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If Earth instantaneously reverted back to when the only organisms were small microscopic creatures in the sea, would evolution happen the same exact way creating the same animals and species? Was chance a factor in the world as it is today? How drastic could that change be?

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    $\begingroup$ Comments are not for extended discussion; this conversation about whether and how this question can be answered has been moved to chat. $\endgroup$ – Monica Cellio Jun 21 '18 at 17:49

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The first thing that it's important to understand is that evolution is a by-product of natural selection, not the driver of it. That is to say, every change introduced by DNA combinations and mutations over time is (more or less) random, although the selection of which will become more common over time is not.

Ultimately, the most important factor in the evolutionary development of life on Earth is the environment in which that life exists. We know that life has gone through several mass extinctions in the past, and that the environment of the earth has been very different at different geological periods. Some periods have been warmer than now, some colder, some have had atmospheres with higher oxygen concentrations... The list goes on.

If life on this earth started over tomorrow, the selection of traits via natural selection would be based on today's environment, not the one in which early life formed. In the billions of years that life could potentially survive on the earth, its environment will change to be unrecogniseable from the former environment many times. As such, it's highly unlikely that life would evolve along similar lines to the past.

For one thing, early life had to survive in an environment where oxygen didn't exist as a freely available atmospheric gas. It was a form of life (probably cyanobacteria) that triggered the Great Oxygenation Event that made more conventional life possible at all. That step wouldn't be required the second time around meaning that the forms of life that allowed this to happen aren't necessary, so the divergence of species is starting from a completely different trunk to begin with, in a different environment that is going to 'reward' different traits.

I wouldn't say we are what we are today by 'chance', but it's clear that we could have evolved down a completely different path had environmental factors been even slightly different in certain quarters. The path to us leading from the past is clear, albeit fragile. If life was to begin that path again, it would take a different route because the starting conditions are different.

Add to that environmental factors that cannot be controlled directly by life on the earth (like the meteorite impact of the late Cretaceous) as massive disruptions to the status quo, and the probability of parallel development falls to so close to zero as to be certain not to have happened.

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    $\begingroup$ While natural selection in the aggregate can average out to something reasonably deterministic, each individual organism's survival is highly dependent on chance. $\endgroup$ – Acccumulation Jun 20 '18 at 16:24
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    $\begingroup$ Natural selection might be "reasonably deterministic", but the mutations which drive natural selection aren't. At all. $\endgroup$ – Beanluc Jun 20 '18 at 23:37
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    $\begingroup$ you would also end up with different mass extinctions, so you are changing several very important events in the evolutionary history. $\endgroup$ – John Jun 21 '18 at 14:18
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    $\begingroup$ @Beanluc Not to mention that the 'chapters' of evolution are driven by unpredictable events such as the Ordovician–Silurian extinction. The environment that life evolves in is largely about the other life that exists in it. Flipping the chessboard every so often dramatically changes the direction of evolution. $\endgroup$ – JimmyJames Jun 21 '18 at 14:20
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    $\begingroup$ I do not think this is answering the question that the OP intended to ask. From the second comment on the question: "reverting back as in going back to that time". So your statement "If life on this earth started over tomorrow, the selection of traits via natural selection would be based on today's environment, not the one in which early life formed" is correct, but not relevant to the question being asked. $\endgroup$ – Robin Saunders Jun 21 '18 at 14:36
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No, but...

If you restart evolution, you would end up with new species. But some of their features may be very similar to what we know in our world.

The Nobel Prize winner Jacques Monod said that evolution is based on chance and necessity. According to him, there is no final causality that would lead evolution towards a specific goal (like creating Humans or sapient species).

Chance is the random part of the equation, that will bring you different results every time you roll the dice. During a long period of time, Life throws so many dice, so many times, that there is no chance that you could end up with the exact same results.

But necessity is what causes natural selection, and under the same conditions, similar features may be selected. This process leads to "convergent evolution". It means that different species may develop analogous structures independently from each other (in a sense that they don't inherit that feature from a common ancestor).

To get a good example of this process, you can compare the anatomy of dolphins, sharks and ichthyosaurs. They are very similar, even if dolphins are mammals, sharks are fishes and ichthyosaurs were reptiles. But life in similar aquatic environment gave them similar shapes.

A shark, an Ichthyosaur and a Dolphin

So, if you restart evolution, you will get very different species, but some of them may look familiar.

