6 Civilizations Past: 1.1 Million
To find the number of civilizations that have appeared in the past, we need to know the span of time over which civilizations may appear. The basic assumptions that can be used are as follows. The Solar System, including the Earth, is 4.5 billion years old. On the assumption that the Earth is a typical living planet, I assume that it takes 4.5 billion years for a civilization to evolve. The Galaxy is 10 billion years old. When the first Galactic stars condensed, planets started to form around them; so the first civilization emerged after 4.5 billion years. That is, there are 5.5 billion years for the subsequent population of the Galaxy with civilizations. We arrived after 10 billion years.
The assumptions have been modified by doubling the civilization lifetime to allow examination of the relation between concurrent civilizations and cumulative numbers. (It also allows for the possibility of Earth 2.0.). The total number of civilizations emerging in the Galaxy is shown in Table 4. While it is unrealistic to expect civilizations to emerge strictly on after another, it is important to get some sort of idea of how many might exist at the same time on the average. This is done here by dividing the total time available by the number of civilizations to find out the average time slot available to a civilization. Dividing the civilization lifetime by the length of the time slot gives the average number of civilizations in that time slot. Under the assumptions of Table 4, there are two concurrent civilizations on the average.
This calculation, with its low civilization lifetime, suggests that over a million civilizations have passed out of existence, while only two currently exist. Us and Earth 2.0.
Table 4: Cumulative Number of Civilizations, Assumptions 2
| Civilization Time Available, years | 5,500,000,000 |
| Planet Civilizations Emerging, per year | 0.0002 |
| Total Number of Civilizations | 1,100,000 |
| Available Sequential Time-slots | 5,000 |
| Assumed Civilization Lifetime, years | 10,000 |
| Concurrent Civilizations | 2 |