What can happen depends on what has happened before

kw: book reviews, nonfiction, evolution, theory of facilitated variation

Imagine a large, snowy mountain. One so large you'll never get off it. This ski run will last forever. At the summit, which direction will you go? Ridges and canyons stretch in all directions; gentle slopes, steeper slopes, crags and cliffs. It's not downhill everywhere. There are pockets and swales to be had, and the occasional deep, blind pocket. Some look inviting, perhaps places to rest, but there is a catch: you depend on gravity and inertia alone. If you come to rest in a low spot, that is the end of the run, forever.

You pick a direction and go. Soon you are schussing at a good clip, and it runs here and there, sometimes branching; you choose this branch, then that, reacting, barely thinking. At one point, a steep section levels off to a plain. Choices abound, here. You can creep along, almost at a standstill, pick a direction, and go off again.

Some runs are steep and rough, but single. There are no choices for a stretch, then choices abound. At some point, you think, "What if I had taken a different branch back there?" You are more likely to think that if a run levels out onto a pocket plain, surrounded by upslopes on all sides. There's no getting out of this one, no escape, no further progress. You're stuck. Eons pass, with no change. Perhaps you're lucky...slowly, the mountain slopes shift and change. Rare, sudden changes shift the landscape a little more. One day, there is an ever-so-narrow path that leads downward.

The inertia of being stuck so long is hard to overcome, but you do so. You glide slowly through the new gap that leads onward, then drop into a steep run, and you're off! Where to next?

Where indeed. We see around us the results of trillions of choices, made throughout the eons that life has existed here. The choices were not made by a brainy skier, but by happenstance. Once living things came into being—by what means we don't yet know—they multiplied, perhaps rather slowly at first. They could afford some slowness; "at first" there was probably little competition. But any pocket of living cells, perhaps remotely similar to bacteria, became the first few billion skiers dropped on the top of that mountain. When you are the first life on a planet, every direction represents progress.

It may be that RNA/DNA comprised the first and only genetic storage system that came about. Maybe not; it may be the sole winner of the oldest contest. But at some time, a genetic storage system gained a way to exist in a capsule and carry on metabolic activities (i.e. energy consumption). This probably happened nearly four billion years ago. We may imagine that these first living cells, however primitive, didn't stay that way very long. Light capture, by rhodopsin and chlorophyll, arose almost at once.

Many kinds of metabolism arose, specialized for different chemical regimes, gaining energy by reducing sulfates, or iron oxides, or (with light's help) carbon oxides. Some kinds got bigger, some stayed small or got smaller.

What took a long, long time, it seems, was for a bigger cell to engulf a number of smaller ones and enslave them. Little, single-purpose bacteria and photo-bacteria are very efficient at energy production and conversion. Somebody big was able to take advantage of their efficiency, to turn a single cell into a community, a "bacterial village" consisting of specialists in food gathering, energy production, DNA decoding, and many other tasks. As a result, that "somebody big" got bigger. Today's protozoa are the highly-advanced descendants of "somebody big". We'll give it a name, "Fat Albert." Or, just Albert. Albert isn't a single cell, but the presonification of the first Eukaryotic species. Eukaryotic cell fossils aged two billion years are known, and the innovation probably occurred within half a billion years before that.

Think of the mountain. Once Albert arose, he found himself on the slopes, in a broad, steep valley of his own. Stretching away to the sides were other ridges and valleys, covered with the little skiers, cousins of Albert's internal specialists. All were headed down the mountain, finding their way into this rille or that crack, past ridgeline and rock, all making what progress they could. Albert had his own valley. Nobody would enter it again, and he would never pass into any of the runs to either side. There was nowhere to go but down. Fortunately, because of the greater efficiency of the Albert-village, there were lots of ways down. Albert became Bertha, Carrie, Daniel, and many others, and took them all. A species can do that. Not all led to continued progress. Many got stuck, but many didn't.

At some point, a collection of Albert-villages—or maybe Valerie-villages—took a slope that led to numbers of them sticking together. A group became a whole. A lot of Valeries took a new name, Dallas, and became a small city composed of several villages. There were plenty of downhill directions Dallas could go. Of course, she took them all. This was probably 1.5 billion years ago.

As it turned out, there were so many, many ways Dallas and her siblings could go that for a time it seemed there was one member of each species on Earth. Various levels of efficiency, in the various environments in which Dallas-cities could live, resulted in quite a sorting-out, until just a few hundred varieties remained.

At some point, probably not long after the first Dallas was assembled, the development program in the DNA for some little wormy thing became well-enough organized that it could direct the staged development of a bilateral body, with certain intercellular messages used in a variety of ways in a segmented, or at least compartmentalized, body. Most of the naked-eye-visible animals we encounter today arose from little XingXing*.

