GMAT Critical Reasoning (CR) | Re: For nearly a century, physiologists erroneously believed

I went with D because of the second part of the boldface but can please someone explain to me how the first boldface is an Evidence?
"Scientists believed that...". I mean people can believe anything the want to, there is no clear evidence that this is established.
Is it that we chose D because it is the only one with a correct second bf?

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