kw: book reviews, nonfiction, science, biology, physics, cosmology, genetics, first contact
To put Sara Imari Walker's last question first, will first contact with a true alien occur in a laboratory? Dr. Walker's book Life as No One Knows It: The Physics of Life's Emergence is a heady brew of ideas. She has a wonderful kind of mind that looks at things from angles others would never imagine existed, and she has managed to find a number of kindred souls with a similar talent.
Dr. Walker and her colleagues propose Assembly Theory, a new view of evolution in which technology has become an agent of evolutionary change, and "selection" takes on a meaning that would mightily bemuse Darwin. Her explanations make sense. I don't yet understand enough about assembly theory to attempt an explanation. I must content myself with a few bullet points gleaned from the third chapter ("Life is what?"; intended to be pronounced with a distinct upward lilt, as, "Life is WHAT??"). These items do not describe life, but rather objects. Objects as the new theory envisions them:
- Objects are finite and distinguishable
- Objects are breakable (I would have said, "can be disassembled")
- Objects exist more than once
- Objects are lineages
- Objects form via selection
Some objects are living things. Some are not. A wooden table of the kind I built long ago to hold my mouse pad next to my desk is an example. I constructed it using nine pieces of wood, eight large wood screws, 24 small nails, and some glue. The top is plywood, which itself was fabricated by someone else before I bought it from a lumberyard and cut it to size. We can inventory the statements.
- My little mouse-pad table is finite (less than a meter tall) and is distinct from other items of furniture in this room, some purchased (and thus built by someone else) and others built here, by me.
- It can be taken apart, by removing the fasteners and breaking the glue joints (no doubt tearing wood in the process). I don't plan to do so.
- I have made other similar tables, which I suppose qualifies at "exists more than once." Worldwide, there are many tables of many designs, but all related in general shape and function.
- Is it a lineage? The idea of "table", generalized from "supporting platform" is certainly a lineage, going back to the first table or table-like platform built by a human or prehuman creature.
- "Selection" in such a case means that the I selected to make a table rather than a cantilevered shelf hung off my desk, or other possible means of supporting my mouse pad.
Consider a screwdriver. The author writes, "An evolutionary chain of objects is necessary to assemble screwdrivers into existence." In other words, the forebears of screwdrivers include machine shops, mining and metals productions facilities, and creatures like us to think them up. In the case of living things, the "lineage" of everything alive today goes back about 3.9 billion years. All known life comes from life. What came before?
This is the crux of the matter for assembly theory. The theorists use these concepts to imagine life as we don't know it, life as no one knows it.
Side question: Can objects be produced by other objects that are not living? They can, which is apparently where all nonliving and never-previously-living things arose. For example, a planet such as Earth was assembled from dust, rocks, etc., under the influence of gravity and electromagnetism and sundry other possible forces. Also, natural, nonliving actions produce raindrops and snowflakes in clouds, sand and gravel from rocks, and so forth.
However, the production of objects by nonliving processes yields objects with very few unique parts. Living objects tend to be much more complex, as do many of their products. Thus this conjecture, stated at least a couple of times in the book, "Life is the only thing in the universe that can make objects with many unique parts." The author notes that many kinds of minerals don't form in the absence of life. I recall taking a class in minerology, in which the professor stated, "There are about 1,000 mineral species known in the earliest rocks, before there was much life on Earth. There are now more than 4,000 minerals, and most of them can only form because there is life on Earth." Today the number of catalogued minerals is almost 7,000, and that may grow to more than 10,000 as busy geologists keep finding new stuff.
This principle yields the basis of the author's "assembly index", a measure of the minimum number of steps needed to produce an object. This gets us away from looking for life that is too chemically similar to "Earthlings" (from microbes and viruses to whales and forests, with us in the mid-range). If the assembly index of numerous objects collected on an exoplanet is large enough, we could conclude that life most likely produced them. The critical number is, according to the author, fifteen.
Frankly, I don't know how assembly index is calculated. I read that the assembly index of the molecule ATP is fifteen. This molecule has 47 atoms. Perhaps the calculation allows the synthesis to begin with smaller molecules that natural processes have already produced: water, carbon dioxide, ammonia, phosphorus oxide, and perhaps even small hydrocarbons such as ethane (ethane can result from abiotic, or nonliving, processes, but in the presence of a living biosphere we never observe it).
Therefore, the number 15 seems to be a good "filter" to discern objects produced by life.
A second prong in the approach by proponents of assembly theory is the development of a "chemputer", a semi-automated way to sort of "3d print" molecules, designed to have certain properties, in an attempt to produce a chemical system that takes on the attributes of life: reproduction, ingestion of supplies, elimination of wastes, relationships (very broadly construed), for example. Were such an effort to succeed, using large numbers of chemputers to explore the "assembly space" of small-to-medium sized molecules, there might indeed be alien life produced in the laboratory. It would be as alien as anything we might find on a planet far away, perhaps even more so.The key to grasping assembly theory is the claim that all things life can build are historically contingent. We can see this in the visible relatedness between the wide range of animals known as "tetrapods". They all have four limbs. The mythical flying horse Pegasus cannot evolve from a horse, because to do so would require adding two new limbs (to become wings) to a body that already works well with four limbs. The intermediate steps required don't make sense (and let's ignore the physics of wings long enough to support a half-ton animal). In fact, mammalian hexapods in general aren't likely to arise, because the competition from extremely well-adapted tetrapods has already pretty much filled all available ecological niches for critters bigger than a cockroach…on land, at least.
Historical contingency is a key concept. Finger-and-toenails didn't evolve all at once. They descended from claws. Claws came from something else. All living things trace back to a single-celled creature called LUCA, the Last Universal Common Ancestor. LUCA may not be the first cell. At one time other living things could have existed alongside LUCA that may have had quite different chemistry and cellular mechanisms. But only LUCA has descendants: us, and every living thing in the biosphere of Earth.
Our chain of imagined forebears stops with LUCA. Our knowledge is further constrained, because it is extremely unlikely that any creature now living is descended without change from LUCA. When I say "extremely unlikely", that middle E needs to be drawn out, "Extreeeeeeeeeeeemely!" Meaning, utterly impossible unless the universe is truly infinite, with an infinite number, not just of planets, but of biospheres. Even if we somehow track back the chain of biochemical contingency to "show" us a robust model of LUCA, the chain stops there.
I like the idea of the chemputer. But I don't hold out much hope. The assembly space of "small molecules" is too big. For example, if we "restrict" ourselves to the twenty most common elements, all of which are found in known life, and all of which are likely to be in any possible kind of life, and further, we call a molecule "small" if it has fifty or fewer atoms (just a tad bigger than ATP), the number of possible chemical species (most of them quite chemically unstable) is close to ten-to-the-power-of-65. A 66-digit integer. How large is that number? The number of stars in the visible universe is thought to be a 22- or 23-digit number. Could the average number of planets be as great as ten? Even if that were so, the number of planets in all the galaxies in the visible universe is no larger than the largest 24-digit number. It is hard to think about 66-digit numbers in any useful way.
Dr. Walker dreams of being the researcher in the cartoon above, "meeting" the first true alien in the laboratory. I hope her dream comes true. But I think it more likely that we'll come across something, somewhere, soon, that proves life as we don't yet know it does in fact exist.
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