MadSci Network: Evolution |
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?
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.
Neil
Try the links in the MadSci Library for more information on Evolution.