The Big Picture
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into existence from non- life. For compartmentalization, we need to under-
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stand how we got to bilayers made of phospholipids, and the answer might
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be found in fatty acids. For metabolism, we need to know how we got to
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cells driven by the proton- motive force, and the answer might be porous
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chambers in alkaline vents. For replication, we need to know how we got to
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DNA, and the answer might be RNA.
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The relationship of RNA to DNA is like the relationship of an oral tra-
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dition of poetry to words written down in books. The same information
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can be conveyed, but DNA is much more reliable and stable. Yet it is suffi-
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ciently sophisticated that it’s hard to see how it could have come into exis-
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tence by itself. When DNA gets copied, an important part of the work is
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done by proteins. But the proteins are supposed to be constructed using
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information encoded in the DNA. How could either one arise without the
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other already being present?
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The favorite answer among abiogenesis researchers is a scenario called
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RNA world. The basic idea was proposed by a number of people in the
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1960s, including Alexander Rich, Francis Crick, Leslie Orgel, and Carl
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Woese. DNA is good at storing information, and proteins are good at per-
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forming biochemical functions; RNA is able to do both, although it’s not
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as good at either one. RNA could have come along before either DNA or
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proteins, and served as the basis for a primitive and less robust form of early
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life, before evolution gradually distributed responsibilities to the more ef-
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fective DNA and proteins.
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The role of RNA in extracting information from DNA was recognized
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fairly early on, but it wasn’t until later that biologists verified that RNA
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could also act as a catalyst, expediting and governing the rate of biochemi-
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cal reactions. In particular, ribozymes, discovered in the 1980s, are a par-
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ticular kind of RNA that can catalyze their own synthesis, as well as that
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of proteins. The word “ribozyme” is annoyingly similar to “ribosome.” It
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turns out that the crucial part of the ribosome complex consists of ribo-
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zyme RNA. That is, the ribosome is mostly ribozyme. (It’s jargon like this
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that turns young scientists toward physics and astronomy.)
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Further investigations have shown that there are a number of different
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types of RNA, responsible for a variety of functions inside the cell. In ad-
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dition to messenger RNA and ribosomal RNA, we also have transfer RNA
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that brings amino acids to the right place to be made into proteins, regula-
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tory RNA that helps guide the expression of genes, and more. These discov-
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eries have helped popularize the RNA-world hypothesis. If you want to get
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life started from a replication- first perspective, you need a molecule that can
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carry genetic information without relying on other complex mechanisms
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to reproduce itself. RNA seems to hit the sweet spot.
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The idea that RNA may have been the first carrier of genetic information,
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and was able both to self- reproduce and to assemble other biochemically
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useful structures, is compelling and beautiful. Like any good paradigm, one
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of the great features of the RNA world scenario is that it has spurred a
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tremendous amount of exciting research.
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Consider the fact that RNA can be an enzyme: it can catalyze chemical
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reactions, both for self- assembly and for protein synthesis. Where did that
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ability come from? It’s pretty clear how a string of nucleotides can store
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information, but acting as an enzyme seems like a completely different kind
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of talent.
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This question was addressed in an interesting experiment by David Bar-
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tel and Jack Szostak in 1993. (Szostak shared the Nobel Prize in 2009 for
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his work on how chromosomes are protected when DNA divides.) Their
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technique was basically a human- aided version of Darwinian evolution.
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They started with a large amount of random RNA: trillions of molecules
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with no particular sequence to their nucleotides. They then picked out a
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fraction of those molecules, the ones that seemed to be associated with
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somewhat higher rates of catalysis, and made many copies of those. This
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procedure was repeated several times: look for RNA that seemed to be
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catalyzing certain reactions, and make copies of it. At each copying stage,
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random mutations occurred, which occasionally led to the copied RNA
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being a better catalyst than its precursor. After just ten iterations of this
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procedure, the results were clear: the last pool of molecules was approxi-
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mately 3 million times better at catalyzing reactions than the original sam-
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ple. It’s a vivid demonstration of how undirected, random mutation can
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lead to enormous improvements in the ability of chemicals to perform bio-
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logically useful functions.
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Another exciting development came from biologists Tracey Lincoln and
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Gerald Joyce in 2009. They were able to create a system of two RNA en-
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zyme molecules— ribozymes— that together underwent self- sustained rep-
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lication. Without any help from surrounding proteins or other biological
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structures, these molecules are able to completely duplicate each other in
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about an hour. Even better, the molecules occasionally mutate, and there-
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fore undergo Darwinian evolution, with the more fit structures preferen-
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tially surviving. It’s not a cell by any means, but you don’t need to strain to
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see how it could be one of the steps along the road from chemistry to life.<
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Even if RNA played a central role at the origin of life, we don’t yet have
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a complete picture. Compartmentalization, metabolism, and replication all
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have to be brought together. RNA and bilayers made from fatty acids may
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be symbiotic— they could help each other flourish in the rough- and- tumble
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environment of the early Earth. A membrane can shield the fragile RNA
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from external commotion, helping it survive long enough to reproduce. An
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RNA molecule, meanwhile, can attract other biological molecules into the
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membrane, helping it grow to the point where it will naturally split in
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two— a primitive form of cellular division.
