Life, What's Luck Got to Do With It?

Paleontologist Stephen Jay Gould, in his book Wonderful Life:  The Burgess Shale and the Nature of History (1989), introduced a broad swath of the reading public to the concept of historical contingency.  In the book, he explores how the world of living organisms could move from the many "weird" forms of Cambrian life (at a juncture some 530 million years ago) fossilized in the Burgess Shale, to creatures such as us.  He states that there are no natural laws requiring a particular outcome (e.g., us), rather, he posits, what's at work, and what shaped our world, is historical contingency which is:

an unpredictable sequence of antecedent states, where any major change in any step of the sequence would have altered the final result.  The final result is therefore dependent, or contingent, upon everything that came before - the unerasable and determining signature of history.  (Wonderful Life, p. 283)

When considering the final product of such a process, apparently luck, or chance, plays a determinative role.  The previous post on this blog (the one about watches, God, and evolution) draws on some different work by Gould to posit that evolution through natural selection, not some divine force, shaped the biological world around us.  What role has chance played in life, what's the connection between chance and evolution, and what does historical contingency have to say about that connection?  I address those questions in this post through a review of a book which I read only by chance or, more precisely, by mistake.

In that earlier post, I reference Sean Carroll's book The Big Picture:  On the Origins of Life, Meaning, and the Universe Itself (2016) when asserting that the human eye came into being through natural selection.  Afterwards I went in search of other works by Carroll, and came upon the thoroughly enjoyable A Series of Fortunate Events:  Chance and the Making of the Planet, Life, and You (2020)  The title is a nice play on Lemony Snicket's series of books titled A Series of Unfortunate Events.  Wait, I will forestall a rise in your blood pressure, by hastening to admit that it took me an embarrassingly long time to realize that the Sean Carroll whose book I mention in that previous post is a physicist and is NOT the author of A Series of Fortunate Events who is biologist Sean B. Carroll.  A whole different kettle of fish.  Fittingly, had I not made that error, I might never have read A Series of Fortunate Events, at least not when I did, which would have been an unfortunate event.

The biologist Carroll's A Series of Fortunate Events is a brief, graceful, and often funny read about the central role chance has played, and plays, in determining how the world and everything in it - organic and not - has come to be.  Carroll structures his exploration of chance around its role in two broad realms:  the "inanimate" (read cosmological and geological), and the "animate" (read biological).  There are two components to the biological realm:  a part embracing life in general where evolution through natural selection as well as mutation are at work, and a part focused on the way chance affects how each of us individually is conceived, lives, and dies.  Quite ambitious and he pulls it off with surprising ease.

I think it useful at the outset to draw attention to the two pages that mark the transition from Carroll's discussion of the inanimate to that addressing the animate.  In them, he explains what he means by chance:  "a rare, unpredictable, or random event, or one that entails so many variables or forces that is near enough to be random,"  (p. 59)  And, perhaps even more important, he defines the concept of historical contingency:  "a past event or process that was necessary for a particular outcome."  (p. 59)  He captures the significance of the relationship between the two when he writes:

Chance here, then, pertains to an event itself, while a contingency emerges through the benefit of hindsight.  The interrelationship between the two is that a chance event can become a historical contingency through its effects.  Contingency is the aftermath of chance.  (p. 59-60)

Carroll presents various examples of chance in the broad realms around which the book is structured.  I mention a few of the most prominent.  The star of the inanimate realm is cosmological in nature:  the asteroid that smashed into the Yucatan Peninsula 66 million years ago, dooming the non-avian dinosaurs and myriad other species of animals and plants.  This asteroid was a great big ball of chance.  As Carroll point out, a mere 30 minutes was key.  Had the asteroid hit Earth that many minutes earlier or later, it would have landed in deep ocean waters and likely not played havoc with the world.  Instead, the bolide landed on a part of the planet that magnified its most lethal effects.  And, so, the asteroid "hit the reset button" for life on Earth.

Regarding chance in the biological realm for life in general, Carroll focuses on a biologist about whom I knew nothing - Jacques Monod whose work included exploring how genes control enzyme production.  He shared the 1965 Nobel Prize in Physiology or Medicine.  With the publication of his book Chance and Necessity in 1970, Monod achieved an unexpected degree of fame and notoriety.  Carroll asserts that, when Monod wrote about how mutations appeared in DNA, "he delivered one of the most powerful ideas in five centuries of science."  (A Series of Fortunate Events, p. 8)  Monod stated that

chance alone is at the source of every innovation of all creation in the biosphere.