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    $\begingroup$ years ago i saw a documentary that speculated that if dinosours hadn't gone extinct, they would have developed into a human like form. $\endgroup$ – eMBee Jun 20 '18 at 16:22
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    $\begingroup$ @eMBee they didn't go extinct; look at an angry goose and tell me that it isn't a velociraptor! $\endgroup$ – Nicolas Budig Jun 20 '18 at 16:32
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    $\begingroup$ @Nicolas Budig: That's not a velociraptor: gannett-cdn.com/media/2018/04/23/DetroitFreeP/DetroitFreePress/… $\endgroup$ – jamesqf Jun 20 '18 at 19:13
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    $\begingroup$ "Very different" could use some very strong emphasis. The reason bony animals are so similar is, their ancestors survived particular extinction events that drastically pruned the tree so much it looked more like a twig. Without those particular events recurring exactly the same way, the results might be nothing at all like life as we know it. $\endgroup$ – cHao Jun 20 '18 at 21:34
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    $\begingroup$ What your answer discusses is convergent evolution. Similar adaptations will appear in similar ecological niches. Because that's what natural selection does. $\endgroup$ – a4android Jun 21 '18 at 6:09
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It is extremely unlikely that starting over the evolutionary path we might end up in the very same situation we have today.

Evolution is determined by the interaction of the organisms with the surrounding environment, and though I have no data to support my affirmation, I am convinced that this is a chaotic system, meaning that small differences in the initial conditions would lead to dramatic differences in the evolution of the system itself.

Think, just as an example, at how can you have evolution of mammals in a world where the dinosaurs don't go extinct because there is no massive volcanic eruptions and meteorite strike.

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It depends on what you mean by "reset".
If you can somehow reset it perfectly so that all elementary particles are exactly the same as they were at some point in the past, and all input (cosmic radiation) happens exactly the same again, and so on and so forth, then you have a chance of having thing work out exactly the same again.

But if anything is different then Chaos will take over and who knows what you'll get.

Not to get crude (or personal), but if a different one of the thousands of sperm had made it to the egg first, you wouldn't exist. If you go back before someones birth and change even the smallest thing, the chance of them being born is vanishingly small. Some baby might be born, and might even get the same name, but it wouldn't be the same person.

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Genetic mutation is a product of quantum (e.g. true) randomness.

DNA mutations happen at the molecular level, making them subject to quantum mechanics. Thus, evolution couldn't possibly happen the same way, because it's wired into the only true source of randomness in the universe. In my humble opinion, evolution is, in a way, one of the great manifestations of quantum randomness at the macro level. But here's some research:

Duke University researchers have witnessed DNA bases making the slightest of changes -- shifting a single atom from one spot to another or simply getting rid of it altogether -- to temporarily mimic the shape of a different base. These "quantum jitters” are exceedingly rare and only flicker into existence for a thousandth of a second, and yet have far-reaching consequences.The study, which appears March 12 journal Nature, indicates that these jitters appear at about the same frequency that the DNA copying machinery makes mistakes, which might make them the basis of random genetic changes that drive evolution and diseases like cancer.

Source: https://today.duke.edu/2015/03/quantumjitters

See my own question here: As a time traveler, how would I see quantum randomness change history?

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    $\begingroup$ Everything is a product of quantum randomness, yet some processes are deterministic at macroscopic scale. The enormous changes in the environment between day's Earth and the one billions of years ago when life started swamp any quantum considerations. $\endgroup$ – Schwern Jun 20 '18 at 19:35
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    $\begingroup$ There may be quantum randomness involved, but the mutations themselves aren't completely random. Cells contain repair mechanisms, but they don't respond to all sequences the same (as an analogy, we have a lot of difficulty with centromere sequencing. While bases may mutate randomly, the repair machinery's ability to fix this before the next replication varies quite a bit $\endgroup$ – Punintended Jun 20 '18 at 20:05
  • $\begingroup$ @schwern - processes aren't deterministic at macroscopic levels. They are approximately deterministic, to the point where deterministic models are typically accurate enough for any measurements humans are likely to make. Humans don't have a sufficiently powerful computer or sufficiently advanced model to tell what would happen if we went back in time and quantum randomness went a different way. That bowling ball will fall in a manner that looks deterministic - but maybe quantum randomness alters its path the tiniest amount, which causes vibrations in the air and sets off a hurricane. $\endgroup$ – Scott Jun 21 '18 at 6:13
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    $\begingroup$ There's no evidence that quantum mechanics are true randomness, except from the limited human perspective $\endgroup$ – Callum Bradbury Jun 21 '18 at 11:20
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    $\begingroup$ @CallumBradbury Until you can tell me a photon's location and momentum deterministically instead of probabilistically, Bell's theorem stands. God does, in fact, play dice. $\endgroup$ – ognockocaten Jun 21 '18 at 14:22
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This is secretly a physics question, and the answer is we don't know. A lot of quantum physics seems to suggest that the laws of nature are probabilistic at core, but deterministic "hidden variable" or pilot wave theories are still kicking, if unfashionable in the wake of Bell's theorem. In fact, NASA's proposed EM-drive propulsion system seems to depend on a pilot wave theory. (Unfortunately, independent testing has implicated the Earth's magnetic field, and not quantum weirdness, as the source of observed thrust in EM-drive tests.) Most physicists, however, will assure you that the laws of nature are fundamentally probabilistic and therefore in a scenario like this you could expect evolution to take a different course rather than unfolding exactly the same way as in the first trial.

