We test the predictions of the ToE. IOW, if the ToE is correct, we should see a tree-like pattern of relationships between existing organisms, and we should see a progressive sequence of snapshots of the development of this tree in the fossil record, and so on for each of the predictions we can make based on the ToE. Assuming a fairly constant mutation rate (averaged over evolutionary timescales), the differences and similarities in DNA between humans and fish match the tree-like ToE relationship and the estimated time back to the common ancestor pretty well.
One of the best tools for verifying the relationships between organisms is their DNA. Not only can we see that we share the majority of our DNA with fish, but we can see the relationship predicted by the ToE reflected in the DNA similarities and differences. One key indicator is called an endogenous retrovirus (ERV). This is a chunk of DNA inserted into the host organism's DNA when a retrovirus infects it. The insertion point of an ERV in the host DNA is random. Occasionally these pieces of viral DNA are inserted into the DNA of germ cells, so they are inherited by the offspring and effectively become part of the DNA of that species.
Now that we can sequence DNA, we can spot these ERVs, and compare them with the ERVs in other creatures. For example, if we compare our ERVs with those in chimp DNA, we find about 200 matching ERVs in exactly the same places in their DNA as in ours. We also find a few ERVs in us and them that don't match. The only way we can share those matching ERVs in identical positions is if we both inherited them from the same ancestor. The ERVs that don't match are more recent ones that were inserted after our two lineages branched off from that ancestor. We can roughly estimate the rate at which ERVs get inserted into our DNA, and that too is a good match for what we'd expect if humans and chimps had a common ancestor.
We can repeat the ERV matching process with creatures that are less similar to us than chimps, and the matches we find there confirm that we share more distant ancestors with them. Combining the shared DNA similarities, the matching ERVs, the fossil record that matches the same timescales, and the other indications of shared ancestry (some of which I mentioned in an earlier post), common ancestry of mammals (including humans) and fish is beyond reasonable doubt.
As I understand it, there have been a number of discoveries that suggest that the creatures found in the 'Cambrian explosion' can be connected to Precambrian creatures, but that very few persisted after the Cambrian. It appears that rather than a single rapid diversification, there may have been a number of 'mini-explosions' occurring over a relatively short geological timescale. But fossil evidence of that period is scarce because it seems to be the time when armoured body parts first evolved, so there are bound to be fewer fossils of the preceding soft-bodied period.
'Truth' is a slippery customer. Science aims to find the best way to describe and explain our observations as models that can explain what we've observed in the past and predict what we're likely to observe in future. 'Truth', in that respect, is an ideal that impartial observers will eventually converge on, but we can never be sure we have it. We can only confirm that we have a better model of the world than any previous model.
'Devolution' isn't really a useful concept. Evolution is just change over time - in biology it's genetic change over time. There are plenty of examples of creatures that have evolved to become less complex or less capable over time because they no longer need the extra complexity, they can be more efficient and successful without it - a well-known example is creatures that lose their eyesight when they breed in dark environments (e.g. caves).
What happens is that when eyes (or whatever) are no longer an advantage, creatures with mutations that interfere with sight can survive just as well, so the number of partially sightless and sightless creatures will increase in the population. If having sight has a cost (e.g. maintaining a redundant energy-hungry piece of the brain), then there will be a positive disadvantage to having sight when it's useless.
See above and previous answers.
If you still don't understand it, please ask a specific question about the part you're having trouble with.
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