What Technology Wants (17 page)

Other physical constants run through the biological world. Bilateral symmetry (mirrored left and right sides) recurs in almost every family of life. This fundamental symmetry seems to bring adaptive advantage on many levels, from superior balance of movement to prudent redundancy (two of everything!) to efficient compression of genetic code (just duplicate the first side). Other geometric forms, like a tube for nutrient transport in plants or animals (a gut) or legs, are just plain good physics. Some recurring designs, such as the arboreal splay of branches in a tree and coral or the swirling spiral of petals on a flower, are based on the mathematics of growth. They repeat because the math is eternal. All life on Earth is protein based, and the way those proteins fold and unfold inside cells determines the character traits and behaviors of that creature. Biochemists Michael Denton and Craig Marshall state that “recent advances in protein chemistry suggest that at least one set of biological forms—the basic protein folds—is determined by physical laws similar to those giving rise to crystals and atoms. They give every appearance of being invariant platonic forms.” Proteins—the essential molecules of life's diversity—are also ultimately governed by a limited set of recurring laws.

If we made a large spreadsheet containing all the physical characteristics of all the living organisms on Earth, we would find many blank white spaces for organisms that logically “could be” but aren't. These fill-in creatures would obey the laws of biology and physics, yet were never born. Such “could be” life forms might include a mammalian snake (why not?), a flying spider, or a terrestrial squid. In fact, some of these could still evolve on Earth if we left the current flora and fauna alone long enough. These speculative creatures are entirely plausible because they are convergent, recycling (but remixing) morphological forms that repeat throughout the biosphere.

When artists and science-fiction authors fantasize alternative planets full of living creatures, try as they might to “think outside the box” of earthly constraints, many of the organisms they envision also retain many of the forms found on Earth. Some would chalk this up to a lack of imagination; we are constantly being surprised by bizarre forms found in the deepest part of the oceans on our home planet; surely life on other planets will be full of surprises. Others, myself included, agree that we will be surprised but that given what “could be”—that vast imaginary space of all possible ways in which one could arrange atoms into an organism—what we will find on another planet will only fill one small corner of what could be. Life on other planets will be surprising because of what it does with already familiar forms. Biologist George Wald, who won a Nobel Prize for his work on eye retina pigments, told NASA, “I tell my students: learn your biochemistry here and you will be able to pass examinations on Arcturus.”

Nowhere is that physical constraint of the infinitude more evident than in the structure of DNA. The molecule of DNA is so remarkable that it is in its own class. As every student knows, DNA is a unique double-helical chain that can zip and unzip with ease and of course replicate itself. But DNA can also arrange itself into flat sheets or interlocking rings or even an octahedron. This singular gymnastic molecule serves as a dynamic mold that prints the stupendously large set of proteins responsible for the physical characteristics of tissue and flesh, which in turn, by mutual interaction, generate vast ecosystems of complexity. From this single omnipotent quasicrystal the awesome variety of life in all its unexpected shapes springs forth. Subtle rearrangements along its tiny, ancient spiral will produce the majesty of a strolling sauropod 20 meters high, and also the delicate gem of an iridescent green dragonfly, and the frozen immaculacy of a white orchid petal, and of course the intricacies of the human mind. All from such a tiny semicrystal.

If we acknowledge no supernatural force working outside evolution, then all these structures—and more—must in some sense be contained within the structure of DNA. Where else could they come from? The details of all oak lineages and future species of oak are resident, in some fashion, in the original acorn of DNA. And if we acknowledge no supernatural force working outside evolution, then our minds—which all descended from the same original first cell—must also have been encoded implicitly in DNA. And if our minds, then what about the technium? Were its space station, Teflon, and internet also dissolved in the genome, only to be precipitated later by constant evolutionary work, just as an oak tree is finally manifested after billions of years?