Today we recognize at least these five major stages in the evolution of bilateral animals: Prokaryotic cells arose from whatever went before (we have little clue so far), Eukaryotes came a billion years later, Metazoans another half-billion or less, and Bilateral animals that today comprise about thirty phyla a billion years or more ago. All "bilaterians" share a group of genes called the Homeobox, or just Hox, that regulate the development of a fertilized egg cell into an organized body. Non-bilaterians such as jellyfish have a very distant relative of Hox comprising two or four genes, half the minimum size of the Hox cluster.

The key feature of the mountain metaphor is, you can't go up, only ski down. Once you've entered one run, you'll never ski the run to its right or its left. Critics of evolution used to say things like, "You don't see rabbits evolving into cats or monkeys, do you?" Now, most of them know how abysmally stupid that question is. I think we understand the principle, that what we can do today depends on what we did in the past. If you were educated or trained for business, and you excel at marketing, you are unlikely to become an engineer, and vice versa. If you first begin a little recreational fencing at age thirty, you may become pretty good on the mat, but you'll never win Olympic gold for fencing; there, you'd face kids half your age who've fenced since they were four.

All this rumination was triggered by reading The Plausibility of Life: Resolving Darwin's Dilemma by Marc W. Kirschner and John C. Gerhart. I have read millions of words by and about evolutionary biologists (I know nearly all biologists are evolutionists, but some specialize in evolution itself). S.J. Gould was, to me, the best. He and Niles Eldredge developed "punctuated evolution" to explain why species seem to persist unchanged for periods of a few million to tens of millions of years, then either vanish or (rarely seen in the fossil record) change rapidly into a different species or radiate into several related species. This idea is typically discussed in anatomical or palaeontological terms. There has been much discussion of possible underlying mechanisms, including much by Gould in his columns (collected in a great series of books, the most memorable to me being "The Panda's Thumb"). Kirschner and Gerhart have provided a very plausible mechanism.

After noting the title and reading the introductory blurb, I was prepared to dispute the idea. They call their idea "facilitated variation". I immediately thought, "Facilitated by whom?" I am a facilitator of process modeling as part of my work. So I bring my history to the word. Having read the book, I understand, the denotation they are using is not my kind. Facilitation of the evolutionary process is active, but not conscious, nor purpose-directed.

Here was my epiphany: Evolvability is also subject to evolution. Variation of the genetics in a population is the source of the bodily variety among which natural selection makes its selections, and the ability to both vary and to tolerate greater degrees of genetic variation is also something that natural selection can work on. I saw this about halfway through, and found that the authors discussed it in their last two chapters. Oh, well. I had the brief illusion of a new idea...

Just seeing this was a result of some history. I have for a decade or so been considering the phenomenon of gene regulation. I did some work in the 1990s with chemical kinetics in interlocked networks of reactions, particularly the feedback loops that result. Then I connected the ideas to gene regulation. Here is the overview chapter in Wikipedia's article Gene regulatory network:

"A gene regulatory network (also called a GRN or genetic regulatory network) is a collection of DNA segments in a cell which interact with each other and with other substances in the cell, thereby governing the rates at which genes in the network are transcribed into mRNA."

I thought, are regulatory networks freestanding, or more deeply regulated? Regulating a gene depends on a tag near the "start" signal, that is recognized by a regulatory protein. A gene can have several of these. They are in what once was called "junk DNA." I can think of lots of uses for that "junk", but that's another rant!

After a lot of reading, I've concluded that there are at least three, and perhaps five or six, levels of gene regulation. Also, the machinery for exon/intron splicing, plus other protein or RNA splicing that combines smaller segments into larger proteins, explains how the human complement of about 23,000 "genes" can produce millions of proteins.

Thus, about fourteen human Hox genes suffice to regulate the development of our entire body. Mechanisms that are capable of development under the direction of relatively few regulatory signals can be trusted to grow bodies that are at least roughly proportionate, so that people have quite a range of variation (which keeps ergonomics folk in business), but not an infinite range (no 30-foot people, or 4-footers with 8-foot arms).

Kirschner and Gerhart have a compelling message. I think they have it right. Time will tell. They compiled and sorted a great mass of material to deliver a book that, while I had to read it slowly, kept me captive with clear examples and forceful logic. They go as deep "in the weeds" as they need to, to show how powerful and robust the evolution of the ability to evolve safely has rendered the living species with which we share the Earth today.

*Xing-Xing is Chinese for the Gemini.