First, he points out that humans suffer from bad backs. He says that's a sign we evolved on another planet with lower gravity than here on Earth.
Another piece of evidence -- the fact humans easily get sunburned, suggesting we're from a planet with less sunlight.
So how did we get here? Dr. Silver suggests eErth is likely a prison planet since humans are naturally violent and we were likely placed here by another alien race tens of thousands of years ago until we teach ourselves how to behave.
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