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The protein source （that） these scientists studied was human hair.
that these scientists studied是一个定语从句，因为指示代词在从句中作宾语，所以可以省略
主句为：The protein source was human hair.

主语从句中的that是绝对不可以省略的，因为句子不能作主语，所以需要由that引导从句。
That the earth goes round the sun is known to us all.
That the earth goes round the sun即为句子主语从句。
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