Suppose we wind time back to the instance when life emerged on Earth and let evolution start over again, will human beings or any other kind of self-conscious animals evolve ultimately, inevitably?

Or do these sorts of animals just evolve as a result of some randomness processes in evolution, if so, what's the probability? Is it great or small?

If randomness does exist, then to what degree will it affect our ability to depict evolution? Will we still be able to describe the general trends of evolution, for example, the trend of more and more complicated animals evolving?

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    $\begingroup$ there is no way to answer this question, because we don't have other systems to observe. $\endgroup$ – MattDMo Jan 12 '14 at 16:10
  • $\begingroup$ @MattDMo But observing our own system is enough. I mean if we repeat our own system again and again, will there be human beings or similar creatures eventually? $\endgroup$ – user5407 Jan 12 '14 at 16:20
  • $\begingroup$ @Peterchar I hope my answer will help you understanding why your question is not answerable. $\endgroup$ – Remi.b Jan 12 '14 at 16:37
  • $\begingroup$ @Peter It is obvious that some of us (well, me) interpret your question differently. Are you primarily interested in evolutionary processes 1) in general, 2) from Earth (4bn years ago)-like circumstances, or 3) from the exact "state of the universe" when life took off on Earth? $\endgroup$ – user3395 Jan 13 '14 at 18:41
  • $\begingroup$ For the second one, yes. For the first one, I'm just asking if we could predict the general trend or even particular spices under the effect of randomness(if exist). For the third one, what does 'state of the universe' mean? Thank you! $\endgroup$ – user5407 Jan 14 '14 at 2:57

Evolution is largely random, because most of the processes that drive evolution are random. A few ideas you should understand to realize why it is so random.

Most people are only aware of natural selection when it comes to evolution, and think that natural selection has a goal of creating new, increasingly sophisticated forms of life. None of this, of course, is true. Evolution is not only natural selection, there's also genetic drift, gene flow, and mutation, which are random. Natural selection is not technically random, but it's not the biggest player. Secondly, natural selection does not have any goals and does not move forward, create more complex life, invent adaptations, or does anything of this sort. Natural selection is not designed to produce perfection, it just acts upon whatever variation is present in population, and favours individuals that are more successful in reproduction. Case study: 98% of our DNA doesn't code for any proteins, and presumably most of this DNA is junk. Does it sound like perfection to you, like we are the ultimate species?

The reason I tell you all this is because, in my opinion, when asking this question of "inevitability", you assume that there's some guiding hand that leads evolution towards some end-point, a climax. But there is no such guiding hand, and it takes a number of incredibly unlikely events to occur in a specific order for: Earth to be created with such conditions, life to start on Earth, collective learning to evolve in some form of live, agricultural revolution to occur so this form of life can thrive, etc. If at any stage something went different, everything would be different.

Final thoughts, why are you so sure that humans won't go extinct just like 99.9% of all the species that ever existed on Earth? A typical species goes extinct in about 10 million years, and we've been around for only 200,000 years. If you accept this possibility (which is real, in my opinion), suddenly, it doesn't seem that intelligent life, and we as its representatives, is some inevitable end-point in evolution, does it?

Edited: As to your last (new) question, "if randomness in evolution exists, are we still able to describe the general trends in evolution?". Firstly, of course randomness exists in evolution! Mutations are caused by errors in the process of DNA replication, genetic drift is caused by random sampling by definition. Secondly, there is no universal trend to more complicated organisms being evolved, as I mentioned above. Some organisms do get more complex, but some organisms haven't changed much over very long time, some are "living-fossils". Once again, evolution has no goal to create more complex life -- if, say, nautiluses or horseshoe crabs are capable of surviving and reproducing in the niches they occupy -- there's nothing else needed for their existence. Furthermore, fitness is linked to the environment, which is dynamic -- many complex species went extinct during global changes in the environment, whereas what you might call as "primitive" species of unicellular bacteria survived.

