I've read that around 20,000 years ago, Chicago was covered by 2 miles of ice. Perhaps smaller ice ages occur at less frequent intervals than the bigger events.

Regarding the DNA aspect, the flood story in Gilgamesh (which closly resembles the Bibles), reads that the "seed" of all living things was placed in the ship. I've often wondered if they could have been talking about DNA. There's a lot we don't know about our past. For those who don't know, Gilgamesh was deciphered from Sumerian text, and predates the Hebrew scriptures by about 2000 years.

Billion
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