The Nested Derivative Descent theory (NDD) suggests that evolution ("tree of life"), here on Earth, is a massively parallel computation involving millions of genetic systems we call species. Each such system consists of a population of millions or billions of individuals each being "built up" by a biological program driven by the individual DNA, that is, its genetic code. A species individuals forming a kind of cloud in the DNA code space. Such clouds exhibiting a central tendency. I.e. a tendency against dispersing in code space. This central tendency is largely due to the averaging effects of sexual reproduction and insures a high degree of coherency for the species.
Each species building (embryogenesis) program having a unique control architecture reflecting the evolutionary history of the species. This program, the BioPert, is driven by the species gene control networks (GeNets) and related precedence relationships (PRs). The effect of the evolution computation is to build up, subsequent more complex BioPerts from earlier more primitive ones, by primarily appending new GeNets and related PRs.
In other words and generally, once evolution discovers how to build a particular subsystem (organ or body part) it keeps using essentially the same process in derived species. We can try, with this picture in mind, to speculate how potential evolutionary processes could take place on other planets.
In the first place it is very likely that the early phases of the "tree of life" will be a rather messy bush much more like a network than a tree. at least that is what researchers are finding in the case of Earth life. Secondly, given that a planet is in the right "life friendly" zone around its star, to enjoy liquid water temperatures (32 to 180 Fahrenheit or 0 to 100 Centigrades) and executing a nearly circular orbit around the star, so not to suffer extreme temperature variations, it is, we believe, a near certainty that some very simple life forms will emerge. For example, it took less than one billion years for simple life forms to emerge on Earth. Therefore, we predict that simple life forms should be quite diffused around the Universe and, in particular, we should soon see some concrete evidence of extinct life forms from Mars and very likely live forms from some of the larger moons of Jupiter, Saturn and Uranus, at least for those moons which have oceans, especially if the oceans are water based and not ammonia based. The bottom line is that life, at least in its simpler manifestations, should prove out to be a relatively frequent occurrence.
The existence of complex life forms is another issue altogether. In fact, the NDD indicates that the progressive evolution of increasingly complex BioPerts takes much longer and involves increasingly smaller "a priori" probabilities. In other words, the more advanced embryogenesis programs would be increasingly differentiated from one planet to another, since the probability that two complex evolutionary histories would replicate each other is vanishingly small. However, while we can not say much about such evolutions, we can certainly compute some interesting bounds for their existence. In fact, we can set out to compute the minimum average distance between planets achieving a certain level of evolutionary complexity.
For example, if we are interested in estimating the minimum average distance between planets harboring life forms of a "human" complexity, we can do so if we can give an upper bound for the "a priori" probability of "discovering" the human BioPert. We can. for example, posit that such probability is certainly no greater than one in a billion or 10**(-9). A probability of one in a billion means that, assuming optimistically that all stars have a life abode planet, one would find a planet harboring a human level life form in one star out of each billion stars. Since a galaxy has around 100 billion stars a galaxy would have no more than about 100 "human" level life forms. Next we estimate the average distance between such "special" planets. This is easily done by reflecting that 1 billion stars represents 1% of the star population of an average galaxy. Since galaxies are flattened pancake structures their stars are predominantly distributed along the two dimensions of the pancake, therefore an area of the pancake which extends 10% along each of its dimensions will contain about 1% of the stars. This means that, in the case of our Milky Way, with a radius of 50,000 light years a neighborhood of 5,000 light years is likely to contain another "human" level life form. The conclusion we arrive at is that "human" level civilization are likely to be separated by average distances of the order of 5,000 light years. This means that all the money being spent on the Search for Extraterrestrial Intelligence (SETI) is being wasted, in fact, the most advanced and sensitive technologies for detecting electromagnetic radiations from such putatively "intelligent" planets can barely reach out to 1,000 light years
Please note that if we assume that the "a priori" probability of discovering the human BioPert is smaller than one in a billion, or that not all stars have "life abodes" with long and stable lives, then the above computation would result in much larger SETI distances!
Our guess of one in a billion could easily turn out to be optimistic since, at least judging from the facts of our own case the path that led to the emergence of the human species includes very many highly unlikely steps such as the long trajectory through arboreal forms, the re-adaptation to ground dwelling, the adoption of bipedal locomotion, the use of hands to make stone tools, the impetus for rapid brain capacity evolution due to such developments, the evolution of a larynx particularly adapted for producing complex sounds, the emergence of speech, the evolution of strong sexual dimorphism and consequent extremely drastic sexual selection forces on the part of human females strongly selecting males of greater brain capabilities. All of these are necessary in order to evolve a human being, for example we would not be a technological species if we had not developed our extremely effective prehensive hands. This structure is present ONLY in species which are arboreal or used to be arboreal. Many of these steps are present only in the line leading to us, in other words they do not appear to be high probability steps.