Replaying Life's Tape - No Miracles Required

Stephen Gould'due south influential thought experiment, "replaying life's tape," is usually construed as a version of "sensitivity to initial conditions," where slight differences in initial conditions atomic number 82 to substantial differences in event. I think Gould had something more than in mind; the replay experiment includes sensitivity to initial conditions, just there's more than to it than that. And the boosted part fills a void that, left unfilled, is quite mysterious.

Here'south the well-nigh famous, but somewhat ambiguous (in light of other things he wrote) version, from Wonderful Life:

I telephone call this experiment "replaying life's tape." Y'all press the rewind push and, making sure y'all thoroughly erase everything that actually happened, go back to whatever time and identify in the past . . . . Then let the tape run over again and run into if the repetition looks at all similar the original. (p. 48)

Gould's expectation was that "any replay of the tape would pb evolution down a pathway radically different from the road actually taken" (p. 51).

He strengthened this at 1 bespeak, specifying that the tape is to be replayed from the same event at the same fourth dimension and place to which it was rewound: "from an identical starting point" (p. 14). I'll call this the "identical starting point," or "identical" version.

But Gould seemed to contradict himself in favor of a weaker version when he suggested that we rewind the tape dorsum to some indicate in the past, then "Change whatever early event, ever so slightly and without apparent importance at the time, and evolution cascades into a radically unlike channel" (p. 51). Elsewhere he spoke of "alterations at the showtime" (p. 19). "Alter whatsoever early result," or alter any effect "at the showtime," sounds less similar replaying "from an identical starting point," and more than like replaying from a different (altered) betoken. The "altered starting indicate" version sounds a lot similar – really just like – sensitivity to initial atmospheric condition. In fact information technology sounds so much similar sensitivity to initial conditions that one wonders why Gould didn't merely call it that (if that's really what he meant). But anyway.

Elaborating on the "altered" version, we're supposed to go dorsum to some point in the past, characterized by a particular state of the earth – detail values of particular variables. Then, when nosotros printing the play button, the initial land is somehow dissimilar from the one we rewound to; information technology's slightly altered. A miracle! Well, it'due south a thought experiment after all. Or perhaps we go dorsum to some bespeak in the past, characterized by some land of the globe, and printing "pause" while we alter the values of the variables somewhat, and and then press "play." Still fishy. But once more it's merely a thought experiment.

Maybe information technology was the "altered" version that Matt Groening had in mind in the "Time and Punishment" episode of The Simpsons, with Homer altering the values of the variables. The relevant scenes begin with Homer trying to fix his toaster and inadvertently transforming it into a fourth dimension motorcar ("What the . . . ?!") that takes him dorsum to the age of dinosaurs.

 There he recalls the communication of his begetter:

 If you ever travel dorsum in fourth dimension, don't step on anything. Because the tiniest change tin modify the hereafter in ways yous can't imagine.

"Fine," Homer reflects, "Every bit long every bit I stand perfectly still and don't bear on anything, I won't destroy the future."

Simply of class he doesn't stand perfectly notwithstanding. He swats an insect buzzing around him.

Stupid bug! You get squish now! [GASPS] That was just ane piddling insignificant mosquito. That can't change the future, right?

Oh yes it can Homer. He returns to the nowadays to observe a totalitarian earth in which Ned Flanders is the dictator, carrying-out a brainwashing programme of "Neducation" and lobotomizing everyone who doesn't conform.

Homer returns to the by to make apology for his previous alteration. "This time I'g not gonna touch a thing. Mustn't crush. Mustn't impale. Made it." Simply so a dinosaur startles him, he stumbles backward and steps on a walking fish. "Oh. I wish, I wish I hadn't killed that fish." This time he returns to the present to find information technology pretty great overall except, horror of horrors, no one has heard of donuts.

He keeps returning to the past, altering information technology in some way until the globe is unlike, to be sure, but in a manner that's suitable to him. Namely, anybody has reptilian tongues and can lap up their food without demand of fork or spoon.

