Re: Other Earth like planets in the universe and their inhabitants.

Dear Shawn,

Thanks for your interesting question. We can only speculate about the nature of life elsewhere in the universe, for the simple reason that the only life we know is here on Earth. Let's break down your question into 3 parts:

1. Are there other earth-like planets?
2. Are there processes common to the origin and development of life?
3. Do similar conditions and processes lead to similar evolution?

1. Are there other earth-like planets?
Our best guess as to this answer is "very likely". Theories of planetary formation tell us other solar systems will form wherever there is a sufficient local density of gas and dust to coalesce into stars and planets. We have observed this process and we have also indirectly detected planets around other stars.
Are any of them earth-like? In other words, are they a similar distance from a similar star to our sun, of similar volume/density and do they have continents, liquid water and a nitrogen/oxygen atmosphere? We haven't yet observed such a planet, but the sheer size of the universe means probability alone makes earth-like planets very likely. Remember too that in our own solar system, some reasearchers believe that Venus and Mars were once more earth-like than they are now.

2. Are there processes common to the origin and development of life?
Life on Earth is extremely diverse, but all organisms share biochemical features. Their genetic material is nucleic acid - DNA or RNA. They use proteins to build cellular structures and catalyse chemical reactions. Their cells are bounded by membranes made from lipid bilayers. All of these components are polymers made from simpler, "building block" molecules and those building blocks are common in the universe and easily synthesised by natural processes. So provided that other planets have similar environments to Earth, there's no reason why the basic chemistry could not be very similar.
One problem here is that when we observe organisms, we are witnessing only one stage in the development of life. Nucleic acids encode proteins, but proteins are required to synthesise nucleic acids - a conundrum! Therefore, the first life on earth must have used something other than DNA and proteins - there are several theories as to how this may have worked. What's essential is some form of replicator molecule that can mutate to generate variation. It's quite possible that molecules other than nucleic acids could fill this role.

3. Do similar conditions and processes lead to similar evolution?
This is the most difficult question of all. Evolution can be thought of as an interaction between molecules and their environment. We might have the same molecules, but the environment is unpredictable. Earth, for instance, has suffered several mass extinctions in its history - who can say how life might have evolved had those events not occurred? It would seem unlikely that we would be here today to discuss the question. Remember too that for over half of its history, life on earth consisted solely of single-celled slime coating the rocks. This slime transformed the atmosphere by generating oxygen - another "chance" event with huge consequences for subsequent evolution.
So if we wound back the clock and started again, it's unlikely that we would end up where we are today. On the other hand, if the components of life - molecules and cells - are the same elsewhere in the universe, then we would expect evolution through natural selection to operate in the same way. This is because given the same components, physical constraints are imposed - only certain molecules can interact with each other, only certain mutations in genes can occur, physical structures can only reach a certain size or possess a certain shape. So I think that extraterrestrial organisms could be familiar to us in that they could possess structures that we would recognise - eyes, legs, wings and so on.

It's possible of course that life elsewhere is so different that we would not even call it life as we know it: not carbon-based, not using our biochemistry and genetics, not even motile. Fun to speculate, but firmly in the realm of science-fiction, at least for now.

I hope that this helps with your questions. A web search for "exobiology" or "astrobiology" will find a lot more interesting information for you.