Neil Degrasse Tyson formulated this:
Take the history of Earth and place all aspects viewable while walking along a 100-yard ramp. Walk the entire length. At the end, step back one yard to be at the time of the dinosaur extinction 65 million years ago. Then, step off and place a human hair at the end of the ramp. The hair represents recent human existence – the far side of the hair is caveman era, the right side is us now.
What element is most of the matter in the universe? Hydrogen. Next Oxygen. Next carbon. Next…
What is a human or other Earth organism most made of? Same. Surprising, or obvious?
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Brian Greene:
Travel to distant solar systems must consider generational differences between the two future potential civilizations, assuming these civilizations are advanced enough to send and receive communications. Thus, we send a message or messenger subject to today’s technological status. The time necessary for transmission means that the recipient may have become a much later generation of that civilization, and, in turn, the recipient’s response to us may arrive many generations from now.