The symbol for Earth is a circle with two internal lines at right angles dividing the circle into for pieces. I do not know how old this symbol is, but the ancient egyptians had a similar symbol as far back as 2000 BC, but it was only used in words such as 'estate', 'Thebes', or 'the lower heaven', and not for earth or land.

I can imagine that by the time of the ancient Greeks when Aristotle's 4 elements 'Earth, Air, Fire and Water' were in vogue, that this symbol could have arisen as the four elements joined in unity to represent the Earth, represented as a circle quartered into four internal pieces.

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