Genesis 9:1

 "Then God blessed Noah and his sons, saying to them, 'Be fruitful and increase in number and fill the earth.'"  Genesis 9:1

In Genesis 8:17, God tells Noah that the animals need to come out of the ark so that they can multiply and fill the earth, and now God is telling Noah and his family to do the same.  If he didn't order it, would the men all stay in one place?  The interesting thing is, that the text doesn't say that it was a command, but rather a blessing from God.