Sunsets in a Pre-Cambrian World: the likelihood of life on other planets

If one were to look at the geological history of the earth in terms of potential earth-like analogues for other planetary systems in our galaxy, the probability of finding intelligent life at a similar technological level as we currently have is pretty small.

A random selection of a small period in earth history, would more often than not select a period with single cell life forms. This is based on the geological record and presence of macro and microfossils in that geological record. It is possible to use the history of the Earth as a representation of other planets at different stages of geological and biological evolution. This assumes that the planet would at least have liquid water as the dominant liquid, and a temperature range within the limits reflected by the Earth's orbit and distance from the sun.


A random selection of 50 ages from 0 to 4.5 billion (age of the Earth) results in 88% Precambrian, 12% post Cambrian, and 0% recent (less than 5 million years). A larger selection of 500 ages results in 86% Precambrian, 13.8% post-Cambrian, and 0.2% recent (2.2 million years). Out of the 13.8% post Cambrian 2.2% (11 ages) would fall into the category of less than 100 million years, or advanced life forms (Dinosaurs, Mammals, Birds, etc.).

The scenarios assume the probability of having 4.5 billion planets in the galaxy within the acceptable temperature range and presence of abundant water. Based on this approach and using a random selection of 500, only one planet would exist which most closely matches the Earth in terms of near current age. Out of 70 post Cambrian selections (14%), 11 could be categorized into the more advanced stages of biological evolution (2.2%). A total of 365 planets would have basic cellular life forms (73%) and 65 planets (13%) would not have cellular life forms but may have self-replicating organic molecules.

Drake's equation estimates that there are up to 2 technologically advanced civilizations in our galaxy at any one time. This is not an unreasonable number since we can be assured that there is at least 1 advance technological civilization. Much more likely, a water-rich world could bear fruit to self-replicating molecules and cellular organisms. Even with some good luck, biological evolution could produce multi-cellular organisms. But it would appear that most stable ecosystems do not require interstellar communications to exist, so there is no biological imperative to transcend our earthly bounds.

Although the search for life on other planets is a significant endeavour, we need only look into Earth's geological history to determine that indeed there is a very good likelihood of life existing on other planets. A billion to two billion years ago the Earth had no forests, no birds, no animals. It was mostly water-filled oceans and bare landmasses. Rain fell; rivers flowed to the lakes and oceans, there where sand and gravel beaches. In a few ecological niches there existed stromatolites and algal beds that took on various shapes and forms, but were essentially colonies of single celled organisms.
In the Archean age, the Earth was primarily a planet covered by water and an atmosphere rich in carbon dioxide (CO2), ammonia (NH3), and methane (CH4). There were few landmasses other than volcanic islands and island arcs. Most of the planet was covered in water. Most of life was cellular in form with stromatolites and algal mats occupying ecological niches. Some organisms were using photosynthesis to produce nutrients with oxygen being released. Iron quickly combined with this oxygen to form what geologists call 'banded iron formation'. These iron formations are sometimes associated with large quantities of graphite - likely reflecting and organic precursor, just like coal represents fossilized forests.

Estimating the number of planetary systems where free water can exist over an extended period of time is the key to determining the likelihood of whether life is likely to exist and evolve. It may be quite likely that given the right environmental conditions, organic molecules will form and begin to replicate. This phenomenon may not be any different than crystal growth. Crystals of minerals will grow by duplicating the atomic structure to the point where it manifests itself at a large scale.

Similarly, carbon based organic compounds may be able to grow and result in the duplication of its atomic structure. The main difference between crystals and organic molecules is the limited number of mineral crystal systems that limits the variety of crystal forms. Organic molecules can have a multitude of forms and are not limited in the same way as mineral crystal growth. Some of these organic molecules would be better suited to certain environments. With self-replicating capability the process of natural selection would initiate the long-term process of biological evolution.