What can the Earth’s history tell us about the probability of life on other planets?
Sorry, this is not an article about finding alien fossils next to dinosaurs, or alien portals hidden within the great pyramids of Egypt. However there are conclusions we can draw from Earth’s history that may help us estimate the probability of life existing on similar, nearby (relatively) planets.
[Note: In the course of this short piece I will reference a variety of statistics, all of which will have been severely rounded for the sake of keeping the math simple. Further, rounding the calculations is necessary to remove the implications of accuracy that more precise figures might suggest.]
Life is not rare. It is the conditions necessary to sustain life that are rare. If the conditions necessary for life are present, then life will be present. Therefore we will assume a 100% probability that if a planet contains the conditions necessary for life, then life will be present.
What about ‘intelligent’ life? Let’s define intelligent life as Australopithecus (an early humanoid capable of using tools), or ‘Lucy’ as one such skeleton is known. Lucy appeared around 4 million years ago. Using Earth as our guide, the probability of finding intelligent life on a planet with the conditions necessary for life, is 4,000,000 / 4,000,000,000, or approximately 1/1,000. We can roughly estimate that one out of a thousand planets that contain life will contain intelligent life (assuming evolution would progress at roughly the same pace, and the average number of extinctions would be similar).
What about the closest stars? If we consider 50 light years as a reasonable communication window, we have found approximately 100 stars similar to our sun within range of Earth. If these stars have on average one planet with the conditions necessary for life, then the probability of a civilization existing within 50 light years of Earth is approximately 100 / 1,000,000, or one in ten thousand.
The idea that life on a distant planet would evolve in a manner similar to Earth is bound to make some people scoff. They will argue that our evolutionary path has been influenced by a variety of random occurrences, therefore the progress on another planet will probably be significantly different. It’s true that I’m using earth as both the median and mean of a group with a sample size of one. Based on this, I am assuming that the characteristics of Earth represent the most probable outcome on the bell curve of possible variants.
Consider the impact ‘random’ events have had on the evolution of life on Earth. The Earth’s planetary history is full of cataclysmic events such as giant asteroid strikes and volcanic eruptions that have shaped its evolutionary path. However, over the course of 4 billion years, these events are not ‘random’ at all, but regular. Comets will strike at a given frequency; not with the precision of your Swiss watch certainly but the longer the time horizon becomes, the more predictable the number of cataclysmic events becomes. The same holds true with volcanoes and resulting temperature swings. Similarly situated planets will, over a given space of time, will have a similar number of cataclysmic events of various types, all having similar extinction impacts on their various species.
Two planets 50 light years apart that are similar in size, situation, and age may have very similar evolutionary outcomes. One planet may develop civilizations at 3 billion years old and another at 4. It is also probable that the other planets ‘people’ will look very similar to us. We’re evolution’s best design for this environment, and Mother Nature has had millions of tries to get it right. If the environment is similar, the creatures inhabiting it will be as well.
I don’t expect they’ll speak English.