Pure chance, absolutely free but blind, at the very root of the stupendous edifice of evolution:  this central concept of modern biology is no longer one among other possible or even conceivable hypotheses.  It is today the sole conceivable hypothesis, the only one that squares with observed and tested fact."  (Text is from Chance and Necessity as quoted in A Series of Fortunate Events, p. 8)

Amazing.  Carroll is equating in importance Monod's claim about chance with, say, Darwin's theory of evolution through natural selection.  In fact, the two go hand in hand.  Without the possibility of change in living organisms, there would be no variation for natural selection to work with.  That the variation occurs and is random speaks to the role of chance.  Darwin, Carroll says, had no knowledge of the actual process that generated variations, but characterized them as "accidents."

Carroll devotes a couple of chapters to mistakes or mutations that can occur in the genome, showing that they are random, a matter of chance in how the genetic code might be altered.  The key is that the four bases that constitute the alphabet of which DNA is composed exist in two alternate forms and can shift from one to the other.  If, during copying, the rarer form of a base is present, it will bond with the "wrong complementary base," yielding a mutation.  Carroll labels the shifting between forms as "quantum fibrillation," a feature inherent in the bases, meaning:  "Mutation, then, is a feature, not a bug in DNA."  (p. 95)  He sums up the interplay between mutation (the product of chance) and natural selection:  "Chance invents, and natural selection propagates the invention."  (p. 118)   But propagation may occur only if the mutation offers an organism some advantage in the struggle for survival.

As for the role of chance in my life or yours, Carroll starts with our conception.  It's not just how, out of some 100 million of a male's sperm streaming toward the female's egg, only one will fertilize it.  More impressive is that, based on any male and female's chromosomes, there are some 70 trillion different, genetically unique children that a couple might create in that moment, but only one is.  (p. 131)  Other aspects of chance playing out in our lives that Carroll explores include some unfortunate events, among them having cancer.

He nicely sums up what I believe are the overall takeaways from the book:

I have shown how we know that we are all here, both collectively and individually, through a series of accidents – cosmological, geological, and biological accidents. I have also shown how and why some of us will depart via accident.

Our chance-driven world is a profound revelation. It is astonishing that blind chance is the source of all novelty, diversity, and beauty in the biosphere. I hope that you are wonderstruck at what an asteroid, sliding tectonic plates, and a fibrillating polymer of just four bases have wrought. (p. 164-5)

As much as I liked the book, I do have a couple of quibbles.  First, contingency is at the heart of Carroll's assertion that we, and all life around us, is here by virtue of chance.  It's the sequences of chance (i.e., "rare, unpredictable, or random") events in the histories of all taxa that create the contingencies necessary for the very existence of all types of organisms.  The longer that sequence, I would suggest, the more unlikely the ultimate outcome.  Since this is his argument, I find it curious, to say the least, that Gould doesn't appear in the index (and I don't think I encountered his name in the text) of A Series of Fortunate Events, and that there's no entry for Wonderful Life or anything else by Gould in Carroll's bibliography.  That's a shame because Gould's analysis of historical contingency is exactly on point and, indeed, the metaphor he used to capture its essence is stellar.  Gould asked, what we'd end up with if "we rewound life's tape to a distant past."  (p. 308, emphasis added)  His answer and, clearly Carroll's as well, is that, replaying the tape would lead to a planetary biota quite different from today's and one that almost certainly would not include anything closely resembling us.

A second quibble which speaks to the rewinding and replaying of life's tape.  I think Carroll should have dealt with the implications for chance of convergent evolution, the process through which the same features arise independently in different taxa.  I'm not prepared to treat it here, other than to note that it suggests that some paths that evolution might take may be more likely than others, regardless of the starting point.  There may, in fact, be guardrails for the development of life, so, maybe, not anything goes.  [Later edit:  I discovered several weeks after writing this post that I'd already written about convergent evolution and its import for Gould's thought experiment of rerunning the tape.  It was in a post reviewing Jonathan Losos excellent book Improbable Destinies back in 2017.  Losos came down on Gould's side with caveats.]

That said, I have to salute the wonderful outcome of having misidentified the author.

Evolution and Historical Contingency ~ A Review of Improbable Destinies

Stephen Jay Gould threw down the gauntlet when he argued in Wonderful Life:  The Burgess Shale and the Nature of History (1989) that, were we able to replay the tape of life – that is, erase the metaphorical tape back to some point in the distant past – then “any [subsequent] replay of the tape would lead evolution down a pathway radically different from the road actually taken.”  (Wonderful Life, p. 51)  Evolution, in Gould’s view, is clearly not deterministic, rather, historical contingency holds sway.  By historical contingency, Gould meant that any particular state (of history, of the evolutionary paths of species) was “dependent, or contingent, upon everything that came before – the unerasable and determining signature of history.” (Wonderful Life, p. 283)

Thirty years later debate over the validity of Gould’s proposition continues.  Are the various constraints on evolution through natural selection such that, regardless of where we re-start the tape, the results shown in the replay eventually would be remarkably similar to what we see around us today, or does contingency play such a powerful role that ending up in the same place is not only improbable but perhaps out of the realm of possibility, dependent as it is upon myriad events occurring in just the right order?

