes through your stomach and gets into your small intestines, it’s been reduced to a soupy solution of its basic ingredients, none more special than those you’d find in a high school chemistry lab: simple sugars, amino acids, miscellaneous other small molecules, and a variety of elemental atoms (like calcium, sodium, and potassium).

Now something interesting happens.

While they’re in your digestive system, all those ingredients are (technically speaking) outside of your body. In fact, from a biological perspective, they’re as much outside of your body as they were when you were holding the fish and vegetables in your hands.

The ingredients aren’t actually inside your body until they pass through the layer of epithelial cells covering millions of tiny finger-like projections — called villi – that line your small intestines. Once inside the villi, they’ll get absorbed into the capillaries nested there. That will begin their journey around your body. As they travel, they’ll leak into every nook and cranny, including around your developing embryo. There, the ingredients will begin spontaneously interacting with the chemicals that preceded them, guided only by the basic laws of physics.

Capillaries in the villi of your small intestines. This is where the dead stuff gets inside your body.

As the chemistry proceeds, chemical bonds will be made and broken, and molecules will be reconfigured. Some will be dismantled into their component parts, and others will be assembled into progressively more complex arrangements.

Although undirected in the evolutionary sense, none of these processes are random. In fact, the’ll be strongly constrained by several critical factors. First, the mix of biological ingredients is quite limited: about 96% of your body weight — and most of the food you eat — consists of just four elements: oxygen, carbon, hydrogen, and nitrogen. The rest is composed of a small number of additional elements, mostly in trace amounts.

Second, due to the physicochemical properties of the atoms and the resulting configurations of the molecules they construct, the ingredients can only participate in a limited range of interactions. Although functionally diverse, biochemical transformations are governed by a small set of reaction types — typically classified as just seven.

Third, and most critically, the formation of molecules proceeds incrementally from simpler, already-constructed units. Consequently, each molecular structure has an assembly index — a measure of how many distinct steps are required to construct it from basic building blocks. As such, biological complexity does not emerge through unconstrained or purely random reactions, but rather through a historically contingent, non-random process. New molecular arrangements can only emerge from what already exists, with each step adding to a growing lineage of structural complexity. This history-dependence imposes a strong directionality on biochemical processes, distinguishing biological systems from random chemical networks.

Consider Sara Walker’s Lego® analogy. Imagine you have a box of blue and yellow Legos. I ask you to build some structures by randomly selecting two pieces, connecting them in any way they fit, and then putting them back into the box.

First, what you can build will be limited by the materials you have — you can’t build anything red because you don’t have red Legos. Second, you’re constrained by structure — Legos only connect in certain predefined ways. Similarly, atoms form bonds based on fixed physical and chemical laws which limit the kinds of molecules they can form.

Third, and most importantly, you're building in a historically-dependent way. Each time you toss a newly made structure back into the box, it becomes part of the building pool. Next try, you might select a previously built substructure and combine it with another piece or structure. Over time, this iterative process allows for the construction of more complex assemblies — but crucially, each new structure is limited by what’s come before. This makes the process path-dependent and historically contingent

So, while the number of possible structures may grow rapidly, they don’t emerge randomly from all possible combinations. They emerge in a constrained, cumulative, and directional manner, just like complexity builds in living systems.

As they proceed, these historically contingent chemical interactions will give rise to complex molecular structures that exhibit emergent properties, properties not found in their component parts. For instance, when an adenosine molecule (composed of adenine and ribose) combines with three phosphate groups (all derived from that dinner you ate), they create an adenosine triphosphate molecule which has the remarkable ability to store and transfer energy within cells, a property none of the individual components has on its own.

As these molecular assemblages and their interactions become increasingly complex, they begin to exhibit a remarkable degree of organization, self-regulation, and other unique emergent properties. When a sufficient number and variety of emergent properties become evident, we’ll say that the system, in its totality, is ‘living’.

In other words, ‘life’ is not a property of any individual component of the system, nor of the chemistry in which they participate (what we call metabolism). Rather, ‘life’ is a term we use to describe a system when it displays a particular set of emergent properties.

This conceptualization of ‘life’ is a little different than Sara Walker’s, but not by much. We both see life as an emergent, selected, historically contingent process. However, Sara sees it in chemical structures that are too complex to fluctuate into existence spontaneously (have a high assembly index), and which exist in high copy number (there’s a lot of them). I’m not ready to call something an exemplar of ‘life’ at the level of the complex molecule, but I certainly understand her point. I’m reserving that claim until things get more complex, and the emergent properties more robust.

