How is the Word the light of humanity?
In "Biblical Perspective" Gordon Snider writes concerning John 1:4:
John was fond of borrowing words from Genesis 1 and infusing them with new spiritual meaning. Life here is more than biological life, but the life principle itself. Life is a key word in this Gospel. But whereas John usually used it to refer to spiritual, eternal life, here the term is broader. The life was the light of men. To understand this phrase, the reader must review the occurrences of light in this Gospel (8:12; 9:5; 12:36, 46). John said that light flows from the life that is in the Word. One way of understanding this is to remember that the Word is life because the Word communicates the nature of God to us. We receive light — or truth — as we see the glory of the Father revealed in the Son.
Source: Studies in Romans: Adult Teacher's Insights, page 23.