"It is potentially possible that our machines will someday achieve the properties of life, but they are by definition artifacts of carbon-based life forms: they can exist but they could not have arisen without our intervention. "

Humans couldn't exist without microbial intervention- and continue to be utterly dependent upon microbial "support".

At least with machines made by humans, the machines could in theory be independent of their makers.

Or carbon-based microbial organisms could possibly get enough collective intelligence without actually forming into things we call higher life forms [humans, birds, worms] and instead of creating rather complicated things as humans, they could make machines which are intelligence. Now, it doesn't seem like a very practical thing to do, but perhaps it's just one of their hobbies.