In its 4.6 billion years circling the sun, the Earth has harbored an increasing diversity of life forms:
- for the last 3.6 billion years, simple cells (prokaryotes);
- for the last 3.4 billion years, cyanobacteria performing photosynthesis;
- for the last 2 billion years, complex cells (eukaryotes);
- for the last 1 billion years, multicellular life;
- for the last 600 million years, simple animals;
- for the last 550 million years, bilaterians, animals with a front and a back;
- for the last 500 million years, fish and proto–amphibians;
- for the last 475 million years, land plants;
- for the last 400 million years, insects and seeds;
- for the last 360 million years, amphibians;
- for the last 300 million years, reptiles;
- for the last 200 million years, mammals;
- for the last 150 million years, birds;
- for the last 130 million years, flowers;
- for the last 60 million years, the primates,
- for the last 20 million years, the family Hominidae (great apes);
- for the last 2.5 million years, the genus Homo (human predecessors);
- for the last 200,000 years, anatomically modern humans.
From my ignorant perspective. modern humans cannot be much older than the cave paintings at Lascaux, which makes the age of humans to be roughly 30,000 years.
Arbitrarily taking the start of life on earth at 3.6 billion years ago as our starting point, and dividing (I just made this up for the sake of convenience) 36,000 years of intelligent human life (3.6 x 10 to the 4th) by 3.6 x 10 to the 9th years, we get: 1 x 10 to the5th. That is to say, intelligent human life as we understand it is 1/100,000 the length of life on earth.
Which means that intelligent alien life might flourish for longer than any known human civilization and have perished thousands of years before we even got started. Space is big and time is long.
* “flash in the pan” – Old fashioned muskets had a priming pan outside the barrel. The flint striking mechanism would send a spark into the pan, which would ignite the powder in the pan, and then burn into barrel, most of the time. A “flash in the pan” signified the powder in the pan exploding, without igniting the gunshot.