Richard Leakey and Roger Lewin

[T]he human mind is used to thinking in terms of decades or perhaps generations, not the hundreds of millions of years that is the time frame for life on Earth. Coming to grips with humanity in this context reveals at once our significance in Earth history, and our insignificance. There is a certainty about the future of humanity that cheats our mind’s comprehension: one day our species will be no more.

Richard Leakey and Roger Lewin, The Sixth Extinction: Patterns of Life and the Future of Humankind, New York, 1995, p. 224