Life has evolved and changed over time on Earth, with some organisms like stromatolites and chitons changing very little while others like horses have transformed significantly from ancient to modern forms. The requirements for life as we know it include carbon-based molecules, an energy source like the Sun, and water, with DNA serving as the basis for heredity in all life on Earth. The document discusses whether life originated on Earth or was transported here from other worlds, alluding to the possibility of exogenesis.

1. Life in the Universe

6. Alien life forms?

7. Life has changed over time – evolved. New species have evolved…while some organisms have hardly changed at all.

8. Stromatolite

9. Ancient trilobites & their present-day cousins, the chitons

10. Cockroaches Jurassic & Modern

11. Natural variation

12. Environmental Pressure & Survival of the Fittest

13. Horses – Eocene to modern

14. Requirements of life, (as we know it) Carbon-based molecules Energy source Water

15. DNA Present in all of Earth’s life forms

17. The Goldilocks Principle “ This porridge is too hot.” “ This porridge is too cold.” “ This is just right.”

18. Why does life exist on Earth ? Life began here Life traveled here from other worlds - exogenesis

19. Exogenesis – Did life travel here from a distant world?