insights into life at that time rely on researchers’ imaginations
Where is this 600M years taken from? big claims based on assumptions, I don't like this kind of science, building on too many assumptions and you are going too far from the truth and this is the opposite of what science tries to achieve.
Since the researcher was reconstructing the evolution of a protein I would say the date was determined using a technique called the molecular clock: https://en.wikipedia.org/wiki/Molecular_clock
That and the rise of multi-cellular life marks the beginning of the Cambrian explosion (542M years ago). That gives 60M years for the mutation to spread and specialize.
The Cambrian explosion is late in this game; this would be about the emergence of the Ediacaran biota (largely fractal-like body plans, followed by organisms with more specialised structural groups and less-local symmetries) and simpler aggregate colony organisms like sponges, which seemed to come out of nowhere quickly after the Cryogenian. The Cambrian explosion seems to have been mostly about riffing on the bilateral symmetry lick.
Not at all. An assumption is "a thing that is accepted as true or as certain to happen, without proof" [1]. The goal of science is minimize assumptions and maximize knowledge (justified, true belief) [2].
One easy way to do this is to make predictions: "If $THEORY is correct, then under the same conditions, I will observe $OUTCOME. If $OUTCOME does not obtain, I either haven't properly conducted the experiment, or $THEORY is wrong.
In response to your rephrasing of "if...then", to "assuming...then", consider that we actually do (or do not) observe $OUTCOME -- we don't assume that we observe it. To use an outrageous (but hopefully clear) example, do we know decapitation is fatal, or do we merely assume it? A long time ago (but in this galaxy) someone had a theory that decapitation was fatal. Experimentation gave us knowledge on this issue. Doubtless, they did not have The Scientific Method in mind, but they used the same process.
If I understand you correctly, you're saying all knowledge is just a bunch of assumptions. This raises the question, how do you know that statement is true? On that view, you can only reasonably say, "I've assumed it." The belief that all knowledge is based on assumptions is just a rephrasing of the postmodern view that there is no objective truth (is that true?).
I don't want to sound like I'm attacking you here, but our beliefs about knowledge and truth are very consequential. If we cannot know anything aside from our assumptions, then all assumptions are equally valid (since we can't know which to discard). This naturally leads to situations where, e.g., racism is assumed to be good by one person, assumed to be bad by another, and there is no way we can determine who's right. In fact, "right" and "wrong" become meaningless because they're only assumptions, as does all language.
Where is this 600M years taken from? big claims based on assumptions, I don't like this kind of science, building on too many assumptions and you are going too far from the truth and this is the opposite of what science tries to achieve.