Your hairline goes further back, even further back than the Precambrian Time.
Precambrian Hairline
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Explanation
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Explain Bear
Listen up, buttercup. This joke is saying your hairline is so far back, it's like your hair is ancient, even older than the Precambrian Time, which was like, billions of years ago. Basically, you got a big ol' forehead, you Neanderthal.