Last update: January 25, 2017
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
which estimates the number of communicating civilizations in our galaxy.
OK, here's my list:
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.)
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.)
Right amount of atmosphere:
Too much or too little gas is bad either way, before we even start to
talk about the component molecules.
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.)
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.)
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.)
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
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...
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
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.)
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.)
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.)
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
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.)
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?