Of course, merely inspecting this molecule reveals none of this cornucopia; we seek in vain to find a giraffe in the spiral ladder of DNA. But we can seek alternative “acorn” molecules as a way to rerun this unfolding to see if something else besides DNA could generate similar diversity, reliability, and evolvability. A number of scientists have searched for alternatives to DNA in the laboratory by engineering “artificial” DNAs or constructing DNA-like molecules or engineering wholly original biochemistry. There are a bunch of practical reasons to invent a DNA alternative (say, to create cells that can work in space), but so far alternatives with DNA's versatility and brilliance are in short supply.

The first obvious approach in the quest for an alternative DNA molecule is to substitute slightly modified base pairs into the helix (think of different steps in DNA's spiral staircase). K. D. James and A. D. Ellington write in
Origins of Life and Evolution of the Biospheres
that “experiments with alternative base pairing schemes have suggested that the current set of purines and pyrimidines [the canonical base pair types] is in many ways optimal. . . . The unnatural nucleic acid analogues that have been examined experimentally have proven to be largely incapable of self-replication.”

Of course, science is rife with discoveries initially thought unlikely, implausible, or impossible. In the case of self-organizing life, we might want to be particularly hesitant to generalize about alternatives since everything we can say about it is based on a sample size (so far) of exactly one, here on Earth.

But chemistry is chemistry, everywhere in the universe. Carbon sits at the center of life because it is gregarious and contains so many hooks for other elements to bind to. It has a particularly friendly relationship with oxygen. Carbon is easily oxidized as fuel for animals and easily unoxidized (reduced) by chlorophyll in plants. And of course it forms the backbone for long chains of incredibly diverse megamolecules. Silicon, carbon's sister element, is the most likely alternative candidate to produce a non-carbon-based life form. Silicon also is very prolific in its hooking up with a variety of elements, and it is more abundant on the planet than carbon. When science-fiction authors dream up alternative life forms, they are often based on silicon. But in real life silicon suffers from a few major drawbacks. It does not link up into chains with hydrogen, limiting the size of its derivatives. Silicon-silicon bonds are not stable in water. And when silicon is oxidized, its respiratory output is a mineral precipitate, rather than the gaslike carbon dioxide. That makes it hard to dissipate. A silicon creature would exhale gritty grains of sand. Basically, silicon produces dry life. Without a liquid matrix it's hard to imagine how complex molecules are transported around to interact. Perhaps silicon-based life inhabits a fiery world and the silicates are molten. Or perhaps the matrix is very cold liquid ammonia. But unlike ice, which floats and insulates the unfrozen liquid, frozen ammonia sinks, allowing the oceans to freeze whole. These concerns are not hypothetical but are based on experiments to produce alternatives to carbon-based life. So far, all evidence points to DNA as the “perfect” molecule.

For even though clever minds like ours may invent a new life base, finding a life base that can create itself is an entirely higher order. A potential synthetic life base created in the lab might be robust enough to survive on its own in the wild but fail to organize itself into existence. If you can skip the need for a self-made birth, you can jump to all kinds of complex systems that would never evolve on their own. (This is in fact the “job” of minds: to produce types of complexity that evolutionary self-creation cannot.) Robots and AIs don't need to self-organize from metal-laden rocks because they are made rather than born.

However, DNA did have to self-organize. By far the most remarkable thing about this potent nucleus of life is that it put itself together. The most basic carbon-based ingredients—such as methane or formaldehyde—are readily available in space, and even in pools on planets. But every abiotic condition (lightning, heat, warm pools, impact, freezing/thawing) we have tried as a stimulus to organize these Lego-like building blocks into the eight component sugars that make up RNA and DNA has failed to generate sustainable amounts of them. All the known pathways to creating just one of these sugars—ribose (the
R
in RNA)—are so complicated they are difficult to reproduce in the lab and (so far) unthinkable as existing in the wild. And that is just for one of eight essential predecessor molecules. The necessary—and potentially contradictory—conditions to nurture dozens of other unstable compounds toward self-generation have not been found.

