Human Gateways

By Alan Silverstein,
Last update: October 23, 2023

Just for fun, this is a personally-collected list of "knickpoints" in the history of the universe, Earth, and humankind, when things had to go "just right" for us to be here now as we are. Each point is described only briefly; you can web-search for more information.

Yes I know about the anthropic principle. Even the weak form says, "no fair being amazed that we are here to be amazed." Well heck with it, I still find it interesting to ponder a list like the one below.

Note too the related topic of the famous Drake equation, which estimates the number of communicating civilizations in our galaxy.

OK, here's my list:

  1. Basic laws of physics: Underlying constants and rules that permit a long-lasting, expanding universe with galaxies and stars, including later stars enriched with heavier elements from earlier supernovae. (Many sources talk about this subject, plus the anthropic principle.)

  2. Rocky world at the right distance (stable, non-eccentric orbit) from a stable-enough star to neither boil nor freeze liquid water (the "Goldilocks zone"). (Sky & Telescope Magazine often discusses this.)

  3. Right amount of atmosphere: Too much or too little gas is bad either way, before we even start to talk about the component molecules.

  4. Right amount of water: It could be very rare (like 1 in 1000) for a rocky planet to have enough water to make oceans, but not enough to drown all the dry land. (Sky & Telescope Magazine, Jan 2011.)

  5. Reproducers: However it happened, some kinds of chemicals that could reproduce themselves appeared in the primordial soup, jumpstarting evolution. (Many sources talk about this. No one knows if it's extremely rare, commonplace, or virtually automatic with enough time and space.)

  6. Boring history: After the Late Heavy Bombardment about 3.9 Gya, there have been relatively few asteroid impacts, none totally sterilizing the planet; also no nearby supernovae, orbit-messing stellar (including neutron or black hole) close encounters, focused gamma-ray bursts, or other sufficiently large life-destroying events. "Life can hardly survive truly interesting times." -- Philip Morrison (Sky & Telescope Magazine often discusses topics like these.)

  7. Lunar stability: I've read that the presence of an unusually large satellite might have stabilized the Earth's rotation so the north pole didn't wander far from ecliptic north, thereby stabilizing the global climate in a way that made it easier for life to survive and prosper. (Possibly also affected cloud motions such that more heat escaped and the Earth did not overheat like Venus.)

    As a counterpoint, Isaac Asimov's essay, "The Tragedy of the Moon," asserts that having the moon go around the Earth instead of Venus set back civilization by thousands of years...

  8. Lunar tides: Possibly not essential for life to evolve on Earth, but might be pivotal for life moving ashore from the ocean. (Don't recall where I read about this.)

  9. Plate tectonics: Planets that don't recirculate their crusts, and rise up mountains to be eroded down, can lock away essential elements, starving life of them. Phosphorous is a notable example.

  10. O2/ozone: It took half the history of the planet before microbial life churned out O2 (possibly due to a nickel shortage resulting from core cooling) sufficient to drive life towards efficient aerobic respiration. (Sky & Telescope Magazine, and other sources.)

  11. Multicellular life: It took a remarkably long time for single-celled life to get around to forming (specialized) multicelled organisms of any kind. It might have required aerobic metabolisms first (O2-based), but even after the Great Oxidation Event it took something like 2 Gy more for compound bodies to appear. (Various sources.)

  12. Unique human traits: Stereo color vision, opposable thumbs, upright stance (possibly arising from grassy savannah environment; freeing hands for toolmaking and carrying), and other "evolutionary drivers" over the last several million years towards larger, self-aware brains/minds in "generalist" creatures pushed toward complex tribalism and communications. The combination of these features were very unlikely, and did not occur for half a billion years in any species. (See "How the Mind Works", "The Evolution of Cooperation", etc.)

  13. Pure survival: Genetic research shows that humans passed through at least one knickpoint, possibly several, where our entire population might have dropped as low as 10,000 people. How many times did we dodge random bullets?

  14. Agriculture: Only about 12,000 years ago did hunter/gatherers, in just a few lucky spots on the globe, manage to make the unlikely transition to crop-planting and harvesting. This was crazy and risky at first, but ultimately supported our technological drive, urban civilization, etc. (See "Guns, Germs, and Steel", etc.)

  15. Fossil fuels: Only about 200 years ago did we start making serious withdrawals from the Earth's "carbon bank account" to push ourselves into the machine age and then the information age. (Various sources.)

OK, that's my list. Am I missing anything? Email me.