So, claiming that something represents ‘life’ is a bit of a judgment call. That’s made clear by the fact that there’s no accepted universal definition of ‘life’. In fact, it’s impossible to definitively define it. I’ve talked about that previously, as has Sara and many others.

Consequently, people will differ in their opinions about which — and how many — emergent properties must be present before a system is considered ‘life’ (a key factor in debates over abortion), or how many must be lost before the system is considered dead (a similar issue in debates over defining the end of life).

Those controversies notwithstanding, however, I don’t know anyone who would disagree with the fact that the components — the atoms and molecules — that make up living systems are, themselves, not alive.

At this point, some of my students still look puzzled.

“Think about it carefully,” I’ll say. “When our hypothetical embryo is four weeks old, it will weigh only about half a gram. When she’s born, she’ll weigh about 2800 grams. Where did all of that other living stuff come from?” The answer, of course, is that it came from the grocery store.

Once I get to this point, most of my students get it. And, most admit that they’ve never thought of the issue in these terms.

Then, I remind them that each of us goes through these processes on a daily basis — although much less dramatically than making a new human being — when we heal a wound, grow taller, or replace the cells that we lose through normal wear and tear.

“In fact,” I’ll go on, “it’s estimated that every 10 years or so, you’ve actually replaced every molecule in your body with molecules that you bought at the grocery store (or some drive-through fast food joint). That means that the specific molecules that I’m talking to right now aren’t really ‘you’ in any meaningful way. You’re a dynamic set of interacting, ongoing physiological processes that are continually swapping out one dead particle for another to keep the whole system running…for a while. It’s the system’s higher-level activities — the emergent properties like thinking and talking — that define the ‘living’ you. It isn’t the temporary set of molecules I’m looking at now.

“Unfortunately, but inevitably, once the key emergent properties that I recognize as ‘you’ start to disappear, I will no longer be able to refer to you as living, even though your last set of molecules and some of the chemical reactions will remain.”

In the Beginning…

Near the end of my lecture, I’ll be asked, “But what about the beginning of life on Earth?”

“Well, it’s not much different than the processes I’ve just explained,” I’ll reply.

“It doesn’t matter if you envision Darwin’s warm little pond or a more contemporary hypothesized venue for the emergence of life, a deep-sea thermal vent. The basic model is the same.”

It all had to begin with a bunch of nonliving atoms jiggling around in solution, each periodically coming into contact with the others. Some of those interactions — constrained by the same factors I’ve already explained — led to the formation of small molecules. Based on the environmental conditions at the time, some of those molecules will have been more stable and persisted (i.e., will have been ‘selected’ by the environment). Over time, they’ll be many more of the stable molecules than those that break apart, and they will be the building blocks of even more complex molecules, just like in the Lego example.

Lucky for us that those initial chemical ingredients and reactions gave rise to molecular structures that would eventually create biologically viable systems on our planet. What were the chances of that happening? Pretty slim, I guess, given that we haven’t encountered any other planets on which ‘life’ (as we know it) has emerged.

This explanation of life’s emergence is consistent with laoratory experiments that create what are called proto-cells, simple, self-organized, prebiotic cel-like structures that mimic the early stages in the evolution of life. One might say that the proto-cell represents the transition point between non-living and living systems.

“Capisci?”, I’ll ask.

My students usually say “Yes” because class is about over and they’re already zipping up their backpacks.

About a minute before time is up, one of the more astute students will ask, “But how do you know when something is alive?.”

“Ah, there’s the rub,” I’ll say. “There is no bright line. There is no clear definition or set of criteria to define the exact instant when some entity crosses that threshold. It’s a difficult call that will be influenced by your values, presuppositions, understandings of biological complexity, your cultural background, and the ambiguities of your language.”

That brings me to the end of my first lecture. Most students leave with a “Thank you,” and a few ask, “Can I stop by your lab and talk to you later?” Both comments make me happy.

Epilogue

I always tell my students that if they don’t feel a chill running up their spine, or experience a dizzying sense of transcendence when they see the flutter of a butterfly, the bloom of a flower, or the smile of a newborn baby, they don’t yet understand the wonder that is biology.

Although we can come to understand the physical processes from which life emerges, this does not in any way diminish the profundity of, or the inherent spirituality that many of us see in the living world. Knowledge does not supplant but rather enhances one’s appreciation of the miracle that is life. I am deeply grateful that I’ve had the opportunity to experience it firsthand.