Is this episode a case of (repeatedly) rewinding back to some point in the by and replaying from a slightly different, Homer-altered point? No. It rewinds dorsum to the same point, and replays from there. Subsequently, things happen that might well non have. Okay, it's a foregone decision that Homer volition f**yard upwardly in some fashion or other, but not this way vs. that. As one replay gain, Homer has a chance encounter with an insect. Every bit another replay proceeds, from the aforementioned starting bespeak, he has a chance encounter with a walking fish. These seemingly insignificant differences brand very big differences later, which is the sensitivity-to-initial conditions part. Only there is more than sensitivity to initial conditions going on hither. From the same starting point, small-scale differences outcome by chance. Sensitivity to initial weather does not address how the initial, slight differences come nigh. On the "altered" version of the replay experiment, it'south a mystery how the initial differences ascend. The "identical" version, on the other manus, indicates that the initially resulting differences are matters of take chances. This is conveyed by replaying from the same signal to which the tape was rewound and showing that, from at that place, dissimilar trajectories are possible, with initially small differences compounding into larger.

Of course Gould's favorite illustration of the replay experiment was the Frank Capra moving-picture show, Wonderful Life. It replays the history of Bedford Falls with and without George Bailey, in order to show the despondent George that his life was not worthless, far from it. Proponents of the "altered" version of the replay experiment might imagine rewinding back to some point in the history of events that led to George's nascence, then replaying not from in that location just rather from some altered starting betoken that rules out his being born. Whereas, on the "identical" version we would rewind and replay from the same point in the past when the conception of George, or his birth, was possible simply when it was too possible that he wouldn't be conceived or born. Replaying the record would then lead to George sometimes existence built-in, sometimes not, with very big differences in outcomes for the town.

The moving picture doesn't exactly favor one version of the replay experiment over the other. We don't know for sure where the record was rewound to and replayed from. Wherever it was rewound to, it was then fast-forwarded to the climactic Christmas Eve to bear witness George how sadly things would have turned out by then without him. The best case for the "identical" version, where nosotros rewind to the same point – a point at which George might or might not have been born –  is that there are many other points in the picture where George merely happens to be in the right place at the right time. He might not accept been, but he was in a position to see the pharmacist miss-make full a prescription that would take had a lethal effect. He might non accept, only was present the 24-hour interval his little brother barbarous through the ice and George was able to relieve him. He might non have, but did suffer an ear infection from his exposure that solar day, dissentious his hearing and preventing him from going off to war, leaving him dwelling house to keep the family unit savings and loan going and to serve the working people of Bedford Falls in that mode. He might not have been in town – he was supposed to have already departed on his honeymoon – when the run on the Due south&L started; but he was still around and was able to stem the tide. Throughout the film there are events that take identify that might not have; and had an alternative possibility occurred, the divergence in outcomes would accept been enormous. The history of Bedford Falls could take been rewound to nearly any point and and replayed, and would have had a significantly different ending many times if not every time. There's no reason to rewind, alter, and play. Just rewind and play.

The best case for the "altered" version of Wonderful Life is that replaying the history of Bedford Falls for George's benefit was the brain child of his guardian angel, Clarence. Which suggests supernatural intervention. Which in turn makes some sense (I guess) of an otherwise miraculous alteration of the point to which the tape was rewound. And it would perhaps likewise provide a providential interpretation of what would otherwise seem similar chance events in George's life that were highly consequential. I'll render to miracles presently.

A film that very explicitly plays, rewinds, and replays from the very same starting betoken is Run Lola Run. Over and over, at the same place and time, Lola takes the phone phone call from her boyfriend, screams, and starts running to save him. Subsequent gamble happenings are the lilliputian differences that lead to very different outcomes for her and Manni.

Sorry! Back to Gould and the volume Wonderful Life. One of his main objectives was to bring the replay experiment to bear on the supposedly "lottery"-like decimation of organic forms in the late Cambrian, and its consequences. Among other things, he argued that Pikaia, the first known chordate, might well have been among the unlucky losers, in which case there might be no chordates, including us. How would a proponent of the "altered" version of the replay experiment bring it to touch on the Pikaia-human thesis? Presumably by rewinding the tape back to Pikaia's survival among the decimation, and replaying it not from there but from Pikaia's demise instead. That's definitely not what Gould had in heed:

My fundamental experiment in replaying the tape of life begins with the Burgess creature intact and asks whether an contained act of decimation from the same starting betoken would yield anything similar the same groups and the same history that our planet has witnessed since the Burgess maximum in organic disparity. (p. 188)

