How is it possible to switch careers from undergraduate to graduate level in the US? I have read about many astounding career paths of scientists in the US. E.g. someone had a BSc in education and switched to physics in graduate school.
How are such switches possible?

In Europe I guess it's mostly so that such diverse careers can't be pursued without having had a reasonable exposure to the subject in college. One would have too many courses that are missing without prior knowledge so that they can't be put all in a Master's curriculum; thus, having had a specific subject is necessary as a major or minor in college...at least in Switzerland.

How come that in the US such switches are possible?
Or is it just that all necessary courses are put into the graduate(+PhD) program lasting in total almost a decade, in which case I wouldn't wonder that it's possible?
My PhD was in neuroscience. When performing neuroscience research, there's a lot one needs to know:


Biology: Starting with the basics, almost any course in neuroscience will have both macro- and microbiology of the human/animal bodies being studies. This includes topics at the molecular, cellular, and systems level. There's a non-insignificant chunk of anatomy to be learned as well. A specific focus will be spent on the biochemistry of neural transmissions, which involves a lot of...
Physics: The principles of electromagnetism govern the firing of neurons. As such, a thorough understanding of both the chemistry and the physics leads to understanding neurons. To study these cells, people using implanted electrodes need to understand a fair chunk of circuitry, as the quality of their cellular recordings is dependent on them understanding impedance and amplification and such like that. People using surface monitoring (EEG/MEG) need to understand the basics of electromagnetism, which govern how the fields propagate and are measured. As an added bonus, they get to familiarize themselves with the signal processing of source localization as well. Wheee.
Psychology/Sociology Some people study how the brain controls complex behavior, such as motivation, learning, addiction, planning, visual abilities, motor abilities, bla bla and bla. This works for both animals and humans, so there can be a lot of literature to cover.


This doesn't begin to discuss the computational skills required to create and analyze experiments, or the statistical knowledge required to make sure you're not screwing up your analysis, or the writing (yes, that's a skill) knowledge required to write a paper that someone gives a poop about, or the public speaking skills required to give a conference talk.

And that's just neuroscience.

Long story short, successful researchers nowadays BECOME polyglot experts simply due to the highly cross-disciplinary nature of the fields themselves. Obviously, you don't need to be expert in all these fields, but you have to start somewhere. When it's all said and done, the person coming to neuroscience with a biology background is surprisingly not that far behind the person with a computational background, because they both have so much to learn.