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  • $\begingroup$ Since the OP said reset and not 'go back in time', I don't see how Earth2 would be the same. In particular, it wouldn't have a killer asteroid hit at the precise time to disrupt the dinosaur era. $\endgroup$ – Jiminion Jun 22 '18 at 16:56
  • $\begingroup$ I don't agree that it's clear that the asker intended that ONLY THE EARTH be reverted, but if that is what they meant then you're right, even in a deterministic universe Earth's second history would diverge from its first one the instant something extraterrestrial affected it, including asteroids, local supernovae, or even radiation from our own star (which is quite different than it was a billion years ago, since it's been shrinking.) $\endgroup$ – SudoSedWinifred Jun 22 '18 at 17:17
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Evolution is a sequence of random changes that best allowed a life form to survive in a specific kind of environment.

Your question is also ill-posed: nothing happens in 'the next second' unless the destruction was caused by an impact of such magnitude as to delete life at all. There will be always something in the scale of insects, if not small scavengers as well, that will survive the event.

After that, it's all pure speculation. One thing for sure, life will not be what preceded the event

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Evolution is highly noisy and stochastic and also chaotic in the sense that slight change in the initial conditions might lead to a very different results. It is an interesting question whether there are bifurcation points in this process. It is very unlikely that the evolution would take the same course it has taken even if condition would be almost the same (They can't be exactly the same ). Having said that, due to scale separation it might happen that certain things would evolve to be the same. Due to the fact that there is no theoretical exact solvable model to this problem, and there is also no capability at the moment to simulate this, it cannot be answered to a greater certainty.

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The short answer to this question is "no", as previously discussed by all the comments before me. However I want to touch a slightly different topic: Intelligence.

Humans are the most intelligent species on the planet. However, it seems that other species like crows and octopuses developed a good level of intelligence even though they are not mammals.

So drawing the conclusions from the information we have, it seems that intelligence is the end-game of survival (we are the most adaptive species on the planet). And by looking at non-mammalian species developing a medium-high level of intelligence, it seems that if evolution were to take another course and humans and even all mammals would not have existed... other type of species would have been able, in time, to reach the level of intelligence that we humans have.

And looking at the results so far, we humans developed an intelligence maybe 3-5 times higher than the second most intelligent species and we were able to control the planet and break the pyramidal survival equilibrium game other species are playing. What if evolution continued without humans and in 500 million years from now a species have evolved to be 10 times more intelligent as we are?

It can also mean that if life exists on other planets, there is a high chance that life tends towards intelligence as it's one of the best survival traits possible.

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    $\begingroup$ Seems like a good first answer. Welcome to world building $\endgroup$ – Trevor Oct 1 at 21:34
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In short: probably not

Even if this question can be answered only with some wild speculation, it is probably safe to assume that life forms as we know today will not evolve the same way: maybe there will still be mammals but not the one we know today.
Probably though, whatever life form present will tend to evolve to the same ending point we know (but maybe not).

For example there is some speculation about the fact that if the dinosaurs had not died out (for whatever reason) they may have evolved into a humanoid form instead of us, since the mammals would never had the chance given them by the disappearance of the dinosaurs.

But these are, as said, speculations: there is no way to prove them and there is no way to know what will happen if for some reason the time will be reset to 4.5 billion years ago

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Since you commented:

Well no, reverting back as in going back to that time.

There may actually be a chance. In the end it boils down to how (if) time-travel works in our universe.

If it turns out that we live in a truly deterministic universe (so all things, even those that look random to us, are pre-determined or follow some logic), then yes, after reverting to that time, everything would play out exactly the same again. In essence the universe would just be a videotape which you rewind and watch again.

However, it seems more likely from our knowledge of quantum physics that the universe is inherently probabilistic, i.e quantum phenomena are truly and fundamentally random. In this case evolution would not play out exactly the same way, because the primary way species develop is through random mutations of DNA, some of which get selected for. These mutations are for example caused by radiotion and the interaction of ionizing radiation and matter is a quantum phenomenon. After that, add everything said in the other answers.