This is the context for evolutionary ecologist and herpetologist Jonathan B. Losos’ new book Improbable Destinies:  Fate, Chance, and the Future of Evolution, a riveting and remarkably accessible exploration of convergent evolution and its import for Gould’s thought experiment.

Convergent evolution is the phenomenon in which different species evolve similar features, that is, they converge on very similar morphological features, say, wings in insects and wings in birds among other vertebrates.  The list of examples of convergent evolution in the natural world is rich and long and growing.  As Improbable Destinies makes clear, anole lizards, an early example and the centerpiece of Losos’ research, have been joined by a host of other instances of convergence involving many different kinds of organisms, ranging from snails to guppies, from bats to stickleback fish.

The wealth of such examples has led some scientists, paleontologist Conway Morris prominent among them, to conclude that (as Losos writes), “evolution is deterministic, predictable, following the same course time after time.  The reason, they argue, is that there are only so many ways to make a living in the world,” and so the tape actually would end up in the same place every time it is replayed.  (p. 5)

At the same time, Losos shows that the list of “evolutionary idiosyncrasies” isn’t short and often features creatures, such as the kiwi or the solendon, that have evolved on isolated land masses such as New Zealand, Australia, and Madagascar.  There’s a diverse richness here.  Losos notes, “Conway Morris and his colleagues have made long lists of examples of convergence, but it would be just as easy to make comparable catalogs of species without counterparts.”  (p. 87)  He argues that these one-offs may appear because “natural selection is either not as predictable or as powerful as some make it out to be.  That is, even when species experience identical environments, they might not evolve in the same way.”  (p. 88)

Losos describes many research projects relevant to our understanding of convergent evolution.  These include observation of convergence in the wild, particularly in settings showcasing natural experiments (principally islands), deliberate experiments in the wild as scientists transfer organisms to different locations and analyze their subsequent evolution (yes, as he makes abundantly clear, evolution need not proceed at the snail’s pace that Darwin would have had it), and finally experiments in the laboratory, often in petri dishes or vials involving microorganisms, such as E. coli.

The highlight of the book for me (and here it’s a page-turning pleasure) is Losos’ account of his research on the Anolis lizard (commonly referred to as the anole lizard) and his field work in the Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico).  Perhaps their most spectacular feature (reserved in most species to males) is the dewlap, “a flap of skin under their throat,” usually hidden from sight.

But when the lizard has an announcement to make – ‘Get lost, buddy, this is my territory” or “Hey, ladies, come check me out.  I’d make a good baby daddy’ – out comes the dewlap, arching downward from the jaw, forming a semicircle so large that the lizard often has to straighten its legs, pushing its body off the ground, to provide clearance.  (p. 58)

A nice example of the prose that graces the book.  Losos is able to make what is certainly tedious and physically difficult research on these lizards into an exciting narrative, a narrative that shows why the anole lizards are robust examples of convergent evolution.

He found that, although each of the four islands in the Greater Antilles has a different array of anole species, those species generally occupy similar ecological niches (bush, or different parts of trees) across the islands.  Those in each niche have evolved to be so similar from island to island that each of these “habitat specialists” can be mistaken for each other.  Yet all of the species on any of these islands are more closely related to each other than they are to any of the similar habitat specialists across the islands.

There is a great deal to absorb from Losos’ walk through the relevant research from the wild to the laboratory, much of it, particularly those natural and deliberate experiments in the wild, thoroughly engaging.  Yet two aspects of the research covered here are, to me, disquieting.

First, I appreciate what can be learned from projects that involve researchers moving organisms in the wild from one place to another (e.g., guppies from pools with predators to those without, or anoles to islands with no lizards), but, given how complex the array of contingencies is in the natural world, what are the real consequences of this manipulation?  I am reminded of how off putting I found ecologist E.O. Wilson’s research that involved fumigating entire mangrove islands in the Florida Keys to kill off all the arthropods.  Yes, he gained insight into how populations recover and how quickly, but at what cost?  Certainly I’m being overly sensitive about this and I’m sure my concerns would be discounted by those involved.  Still the deliberate interference bothers me.