  • $\begingroup$ Philosophers and physicists argue among themselves whether nature is random or not and even whether this is a proper question. But, assuming for the moment, that nature is not random (a minority position), then "most of the processes that drive evolution are random" would be false. $\endgroup$ – user3395 Jan 13 '14 at 10:32
  • $\begingroup$ @GlenTheUdderboat, my problem with this debate would be: "what evidence do you have to suggest that seemingly random events in evolution are not based on chance but rather follow some predetermined path?", and to my knowledge no one can give me such evidence. Thus, I prefer utilizing the principle of Occam's razor and stick to the simplest hypothesis. $\endgroup$ – Th334 Jan 13 '14 at 10:52
  • $\begingroup$ Including gene flow along mutation, natural selection and drift seems a bit weird to me. Things that are random are subject to probability and you do not adress this part of the question of the OP. There are some stuff that tend to increase through evolution, why would intelligence not be one of them. YOu should argue also according these lines. $\endgroup$ – Remi.b Jan 13 '14 at 10:56
  • $\begingroup$ Of course I have no such evidence. But I can now see that for a biologist by far the most practical way to deal with his or her subject would be to assume a probabilistic world. (But perhaps keep in mind that this isn't a fact like other biological facts.) $\endgroup$ – user3395 Jan 13 '14 at 10:56
  • $\begingroup$ @Remi.b, why is it weird for you to see gene flow, drift, mutation, and selection together? These all are the mechanisms of evolution, and my point was that only the natural selection seems to have no chance involved. Other factors contribute more to the evolution and are random. As to your other point, I addressed his question "what's the probability? Is it great or small?" by saying that it seems "incredibly unlikely", and giving example of the chain of incredibly unlikely events that should happen for modern human to appear. $\endgroup$ – Th334 Jan 13 '14 at 11:05

Problems of epistemology in your question

Two problems of epistemology (which discussion is a matter of philosophy rather than science)

  1. The whole question depends on what you consider being random and what you consider being determinist. The number that come out from a dice is random unless you know exactly how the dice was thrown (and make some calculation about the cinematic of the dice).

  2. Consciousness is in itself rather hard to define as well. Several definitions has been suggested in science and philosophy.

Anyway even with these philosophical issues behind the question, one can assume some common although improperly defined used you may have of these terms and safely answer that no, self-counsciousness is not a necessary consequence of evolution.

Evolution of cognition

The condition under which greater cognitive abilities is beneficial are quite restrictive and even today the way that human cognition evolved is under debate. This wiki article will give you an introduction into this field. The immense majority of all lineage did not evolve great cognitive abilities (Have a look to different tree of life and to the diversity of life).

In humans, a fourth of the total amount of ATP (molecule that is the equivalent of a battery of living organism) is dedicated to the brain. This is a huge cost! It seems quite extraordinary that something so costly can evolve because the benefit are quite hard to grasp. For example. A bird with a heavy brain won't fly well, therefore it is counter-selected. What would a beetle do with a big brain? Well not many things. It would only be a cost with no benefit. Natural selection does not favor cognitive abilities for most of the lineages and as already said, great cognition is favored only under some restrictive conditions. Intelligence is by no-mean the end-point, the goal or the consequence of evolution.

How to answer to "What is the probability"?

In order to calculate the probability of something to happen you need some a priori knowledge. For example, the probability of drawing the king of heart in a deck of card obviously depends on wether you consider that you know how many cards there are in the deck. You might say that you don't know how many cards are in the deck but you know that 10% of all deck in the world have 36 cards and the rest 90% of the decks have 52 cards (it would be much more complex in reality). The probability would be different. What are your a priori knowledge? What is the probability given the that X species exist at a given time, given how the environment changed over time, given that life is made of DNA, given the size of the earth, …? But then even if you give tons of information about your a priori knowledge (which include defining life and intelligence), then it seems to me terribly complicated to determine this probability with theoretical work. Doing so with empirical work is obviously impossible. We would need many earth-like planets and 4000 million years.

  • $\begingroup$ "Once energy and matter after big bang has been created everything was planned and nothing random ever occurred." Are you actually claiming that? 1) It undermines the remainder of your answer. 2) Most physicists would raise their eyebrows upon hearing such a bold statement. And most (but not all) of those who actually have a belief with regards to it, would say it is false. (See here for a brief discussion.) $\endgroup$ – user3395 Jan 13 '14 at 10:25
  • $\begingroup$ Yes, indeed. I did know about that, thank you. But for the point of my question, quite far from the fundamental laws of physics, I think that the all-determinist point of view of energy transfer and matter interactions on earth - processes governing life - would be fine. I'll update my answer to make sure not to include more than what I should in this all-determinist point of view. $\endgroup$ – Remi.b Jan 13 '14 at 10:58
  • $\begingroup$ That's interesting. It would be very nice if you could firm up the idea that quantum mechanical effects (under a probabilistic interpretation) are negligible (they are never zero) when it comes to the (full) evolutionary run. (Is this perhaps even a well-established and well-reasoned viewpoint?) $\endgroup$ – user3395 Jan 13 '14 at 11:08
  • $\begingroup$ I dunno enough about this field of philosophy in order to argue further in any direction. You might throw some question of this kind on phylosophy.SE and add the link here. $\endgroup$ – Remi.b Jan 13 '14 at 11:14
  • $\begingroup$ Your answer still puzzles me. You start out by assuming determinism and then go on to conclude that the probability computation is very complex. But under determinism (of this kind) the probability of a repeat scenario simply equals 1. By definition. $\endgroup$ – user3395 Jan 13 '14 at 11:20