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  • $\begingroup$ I'm pretty sure we've been able to prove that quantum mechanics don't have an underlying (but unknown) set of classical mechanics. $\endgroup$ – Draco18s Jun 20 '18 at 22:28
  • $\begingroup$ @Draco18s You're probably thinking of Bell inequality tests, which have been purported to rule out stuff like locally hidden variables. But, it's pretty much moot; quantum mechanics is entirely compatible with determinism, e.g. de Broglie–Bohm theory or (more comically) superdeterminism. $\endgroup$ – Nat Jun 21 '18 at 10:56
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    $\begingroup$ Regarding the idea that the universe might be inherently probabilistic, that's one of those obnoxious misconceptions that seems too popular to go away. The first issue is a scientific one, i.e. that no scientific experiment ever has tested something like if the universe is inherently probabilistic (though Bell experiments are often misdescribed that way). But the deeper issue is that the notion of something being inherently probabilistic is an inconsistent concept; it demands an undefined frame. $\endgroup$ – Nat Jun 21 '18 at 11:06
  • $\begingroup$ @Nat I've seen the pilot wave theory before, and I'm not qualified to talk on it, but this is amazing to watch. The way the droplets of the oil travel around and behave in quantum-like ways is really cool. $\endgroup$ – Draco18s Jun 21 '18 at 15:53
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If you could find only microscopic organisms then they are protozoans. According to your question if the earth is instantaneously reverted back then the atmospheric conditions will remain the same and the same species will be evolved. But if the earth is destroyed completely and there is no life forms , then if there is evolution ,the species evolved depends on the natural selection , how the genetic material DNA is formed , so the evolution will be different and new species would be formed.

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The evolution from bacteria to humans can be deterministic. Life works like a heat engine. 4 billion years ago, the temperature of the Cosmic Background Radiation was higher, that the temperature difference between Earth's surface and CBR was very low. Now it is nearly 225 degrees Kelvin. The evolution from simple to complex would have happened incrementally with the gradual increase in the temperature difference that make heat engines more efficient, or life more efficient. If we were introduced 4 billion years ago, our brains would be of no use because it will not be able to process anything given the smaller temperature difference. Similarly if primitive life is introduced now, it may not survive. So a going back in terms of expansion of universe will once again bring humans back. Chance has a negative role, it may sometime disrupt this with some unseen events.

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One of Stephen Jay Gould's best-known works, "Wonderful Life: The Burgess Shale and the Nature of History" addressed this question in an interesting way. He talked about "replaying life's tape" rather than reverting it back and restarting. His conclusion is fairly quotable (and has been often quoted):

any replay of the tape would lead evolution down a pathway radically different from the road actually taken.

Gould's thesis has not won universal assent, but the book is a good read and has direct bearing on your question.

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No, but exactly why it is a no depends on what you mean by "reset". For example, let's say just life resets. It would be different to some degree, due to mutations being completely random, but similar mutations would tend to thrive, e.g. wings, because the environment would be almost, (except for stuff caused by photosynthesis in the atmosphere,) completely the same. Let's say the earth as a whole reset. Then it would be completely different, because the natural selection would be different, due to the environment being reset.

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    $\begingroup$ Welcome to Worldbuilding, brad_2_da_mc_boi! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 20 '18 at 13:19
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Absolutely not. For example mammals like us developed due dinosaurs extinction. As we know it, that huge and far more powerful species was erased from Earth by asteroid impact (~160.000.000 years ago). This good example proves that most species we have on Earth in current form is mostly a matter of chance.

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  • $\begingroup$ I'm pretty sure Dinosauria is a clade, not a species. $\endgroup$ – forest Jun 22 '18 at 3:05
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I don't see any answer that mentions abiogenesis.

Evolution is defined in terms of change, so you have to have life before it can supposedly evolve, and that's where abiogenesis comes in. So the start of an answer to the evolution question must depend on a similar question about abiogenesis.

But no one has come up with a plausible theory for how abiogenesis occurred, if it indeed occurred. There are a number of interdependent complex molecular machine molecules which are needed for even simple life. These include polymerase enzymes, DNA Helicase, and ribsomes. These are all encoded in DNA, but they require the molecular machines themselves to synthesize themselves--the original and ultimate chicken and egg problem, which confounds the answer.

On top of the complexity, there's the question of the origin of building blocks: amino acids and nucleotides. Some argue that they've been produced external to life, but they would have to all be produced in sufficient quantity and concentration to enable the original organisms to be built.

So the answer to the original question seems certain to be no.

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  • $\begingroup$ you don't need DNA at all for life and without life to consume them building blocks tend to build up. but most importantly abiogenesis therefore its occurrence is likely. $\endgroup$ – John Oct 1 at 21:36

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