Previously we have considered the issue of biological life in general and then the more specific case of human life. Based on only the first of these discussions, we might forward the following general description of life:
A living organism is a system that can interact with its environment, exchange material with the outside, and is capable of reproducing itself.
On this basis, we might cite bacteria as one of the simplest forms of life on the evolutionary tree. However, scientists have now come to understand that if survival is the sole objective of natural selection, then bacteria may actually have to be put at the top of the tree, not the bottom. Of course, this ignores the important issue of intelligent life, if seen as the real measure of evolutionary 'success'. The reason for highlighting this issue from the outset is because it affects how we reflect on the following question:
"Are we alone in the universe?"
There is often an inference in the 'are we alone' question suggesting that our isolation in the universe is not only being measured against whether any life exists elsewhere, but whether it is intelligent. However, we need to start by first considering the probability of any form of life existing elsewhere in the universe. Based on this broader condition, it might be reasonable to assume that life on Earth is only representative of a very small percentage of life in the universe. We might feel justified in making this assumption on the basis that a visible universe, containing something like 100 billion galaxies each with a 100 billion stars, must have other 'inhabitable' planets that support life in some form.
But what constitutes a habitable planet?
And does extraterrestrial life need to be carbon-based?
If extraterrestrial life started from scratch, then our initial definition of a living organism, as summarised above, does not mandate carbon even though it is the critical building block of DNA. While astrobiologists have considered many other possibilities, e.g. silicon, there is a general assumption that if life exists somewhere else in the universe it will most probably have to be carbon-based because of its ability to build strong molecular chains. Clearly, if this assumption is true, then it places more specific requirements on the environment in which life is likely to have evolved. However, at this point, let us turn our attention to different line of questioning:
Has life evolve independently or spread between plants and solar systems?
One hypothesis, known as 'panspermia' has forwarded the idea that life might have emerged in one location, then spread between habitable planets. However, this idea is not necessarily suggesting that life originated only once and then subsequently spread throughout the entire Universe. However, this does raise an interesting question:
If the constituents of life were unique to one planet, might this explain our failure to detect any signs of other intelligent life in the cosmos?
Clearly, we don't know the answer to this question, as yet, but either the prerequisite building blocks of life are so unique that life has only emerged once in the entirety of spacetime of the universe or these prerequisites must exist elsewhere. However, if you assume the latter, then on the scale of just the 'visible' universe containing some 1022 potential solar systems, probability would suggest that there must have been many moments of genesis, at least, in terms of the building blocks of life coming together. Of course, this perspective does not preclude the distribution of microscopic life within relatively small regions of space via some sort of panspermia mechanism; in fact, there is growing understanding of just how resilience bacteria can be:
The Outer Reaches of Life: John
When a population of bacteria dries out without a protectant, many of the cells break open and release their internal contents. Among these contents are proteins, gums and sugars, all of which are protective. If the population is sufficiently dense, so that significant amounts of protectant are released, material released from the majority which died first can protect a few of their surviving fellows.
Comparable considerations apply to death from freezing. Protective substances such as glycerol are well known and widely used; they are called cryoprotectants. Bacteria frozen without such chemicals leak internal contents, among which are many substances that are cryoprotective
If we assume that bacteria could hitch a ride on cosmic debris and are capable of surviving the extreme conditions encountered along the way, some of their other characteristics might also suggest that colonisation of other worlds is a possibility. For example, a life generation of bacteria can be less than 20 minutes and there can be 1010 bacteria per millilitre, therefore low probability events are not so rare when considered as a whole. If we assume a mutation rate of 1 in 107 cells and an initial population size of 1010 per millilitre, then 1000 mutations could occur every 20 minutes without any need for diversification via sexual reproduction. As a result, adaptation to a new environment, at least in terms of basic survival, could be relatively quick.
OK, so there might be the possibility for microscopic life elsewhere in the universe, but what about intelligent life?
Again, without evidence, we can only speculate. However, we might make an inference based on the axiom that 'what we don't known must be like what we do know'. What we do know is that life, once in existence here on Earth, evolved from single cell eucarya, which existed as little more than cause and effect chemistry, to become intelligent, sentience homosapiens, albeit after some 3.8 billion years. So the suggestion seems to be that if life is possible and evolution is a universal law of life, then intelligent life must also be possible, although not necessarily probable within any given region of space and time. So the question we are now left to consider is:
Well, we might wish to initially consider this question in terms of the life-cycle of all the stars in the visible universe, which is defined in terms of the 'stelliferous age'. This phase of the universe may extend to 1014 years into the future, such that the present age of our universe, i.e. 13.7 billion years, is only 0.01% into this era. In this context, it might be both remarkable and unique that life has already evolved to sentient intelligence here on earth in what may be a very early stage in the 'life-cycle' of the universe. Therefore, maybe we need to consider the possibility that humanity might be one of the very first lifeforms ever to reach this stage; for in the great scheme of things, something has to be first. Given that higher lifeforms require heavier elements that only formed within supernova, it is possible that the first star systems capable of supporting life of any description may have require, at least, 1 billion years to stabilise, which is about 10% of the current age of the universe, but only 0.001% of the projected stelliferous age. If we then consider the number of remarkable conditions that have existed for the last 5 billion years it has taken for planet Earth to evolve sentient intelligence, then the fact that another intelligent life has not been found to-date is possibly not so remarkable, even if we are not the very first. However, the scope of the question above is essentialy the subject of the rest of this section and will start by considering the potential impact intelligent life may have on the cosmos, but then tries to provide some wider perspective of the problems associated with finding evidence of life's existence within the scale of our local solar system and galaxy and then the much bigger universe beyond.