Yet here we are, so we know that these peculiar pathways can be found. At least once. But the supreme difficulty of simultaneous improbable pathways working in parallel suggests that there may be only one molecule that can negotiate this maze, self-assemble its scores of parts, self-replicate once birthed, and then unleash from its seed the head-shaking, eye-popping, mind-blowing variety and exuberance we see in life on Earth. It is not enough to find a molecule that can self-replicate
and
generate ever-larger mounds of increasing complexity. There may indeed be multiple amazing chemical nuclei capable of that. Rather, the challenge is finding one that does all that and can make itself, too.

So far, there are no other contenders even close to offering that kind of magic. This is why Simon Conway Morris calls DNA “the strangest molecule in the universe.” Biochemist Norman Pace says there may be a “universal biochemistry” based upon this most remarkable of all molecules. He speculates: “It seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail. Thus the 20 common amino acids are the simplest carbon structures imaginable that can deliver the functional groups used in life.” To paraphrase George Wald: If you want to study ET, study DNA.

There is another hint of the unique (perhaps universally unique) power of DNA. Two molecular biologists (Stephen Freeland and Laurence Hurst) computationally generated random genetic code systems (the equivalents of DNA, but without DNA) in a simulated chemical world. Since the combinatorial sum of all possible genetic codes overwhelms the time in the universe to compute them, the researchers sampled a subset of these, focusing on those systems they classified as chemically viable. They explored a million variations out of what they estimated to be a pool of 270 million viable alternatives and ranked the systems on how well they minimized errors in their simulated world (a good genetic code will reproduce accurately without errors). After a million computer runs the measured efficiency of the genetic codes fell into a typical bell curve. Far off to one side was Earth's DNA. Out of a million alternative genetic codes, our current DNA scheme was “the best of all possible codes,” they concluded, and even if it is not perfect, it is at least “one in a million.”

Green chlorophyll is another strange molecule. It is ubiquitous on the planet, yet not optimal. The spectrum of the sun peaks in the yellow frequency, yet chlorophyll is optimized for red/blue. As George Wald notes, chlorophyll's “triple combination of capacities”—a high receptivity to light, an ability to store the captured energy and relay it to other molecules, and an ability to transfer hydrogen in order to reduce carbon dioxide—made it essential in the evolution of solar-gathering plants “despite its disadvantageous absorption spectrum.” Wald goes on to speculate that this nonoptimization is evidence that there is no better carbon-based molecule for converting light into sugar, because if there were, wouldn't several billion years of evolution have produced it?

It may seem like I contradict myself when I point to convergence due to rhodopsin's maximum optimization and then to chlorophyll's nonoptimization. I don't think the level of efficiency is central. In both cases it is the paucity of alternatives that is the strongest evidence for inevitability. In chlorophyll's case, no alternative forms appear after billions of years in spite of its imperfection, and in rhodopsin's case, despite a few minor competitors, the same molecule was found twice in an otherwise vast empty field. Again and again evolution returns to a few solutions that work.

No doubt someday very smart researchers in a laboratory will devise an alternative base to organic DNA that is able to unleash a river of new life. Accelerated vastly, this synthetic life base might evolve all kinds of new creatures, including sentient beings. However, this alternative living system—whether based on silicon, carbon nanotubes, or nuclear gases in a black cloud—would have its own inevitabilities, channeled by the constraints embedded in its original seeds. It would not be able to evolve everything, but it could produce many types of life that our life could not. Some science-fiction authors have playfully speculated that DNA might itself be such an engineered molecule. It is, after all, ingeniously optimized, and yet its origins are deeply mysterious. Perhaps DNA was cleverly crafted by superior intelligences in white lab coats and shotgunned into the universe to naturally seed empty planets over billions of years? We would be just one of many seedlings that sprouted from this generic starter mix. This kind of engineered gardening might explain a lot, but it does not remove the uniqueness of DNA. Nor does it remove the channels that DNA has laid for evolution on Earth.