Which are the earliest civilizations in the world?

So far I've read that the Indus civilization is among the earliest civilizations, contemporary to the great Bronze Age empires of Mesopotamia and Egypt. The Olmec civilization in Mesoamerica followed soon after. And it seems in China there was a thriving civilization during the Xia dynasty. The Aegean civilization is also mentioned sometimes.

Is Indus the oldest?

Also I've read that the Indus cilization, the one in Sumeria and Ancient Egypt had contact and traded. Is this correct? If so, is there any possibility they sailed the ocean and had contact with the Olmec civilization? What about the Piri Reis map that shows South America and Antarctica?

I find this all very interesting.