In other words, he intended that nosotros rewind dorsum, across the decimation of organic forms, somewhere closer to the earlier Cambrian "explosion" (increase) of organic forms, and replay from in that location. And so, via multiple replays, we would see that Pikaia might not have survived, just did, with cracking upshot (if you lot remember of united states of america as existence of whatsoever cracking upshot). Why rewind back to some point and change it to get Pikaia out of the flick, when simply replaying from a point prior to the decimation would often have the same upshot, with no miracles required, merely the luck of the draw? What is the signal of altering? I have a suggestion that I'll go to when I return, as promised, to miracles. As well, how does the "altered" version address the lottery-like decimation of forms that Gould emphasizes? It doesn't. Information technology says zip all most whether Pikaia'due south survival was a matter of luck or non. Worse, information technology buries the question and tampers with the evidence. It's the "tampered" estimation.

A reconstruction of Pikaia

A reconstruction of Pikaia

One more affair before I get to "Why, why alter?" and miracles and the similar. Toward the cease of Wonderful Life, Gould lamented that the record can never really be replayed. Okay, but there'due south a decent adjacent-best-affair, namely to comport simultaneous plays/replays of the evolution of initially identical populations maintained in identical (or identically modified) environments. The most enterprising and influential of such efforts is the "Longterm Evolution Experiment" conducted by Richard Lenski and his students and other collaborators. The experiment involves twelve, initially identical (cloned) populations of Eastward. coli , as they evolve in identical (and identically altered) chemostat environments. The investigators have detected a number of differences in evolutionary outcomes amid the twelve lines, differences that cannot be attributed to initial genetic differences, nor to different selection pressures (since the groups have faced identical option pressures in their identical environments), but that seem instead to depend on chance differences in the variations (and order of variations) that take arisen in the different lineages.

One interesting departure in outcomes involves an adaptive opportunity built into the experiment from the beginning. The twelve cloned populations were grown on media that included citrate, which Due east. coli was known not to metabolize. Simply the investigators considered it inside the realm of possibility that the bacteria might evolve the power to brand utilize of citrate equally a carbon source. Every bit of 30,000 generations, none of the twelve populations had done so. Only by 31,500 generations, ane lineage had succeeded.

This left the question whether the 1 population had experienced an extremely rare mutation that would ultimately occur in the other populations every bit well, rendering the difference in outcomes merely temporary. Or whether the population in question had past that time, through a serial of happenstance events, evolved to become uniquely capable of taking the last evolutionary steps in the management of citrate metabolism. The investigators were able to discriminate between the possibilities by employing the frozen " fossil record " of evolution upwards to that bespeak. That is, after every 500 generations, samples of each lineage had been frozen. So the researchers were able to support to a bespeak in time in the history of that lineage, prior to the evolution of citrate metabolism, thaw the ancestors, and replay its development multiple times from in that location. And what they found was that the power to metabolize citrate arose over and over once again, suggesting that, by this indicate, the lineage in question had get uniquely capable of making this evolutionary breakthrough.

A nice combination of replays that diverge and replays that practice non.

The authors' literary conclusion draws from the final passage of the Origin of Species ("Fifty-fifty from then unproblematic a kickoff . . .") and from Robert Frost:

Even from and then simple a showtime, small happenstances of history may lead populations along different evolutionary paths. A potentiated prison cell took the one less traveled past, and that has made all the divergence. (Zachary Blount et al., "Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli ." PNAS 105: 7899-7906, p. 7905)

Lenski's and his collaborators' project is supposed to exemplify a real replaying of life'southward tape. That's why the twelve plays/replays begin from identical starting points. That'southward why they practise not begin from slightly contradistinct starting points.

Why change? Why alter? Why alter? I mentioned miracles. The connection involves causation and determinism. Sensitivity of outcomes to initial conditions – which is at to the lowest degree role of the replay thought experiment – is in keeping with a prominent notion of causation as "counterfactual difference-making." To say that antecedent event A1 caused outcome O1 is to say that, had A1 not occurred – had A2 occurred instead – O1 would not have resulted. The occurrence of A1 vs. A2 makes a difference. Where sensitivity to initial conditions goes farther is to propose that the occurrence of A1 vs. a slightly dissimilar A2 makes a large difference. But that's not the important thing for now.

