23 May 2010

One origin for all

Systematic genetic analysis of a wide range of life-forms has now confirmed what we were fairly sure of all along. All of the life on Earth -- humans, fish, insects, squid, trees, bacteria, all of it -- is descended from a single common ancestor. PZ Myers assesses the results and explains that a possible alternate hypothesis -- that life originated multiple times but the differences were smoothed out by gene transfer between species -- is not credible (in any case, that would require that independently-originated life forms have the same DNA "language" to begin with, which seems unlikely). For all practical purposes it is now a proven fact that life arose on our planet just once, with every living thing today being derived from that common origin.

One might consider this a bit surprising. The Earth is a big place. Why shouldn't primordial life have arisen multiple times? If it had, we would now see several independent and unrelated families of life on Earth, each with a completely different genetic system (or perhaps not genes at all but other systems for encoding hereditary information). Why didn't that happen?

This relates to another question which has long interested me. Given the number of galaxies in the universe, the number of stars in a typical galaxy, and the likelihood of planetary formation, the total number of planets in the universe is probably on the order of a billion billion. That's a huge number. Given so many planets, we have long been told, life must be abundant in the universe, and even technological civilizations must exist in great numbers. If that were so, given that the universe is over 13 billion years old, many of those civilizations would probably be millions of years older than ours -- some even hundreds of millions of years older.

Considering the technological progress we humans have made in just the last 100 years, civilizations so old would be unimaginably advanced and powerful, so much so that they would probably be easily detectable by us, across interstellar or even intergalactic distances. Yet we have never seen any evidence that they exist. No radio signals, no galactic-scale engineering projects, nothing. As far as we can tell, they just aren't out there.

Let's get back to the planet Earth for a moment. Notwithstanding drivel about intelligent design and irreducible complexity, we know that the process of natural selection is fully sufficient to explain all the development of life from the first primordial self-replicating molecule to the present global ecosystem of millions of species including complex organisms such as our noble selves. The bigger problem is that original self-replicating molecule. Its formation must have been a rather low-probability event, given that it only happened once on the whole Earth.

In fact, consider what we're talking about here. For the process of natural selection and evolution to get started, there had to be a molecule, somewhere, whose effect on its chemical environment was to produce copies of itself. That must have been a pretty odd, very complex molecule with unusual properties. And it couldn't have evolved (evolution requires that the ability to self-replicate already be in place). It must have just formed, spontaneously, by some chance combination of atoms that just happened to arrange themselves into a molecule with those properties.

Isn't that really an incredibly unlikely thing to happen? Isn't that a staggeringly low-probability event?

Such a low-probability event, perhaps, that we would expect it to happen only once, even in an entire universe of a billion billion planets?

I think that's it. That's why we haven't found any evidence of life on nearby planets, or seen any signs of alien civilizations advanced enough to reshape whole galaxies to suit themselves. There isn't any other life out there. There never was.

If I'm right about this, it carries one further implication. As human technology advances in the future, we will spread out beyond the world of our origin -- eventually far beyond. The day will come when the universe which is now lifeless will be brimming with life and awareness.

And all that life -- throughout the universe -- will be able to trace its origins back to that one ancestral self-replicating molecule that existed, billions of years ago, somewhere here on Earth.


Blogger Tim said...

It really is amazing. When thinking of all that stuff I can't help but feel a sense of awareness. Kind of like knowledge is within our grasp if not for our brain.
Interesting Post.

23 May, 2010 09:57  
Anonymous Ross said...

"For all practical purposes it is now a proven fact that life arose on our planet just once"

It proves that all existing life derives from one source, but it doesn't preclude the possibility that life may have arisen independently but simply not survived as far as I can see.

I imagine that after life had first emerged and spread throughout the planet it would be difficult for new primitive organisms to emerge and survive because the ecological niches were already filled.

23 May, 2010 10:08  
Blogger Infidel753 said...

Tim: Thanks. As you can see, I like to step back sometimes and look at the big picture. It puts our current political squabbles in a larger perspective.

Ross: That's a good point, but there's no evidence that that did happen. The initial development of life seems to have been quite slow -- eukaryotic cells don't show up until two billion years ago, at least 1.6 billion years after life first appeared. If a second independent origin had occurred during that time, it should have been able to establish itself. Yet that didn't happen.

23 May, 2010 10:16  
Blogger Holte Ender said...

I like this sort of thinking especially as it sends the freaks out there into tizzy.

23 May, 2010 14:54  
Blogger Ranch Chimp said...

Greeting's Mr.Infidel!

Just wanted to say that even though we of course have discussed this before, this is a nice posting and theory, really get's that ole brain blood circulating with "question", even just for fun! I am a strong believer in evolution .... period, been as far back as I can recall (nature) .... it is just fasicinating trying to picture what may have came about over 4 billion year's back .... what this was at first, and that slim chance of whatever happened with those first molecule's that made this whole thing evolve and become more complex in various form's of life that evolved from it. In a way .... I reckon, kind of like human intelligence even.... considering how simple the brain might have been a couple million year's ago in earlier homo's, to what it has become today. Nonetheless .... I can rattle and rattle as you know .... so I'm outta here Guy ...... Thanx!

25 May, 2010 02:20  
Anonymous amadmike1 said...

I'm with Tim! It is amazing to think of the power of the universe. Overwhelming actually.

25 May, 2010 09:14  
Blogger tnlib said...

Awesome but makes me feel insignificant and smaller than a pin head.

25 May, 2010 17:37  
Blogger Infidel753 said...

HE: Certainly those types who think the Earth is 6,000 years old are un-equipped to handle such concepts.

RC: I would hope to make people think. It's worth reading a few good books on evolution (like those of Richard Dawkins) to get a sense of how the world as we know it really arose.

MM and TNLib: See my sidebar -- "in the long run human intelligence is the most powerful force in the universe." The universe isn't gonna know what hit it. Pinheads that can think will rule over galaxies that cannot.

25 May, 2010 19:08  

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