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Fitting in metabolism may be trickier, though Szostak doesn’t think it’s
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a big problem. He envisions a proto- cell, RNA encapsulated in a simple
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membrane, floating in a pond that is warm on one end and cold on the
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other. Convection currents push the proto- cell back and forth between the
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two sides. In the cold end, the RNA grows by gathering nucleotides from
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its surroundings, and two RNA strands huddle together as if seeking
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warmth. When it drifts to the warmer side of the pond, the increased tem-
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perature gradually peels the two strands apart; the membrane accretes a few
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more fatty- acid molecules until it divides in two, and (hopefully, some-
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times) we now have two proto- cells with a single strand of RNA each. They
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both drift back to the cold side of the pond, and the cycle of proto- life be-
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gins again.
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Russell and the metabolism- firsters don’t think it will be nearly that
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easy. They believe that the hard part is assembling a complex system of
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chemical reactions that can take advantage of the ambient free energy, set-
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ting up proton- motive forces in chambers of porous underwater vents.
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From there, they suggest, these reactions will naturally feed on any sur-
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rounding free- energy fuel they can find. That might mean that they break
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free of the rocks by entering fatty- acid membranes, and they keep going by
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regulating their reactions through enzymes, which eventually be-
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come RNA.
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Or maybe both scenarios are right, or maybe neither is.
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There is no reason to think that we won’t be able to figure out how life
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started. No serious scientist working on the origin of life, even those who
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are personally religious, points to some particular process and says, “Here
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is the step where we need to invoke the presence of a nonphysical life- force,
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or some element of supernatural intervention.” There is a strong conviction
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that understanding abiogenesis is a matter of solving puzzles within the
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known laws of nature, not calling for help from outside of them.
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This conviction comes from the incredible historical track record sci-
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ence has established. While there are many questions about life’s origin that
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science hasn’t answered, there are a large number that it has, any one of
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which could have been a problem that science all by itself was unable to
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address. (Recall Immanuel Kant’s confident proclamation that there will
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never be a Newton for a blade of grass.) How do species evolve from earlier
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species? How do organic molecules become synthesized? How do cellular
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membranes assemble themselves? How can complex reaction networks
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overcome free- energy barriers? How can RNA molecules develop the abil-
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ity to act as catalysts for biochemical reactions? These are questions we have
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answered. Our Bayesian credence that this string of successes will keep go-
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ing should be very high indeed.
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This perspective meets resistance in certain quarters, and not only
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among religious fundamentalists. The idea that life could just start out of
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no life at all isn’t obvious. We don’t see it taking place before our eyes, no
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matter what Jan Baptist van Helmont might have imagined. Modern- day
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organisms are mind- bogglingly complex, and made of individual parts that
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work together amazingly well. The idea that it “just happened” is a chal-
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lenging one.
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Fred Hoyle, an esteemed British astrophysicist known for his staunch
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opposition to the Big Bang model, attempted to quantify this unease. He
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considered the configuration of atoms in a biological structure such as a
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cell. Then, in a move taken from Ludwig Boltzmann’s playbook, he com-
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pared the total number of ways such atoms could be arranged to the much
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smaller number that would qualify as a cell. Multiplying together a bunch
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of tiny numbers, he concluded that the chance of life assembling all by itself
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is something like 1 in 1040,000.
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Hoyle was a master of vivid imagery, and he illustrated his point with a
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famous analogy:
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The chance that higher life forms might have emerged in this
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way is comparable to the chance that a tornado sweeping
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through a junkyard might assemble a Boeing 747 from the ma-
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terials therein.
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The problem is that Hoyle’s version of “this way” is nothing at all like
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how actual abiogenesis researchers believe that life came about. Nobody
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thinks that the first cell occurred when a fixed collection of atoms was re-
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a cell- like configuration. What Hoyle is describing is essentially the
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Boltzmann Brain scenario—
truly random fluctuations coming together to
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create something complex and ordered.
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The real world is different. The “unlikeliness” associated with low-
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entropy configurations is built into the universe from the start, by the in-
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credibly low entropy near the Big Bang. The fact that the development of
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the cosmos proceeds from this very special initial condition, rather than
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wandering through a more typical equilibrium ensemble of states, imposes
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a strong nonrandom aspect on the evolution of the universe. The appear-
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ance of cells and metabolism is a reflection of the universe’s progression
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toward higher entropy, not an unlikely happenstance in an equilibrium
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background. Like the swirls of cream mixing into coffee, the marvelous
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complexity of biological organisms is a natural consequence of the arrow
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of time.
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We’ve made amazing progress in understanding what life is and how it
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came to be, and there’s every reason to think that progress will continue
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until we have figured it out. The work ahead will involve chemistry, physics,
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mathematics, and biology, not magic.
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Evolution’s Bootstraps
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n 1988, Richard Lenski had a brilliant idea: he was going to turn evo-
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lutionary biology into an experimental science.
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Evolution is the idea that provides the bridge from abiogenesis to
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the grand pageant of life on Earth today. There’s no question that it’s a sci-
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ence; evolutionary biologists formulate hypotheses, define likelihoods of
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different outcomes under competing hypotheses, and collect data to update
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our credences in those hypotheses. But chemists and physicists have an ad-
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vantage over evolutionary biologists or, for that matter, astronomers: they
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can perform repeated experiments in their labs. It would be very hard to set
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up a laboratory experiment to see Darwinian evolution in action, just as it