If the Martian life form transpires to be eerily similar, this might only show that Life … in reality has very few options. … No sentient forms weaving their existence in vast interstellar dust clouds, farewell to bizarre filamentous species greedily soaking up the intense magnetic fields of a crushingly oppressive neutron star and on even Earth-like planets no forms that we might as well call conceptualized pancakes. … Contrary to received neo-Darwinian wisdom, life on Earth at any level of organization—from molecular to societal— will provide a remarkably good guide as to what ought to be ‘out there’.

So argues Simon Conway Morris, from inside view considerations. I think he’s mostly right, but based on an outside view.

Here it is: when relevant parameters can vary by large magnitudes, the most common type of thing is often overwhemingly more common. For example, processes that create and transmute elements vary greatly in their rates. So even though there are over a hundred elements in the periodic table, over 90% of all atoms are hydrogen, so the odds that two randomly selected atoms are the same element is >80%.

Similarly, since the influences on how many eyes a human has vary greatly across eye numbers, most humans have the same number of eyes: two. Most humans do not have the same last name, however, since rates of gaining or changing names do not vary by huge factors.

The same principle applies to life. Life might have evolved in a great many kinds of environments, based on a great many sorts of elements, and using many types of organization. To the extent that some environments are far more common, or are far more supportive of high rates of biological activity, most biological activity in the universe should occur in the few most common and supportive environments. Similarly if some elements or organizations are far more supportive of biological activity and innovation, most life should use those elements and organization.

I expect cosmic environments to vary enormously in both volume and in support for biological activity. I also expect some types of elements and organizations to be far more supportive of biological activity and innovation. I thus expect most life to be based on similar elements and organizations, to originate and be active and innovative in places similar to where our life orginated and is most active and innovative.

This view is supported by the fact that the assumption that life originates via the entropy of sunlight hitting “dust” predicts many cosmological parameters. In ’08 I reported:

This causal entropic principle so far successfully predicts dark energy strength, matter fluctuation ratio, baryonic to dark matter ratio, and baryonic to photon matter ratio! … A simple reading of the principle is that since observers need entropy gains to function physically, we can estimate the probability that any small spacetime volume contains an observer to be proportional to the entropy gain in that volume. … Exclud[ing] entropy of cosmic and black holes horizons, … ignor[ing] future observers getting far more efficient and aggressive in using entropy, … they estimate that, aside from decaying dark matter, near us most entropy is made by starlight hitting dust, and most of that is in the past.

Our life probably started from sunlight hitting “dust” (including planets). More quotes from Simon Conway Morris:

Will the extra-terrestrials be utterly familiar, completely alien (whatever that is supposed to mean) or is the search a complete waste of time? What will it be? Worlds full of shoppers and celebrities, biological constructions so unfamiliar that they are only brought home by accident and then inadvertently handed over for curation in a department of mineralogy or an exercise in galactic futility as one sterile world after another rolls beneath the spaceship windows. …

Given the likely range of planetary environments, such as a 100 km deep ocean or an atmosphere substantially denser than that of Venus, what fraction of any potentially habitable biosphere is actually occupied? Is the terrestrial ‘habitation box’ only a small proportion of all of biological occupancy space or, alternatively,has life here more or less reached the limits of what is possible anywhere? …

What we find here, therefore, will be a reliable guide to what we will find anywhere. Paradoxically, confidence that this may be correct comes from the dramatic increase in our knowledge of so-called extremophiles. … It may be that the current thermal limit (ca 120◦ C) of microbial activity [10] may not be much exceeded. In part, this is because water at this temperature is necessarily pressurized, and the equivalent limits of microbial habitation in the Earth’s crust (e.g. [11,12]) may not exceed ca 5 km (equivalent to ca 110 MPa; see also below) and an ambient temperature (depending on the local geothermal gradient) of at least 120◦ C. … While the environmental extremes of these and a few other multicellular organisms are impressive [18], the overall size of the habitation box for eukaryotes is unsurprisingly substantially smaller than that of life as a whole. … For life as a whole, there may be no lower limit in as much as at increasingly lower temperatures normal growth then yields to physiological maintenance, and ultimately dormancy where ‘coincidentally’ rates of DNA and protein repair are equal to those of macromolecular deterioration. …

A more fundamental question, however, is whether because of locally contingent circumstances terrestrial life just happens to occupy some fraction, perhaps very small, of the total carbaquist habitation box. As we have already seen, however, in the case of minimum temperatures, pH range, salinities and desiccation, arguably the defined limits for all carbaquists have been reached by life on Earth, and with somewhat less certainty this applies also to hyperthermophiles. …  there is little evidence of microbial viability significantly in excess of the tolerances seen in terrestrial piezophiles. … Given that at least in terms of carbaquist life it is likely that lipid membranes are universal, this suggests that viability may not extend much beyond the deepest oceanic trenches (ca 11 km) or equivalent pressure zone within the crust of the Earth (ca 5 km). But the viability of lipids is not the only problem. Another potential constraint of the habitation box is the behaviour under different temperature and pressure regimes of hydration water essential to biomolecular function. Not only is the optimal zone remarkably narrow, with that for temperature being curiously coincidental in both micro-organisms and homeotherms (ca 36–44◦ C), but the phase diagram for hydration water is circumscribed and little larger than the terrestrial habitation box.