The of import thing is to see how differently the criteria for counterfactual departure-making can be satisfied. Proponents of counterfactual notions of causation are understandably concerned to juxtapose what actually happens or happened with counterfactual situations that are relevantly similar; realistic if non real. To attribute the extinction of dinosaurs to an asteroid impact is to say, among many other things, that had the asteroid been chosen back at the very final 2nd by the extraterrestrials who sent it, and then the dinos would have lasted much longer. But that counterfactual is also unrealistic and is of little help when information technology comes to making sense of what actually happened. At that place'southward a tradition of juxtaposing what happened with counterfactuals that not only accept for granted the actual laws of nature only everything else that has happened in the existent world up to and except for the putative causal consequence. Now, for a determinist, not only is it not the case, but it could not possibly be the example that the events of this world transpire exactly as they have, governed by our laws of nature, up to the event in question, at which point something else happens instead. Only not to worry! The counterfactual world is not our globe; it's an culling "possible world" similar ours in all the to a higher place respects upwardly to and except for the putative causal upshot. Notwithstanding, for determinists, the problem does not go away so easily, considering events could no more play out in this mode in the alternative deterministic world than in our deterministic world. Thus, proponents of this approach attribute to the culling world what they cartel not attribute to ours, namely "miracles" (yes) – "minor miracles" to be sure, but miracles withal. There is a cost to pay for determinism! Indeterminism also has its costs, to be sure, but the indeterminist has no problem with a counterfactual scenario in which events transpire in a specified way (co-ordinate to stochastic laws of nature) up to a point where one of two or more than alternative events could happen adjacent. 1 needn't resort to miracles happening in culling worlds for appropriate counterfactual situations.

Doesn't the determinist'due south pickle sound like the predicament faced past proponents of the "altered" replay experiment, i.e., having to invoke miraculous or otherwise fishy alterations of the events to which the record is rewound, so that the replay begins from a different starting signal? The source of the problem may be the aforementioned. Determinism has been a major motivation for proponents of sensitivity to initial weather condition. Sensitivity makes sense of the practical unpredictability of so many phenomena, but without abandoning determinism. Paraphrasing Edward Lorenz, the present determines the future, its just that the approximate present does non approximately make up one's mind the future.

Proponents of the "altered" version may retrieve they're doing Gould a favor by not attributing to him the kind of indeterminism that the "identical" version seems to embrace. Merely I don't call up he would have appreciated the generosity. Consider his last thoughts on related issues in The Structure of Evolutionary Theory. In the epilog to the terminal affiliate he bemoaned the pervasive conception of scientific understanding that acknowledges the importance of initial conditions, together with laws of nature, but does not regard "the resolution of such details [the initial weather condition] as essential or causal components of the explanation itself" (pp. 1332-1333). Information technology'south not immediately clear what he meant by "the resolution" of the initial conditions, but I retrieve he was saying that it matters to our understanding of the outcome how the initial conditions came almost and especially whether they were matters of hazard.

The line just quoted is followed by a parenthetical diatribe nearly how his undergraduates typically responded to the thought of real take chances by parroting Laplace, insisting that the appearance of chance is just a matter of ignorance (this does seem similar an undergraduate obsession), and moreover that "if science works at all, [it must] be truly deterministic" (p. 1333). To which he responded,

Natural historians accept besides ofttimes been apologetic, but about emphatically should not be in supporting a plurality of legitimately scientific modes, including a narrative or historical style that explicitly links the explanation of outcomes non merely to spatiotemporally invariant laws of nature, only also, if non primarily, to the specific contingencies [happenstance] of antecedent states [initial conditions], which, if constituted differently, could not have generated the observed result. (p. 1333; my italics)

"The specific contingencies of the ancestor states" are not addressed by sensitivity to initial weather. The question of their contingency is not only ignored, but the show is effaced by the "altered" version of the replay experiment. Strangely effaced. And at the cost of realistic counterfactual conditions for understanding what really transpired.

There'southward no demand to rewind, alter in some miraculous or otherwise sketchy way, then play. Just rewind and play. And relish. Just watch out!

D'oh!

Bio

John works in history and philosophy of science, especially evolutionary biology, and also issues concerning scientific authority, and relationships between science and the state. With regard to the problems that matter virtually here - on this wonderful "Extinct" blog - John is especially interested and perplexed by things having to do with run a risk.