Ablaut and Infix Origins

I don't really understand how ablaut and infixing come about. I've been trying
to find information online with good explanations without any significant
success. I found one paper on the theory of infix origins but it was very
Chomskyan. Another summarized the different types but didn't give a me "feel"
for it. There seems to be even less satisfactory information on ablaut origins.
Apparently all the existing ablaut systems came about thousands of years ago.
Any ideas?

Jeff

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