Earth strikes us as a unique planet—primarily because of the oceans right on its surface. The other rocky planets in the Solar System appear lifeless. Yet, according to current data, oceans may well have existed on the surfaces of Mars and Venus—both Earth-like planets—during their first two billion years. Can oceans, then, still be considered unique? Quite the opposite: they appear to be a natural occurrence. Conversely, in its early days, Earth showed no outward signs that it would one day harbor life. Planets, like everything else, do not remain static, and what we see today is by no means the final stage of their existence
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