Second, though much of Losos’ explication of the relevant laboratory research is extremely well done and informative, I had a nagging unease about this research.  It didn’t help that I often lost track of what the various projects were trying to prove and what they did prove.  Perhaps the more substantive part of my discomfort with the laboratory portion of the book has to do with my sense that test tube evolution with microbes is somewhat analogous to running computer programs that seek to replicate evolution:  a limited set of conditions are programmed in, with some random function added to mimic the mutations that natural selection has to work with.  The results can be instructive, yet they are, ultimately, artificial.  The full force of contingency that occurs in the natural world is largely missing in either the lab or the computer program.

None of this is to gainsay the importance of tracking myriad generations of microorganisms in controlled environments in the laboratory, not only to see if identical populations always end up in the same place, but also because, as at least one project was structured, researchers could, literally, replay the tape, taking a population to some previous generation and letting it reproduce from there. The message from such experiments, as I understand it, is that, yes, largely the tape replays exactly as it has run before.  But, and it’s a big but, not always.

Indeed, though most of the research from the field and from the laboratory would appear to support the evolutionary determinist camp – that the replay ends up in the same place – closer examination of the results, often over a longer range of time, shows that isn’t always true.  It’s most likely to be true when the populations begun with are closely related; such relatives are likely to evolve similarly when placed in new environments.  Not so when they are more distantly related.

But I didn’t think that Gould’s thought experiment envisioned the tape replaying exactly as it had before and the only thing in play being how evolution would be affected by random mutations in organisms.  Losos, citing philosopher of science John Beatty, acknowledges as much, positing that Gould actually embraced two concepts of contingency.  The first being that replaying of the tape with nothing changed to see if evolution would or could take a different course.  The second being the replaying of the tape from a specific point with the full panoply of historical contingencies allowed to be at work affecting living organisms.  Though these contingencies may be small events with important or unimportant consequences, they could, as well, be more dramatic things such as a storm wiping out the sole representatives of a species or an asteroid hitting the planet and decimating a host of species.  Each of these specific contingencies cannot be anticipated when we replay the tape – they may or may not occur – and they are likely to make all the difference in the world and we are likely not to end up in the same place with any replay.

In fact, and this needs no spoiler alert given the title of the book, I think Losos comes down largely in the Gould camp though his stance is somewhat nuanced.  As he writes,

So, can we predict evolution?  In the short-term, yes, to some extent.  But the longer the passage of time and the more different the ancestors or conditions, the less likely we are to prognosticate successfully.  (p. 336)

He adds, regarding our species,

If any of a countless number of events had occurred differently in the past, Homo sapiens  wouldn’t have evolved.  We were far from inevitable and are lucky to be here, fortunate that events happened just as they did.  Asteroids, of course, but what other events critically tipped evolution’s path in our favor?  Who knows how slight a difference in the past – a tree falling on great-great-great-to-the-millionth-degree-grandpa Ernie, a forest fire, a mutation – might have snuffed out our future existence?  (p. 335)

As he notes, we weren’t fated to evolve though possibly something like us might have.  Ultimately, no matter the outcomes one might anticipate from evolution, it is highly improbable that any specific one will occur.

In closing, I have to explain why I felt compelled to buy and read Improbable Destinies.  I had no choice after reading his interview in Current Biology (August 21, 2017).  He had me from the moment he answers the question:  How did you get started in biology?

Sure, he mentions dinosaurs as a gateway drug, but he goes wonderfully so much further, offering credit to:

a particular episode of Leave it to Beaver, the one in which the Beaver purchases a mail-order baby alligator.

According to the IMDb synopsis of this particular episode (frankly, I don’t remember this one which explains, I guess, why I didn’t become a biologist), Wally and the Beaver acquire a Florida alligator sub rosa and the laughs (such as they are) stem from their efforts to raise an alligator in the house without their parents discovering.  In contrast, Losos, smitten by the idea, was above board in his campaign to be allowed to acquire this kind of reptile.

. . . I knew that local pet stores sold baby caimans, the neotropical relative of alligators. The question was: how to convince my mother to allow a crocodilian in the house. Fortunately, my mom — not liking to say ‘no’ — passed the buck to the local zoo director, a family friend, expecting him to put the kibosh on the idea. To her dismay and my delight, however, he said that having an alligator was how he got his start in herpetology, and the next thing you know, I had a caiman in a kiddie’s wading pool in the basement.

In closing, I must say that my encountering this interview (which in turn prompted me to read and review the book) was contingent on a long, long chain of events that included spotting a turtle in my backyard several years ago and deciding to write a blog post on the turtle shell.  Current Biology had a relevant article and I actually paid for access to it (shows how desperate I was) which got me snared in the publisher's email web which meant the index to this August’s issue reached me.  Then again, even if I hadn’t seen the interview in Current Biology, given what I read I still would have seen reviews of the book and I might very well have ended up in the same place.