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?
Answer
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.
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