The complex forms of life we see at this moment are the result of billions of years of random mutations. To have the same outcome (or moment - we are not an ultimate outcome) would require the same mutations (or mutations to the same effect) to occur in a similar enough sequence, we also would require the same (or similar) selection being applied and effects of drift (another random process in evolution).

Given the extreme number of mutation events that would be involved in reaching humans (and other complex life) it becomes very clear that it is highly unlikely. Further, because mutation is random, nothing is inevitable. However, the repeated evolution of humans in not impossible (given a parallel system to evolve them within). Evolve and resequence experiments offer some support of this. In these experiments strong selection is applied over a relatively small number of generations with replicate populations and the number of common changes within the replicate populations can be very small. Even over time scales which are several orders of magnitude smaller than the 3½ billion years life has been on earth we see results which vary within a selection regime - i.e. with identical selection and standing variation the populations have little concordance in their response at the genetic level but there are certainly similarities to be found when comparing to other treatments.

The amount of replication needed to evolve something similar to humans starting with the most basic building blocks of life would be unfathomable. The probability of such a sequence of mutations (and patterns in drift & selection) occurring again which are identical to the ones that have lead to humans is so extremely low (beyond calculable) because of the sheer number of possible routes along the evolutionary trajectory that could have occurred.

To clearly answer your question, no we are not an inevitable result of evolution, it is (strictly speaking) possible that we could repeat our evolution but the probability of doing so is minuscule and too small and complex to calculate.

  • $\begingroup$ In my experience, often times after the phrase "imagine a simplified scenario" something simple can suddenly get quite confusing. This seems to be one of such cases. How is it possible for two individuals to go through 100 generations? Your experiment would give us a nice Gaussian distribution, because it's just the classic Galton's board, but does evolution follow this distribution? How can you possibly estimate how many "forks" there are, do you know any evolutionary pathways apart from those that have occurred? I agree with your conclusion, but those "simplifications" of yours... $\endgroup$ – Th334 Jan 13 '14 at 10:05
  • $\begingroup$ Philosophers and physicists argue among themselves whether nature is random or not and even whether this is a proper question. But, assuming for the moment, that nature is not random (a minority position), then "mutation is random" would be - strictly speaking - false. (Perhaps it would only be "random" from an epistemic viewpoint.) $\endgroup$ – user3395 Jan 13 '14 at 10:36
  • $\begingroup$ @herman It is just an illustration of how a path (such as an evolutionary trajectory) with even small numbers of possible deviations and opportunities for divergence (directions and generations respectively) it rapidly becomes unlikely that the same two paths can be repeated $\endgroup$ – rg255 Jan 13 '14 at 10:39
  • $\begingroup$ @herman I do point out that trying to apply such a probability calculation to repeated convergent human evolution would be too complex to calculate $\endgroup$ – rg255 Jan 13 '14 at 10:41
  • $\begingroup$ @glentheudderboat it is widely accepted that genetic mutation is random - I don't see what point you are raising here $\endgroup$ – rg255 Jan 13 '14 at 10:44

I'll have a go at this, from a perspective devoid of any biology (which I know nothing about). You want to wind the time back to some earlier "instant" (when life emerges on Earth) and then run the world from that "instant" on a second time around. I'm taking all that quite literally.

First remark. I put "instant" between thingies because it may be troubling from a general-relativistic perspective. Never mind that.

Second remark. You can't do that. You cannot wind the time back (and start over). Therefore, this is not a question of the normal scientific variety. Never mind that.

According to the laws of physics (as they are currently thought of; and those are freakishly accurate on our scales), if there is (objective/real/fundamental) "randomness" in the world then it can only be of the quantum-mechanical kind. Quantum mechanics typically deals with the very small stuff. (Ignoring Big Bangs and black holes.) Typically no-larger-than-atom stuff. Unless there is some kind of "natural" amplification mechanism at hand,* I don't see how quantum-mechanical randomness (if it indeed exists; there are other hypotheses and interpretations of QM around) with any significant probability can reroute a relatively macroscopic phenomenon such as evolution.

However, if quantum-mechanical randomness exists (e.g., no hidden variables, no superdeterminism, etc.), then (in my tentative** opinion) the probability that it will reroute evolution is not zero. Just very small.

I guess*** that at least 99.99% of all reruns yield exactly the same biological diversity that we have today. (And this will not change unless perhaps we start using quantum-mechanical amplifiers such as Geiger counters for our decisions regarding species demolition.) And if evolution was to be rerouted during the second run, it would most likely be immediately after the start of the second run, when life was barely existing and thus most vulnerable to quantum-mechanical mishap.

*I can only think of some intermediate chaotic process, but even that doesn't do it for me. (It might change the weather a bit, but evolutionary history? Nah, very unlikely.)

**The tentativeness is due to personal uncertainty regarding the causes of DNA errors. However, I now believe them to be (by far) mostly chemically describable in nature (especially those that produce offspring), not only quantum-mechanically describable.

***An actual computation of a good lower bound is impossible. I think I'm on the safe side.

NB: To those who "know" evolution to be inherently random, I would ask: Could you distinguish between evolution based on "true" randomness and evolution based on pseudorandomness? I would suggest that for all biological-theoretical purposes they would be equivalent.

Evolution (assumed true) doesn't come close to proving (or even suggesting) that the world/universe has inherent "true" randomness. Evolution doesn't require "true" randomness. It is perfectly fine (from a consistency-perspective) to accept evolution and be agnostic about "true" randomness.

It is precisely for the purpose of this question (with its "inevitable"), where the distinction between random and not-random matters. The “normal” assumption of randomness is very well suited to answering biological questions, but this question explicitly (and apparently inadvertently) wanders into another domain of inquiry: philosophy.

The OP's question asks about "inevitable". It is precisely in that word only, where the distinction between randomness and, say, pseudorandomness is crucial. If reality is pseudorandom then things are inevitable. If reality is random then things might be less inevitable. So, for the purpose of this question, we need to acknowledge that we really don't know about this aspect of reality (and usually we wouldn't care, because for normal questions it doesn't matter one iota).

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    $\begingroup$ Don't think that I have any kind of educated opinion in this area, but how would you explain pseudorandomness in the Universe? Is someone just running a random number generator, which happens to be our Universe? :-) Even if it is true and everything is pseudorandom in the Universe, it's of no help to us, because we don't know the underlying deterministic process. If this knowledge is useless, we could as well consider everything to be random, because randomness would be our best theory of how pseudorandomness really works, if this makes any sense :-) $\endgroup$ – Th334 Jan 14 '14 at 4:48
  • $\begingroup$ Yes but nature does have amplification on the impact of randomness created by quantum mechanics. You know that the evolution is a huge process, and every tiny step in it can be affected by this randomness. As the errors accumulate, the outcome may be totally different every time. We can certainly call that 'inherent randomness'. $\endgroup$ – user5407 Jan 14 '14 at 5:07
  • $\begingroup$ @Herman Excellent! That is very good. And now have a look at the OP's question, if you don't mind. It asks about "inevitable". It is precisely in that word only, where the distinction between randomness and, say, pseudorandomness is crucial. If reality is pseudorandom then things are inevitable. If reality is random then things might be less inevitable. So, for the purpose of this question, we need to acknowledge that we really don't know about this aspect of reality (and usually we wouldn't care, because for normal questions it doesn't matter one iota). $\endgroup$ – user3395 Jan 14 '14 at 5:34
  • $\begingroup$ @Peter No, I don't think that there is much quantum-mechanical instability in DNA beyond chemistry. I therefore assume that there is no significant accumulation or amplification of quantum-mechanical randomness, if any. $\endgroup$ – user3395 Jan 14 '14 at 6:05
  • $\begingroup$ @Peter Although I would certainly like that particular claim evaluated by a chemical-biologist. $\endgroup$ – user3395 Jan 14 '14 at 6:11