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Austin Cline

Is Life Inevitable or Just a Fluke?

By January 31, 2013

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We keep finding lots of planets beyond our solar system, but so far we haven't found any like ours -- and none yet with evidence of life. So is life rare? Spread thin through the universe? A fluke that only occurred here on our planet?

Alien Life
Alien Life
Photo: Stone/Getty

Perhaps the answer lies in the fact that there are two events to deal with: the original development of life and the later development of complex life out of simpler life. There are reasons to think that the development of simple cells is easy and perhaps common, but the later development of large, complex cells is less likely.

The central problem for large cells is energy: to become a larger cell with a larger, more complex genome you need more energy, but to get more energy you need more copies the genes that control the generation of energy, which creates a vicious cycle that produces diminishing returns.... and no way to evolve into more complex life.

So how did eukaryotes get around this problem? By acquiring mitochondria. About 2 billion years ago, one simple cell somehow ended up inside another. ...these endosymbiotic bacteria evolved into tiny power generators, containing both the membrane needed to make ATP and the genome needed to control membrane potential. Crucially, though, along the way they were stripped down to a bare minimum.

Anything unnecessary has gone, in true bacterial style. Mitochondria originally had a genome of perhaps 3000 genes; nowadays they have just 40 or so genes left. For the host cell, it was a different matter. As the mitochondrial genome shrank, the amount of energy available per host -gene copy increased and its genome could expand. Awash in ATP, served by squadrons of mitochondria, it was free to accumulate DNA and grow larger. ...

These huge genomes provided the genetic raw material that led to the evolution of complex life. Mitochondria did not prescribe complexity, but they permitted it. It's hard to imagine any other way of getting around the energy problem - and we know it happened just once on Earth because all eukaryotes descend from a common ancestor.

The emergence of complex life, then, seems to hinge on a single fluke event - the acquisition of one simple cell by another. Such associations may be common among complex cells, but they are extremely rare in simple ones. And the outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendents could flourish.

This does not bode well for the prospects of finding intelligent aliens. It means there is no inevitable evolutionary trajectory from simple to complex life. Never-ending natural selection, operating on infinite populations of bacteria over billions of years, may never give rise to complexity. Bacteria simply do not have the right architecture. They are not energetically limited as they are - the problem only becomes visible when we look at what it would take for their volume and genome size to expand. Only then can we see that bacteria occupy a deep canyon in an energy landscape, from which they are unable to escape.

So what chance life? It would be surprising if simple life were not common throughout the universe. Simple cells are built from the most ubiquitous of materials - water, rock and CO2 - and they are thermodynamically close to inevitable. Their early appearance on Earth, far from being a statistical quirk, is exactly what we would expect.

Source: New Scientist, 23 June 2012

Everyone learns about mitochondria when they take basic biology in school, but it looks as though mitochondria may be more fundamental to our existence than has generally been appreciated. It's not just that they are needed for our cells to function, but perhaps for complex life like us to exist at all.

Comments
January 31, 2013 at 12:20 pm
(1) Dean J. Smith says:

I would not be surprised if intelligent life is vanishingly rare, such that we may be the only example in our galaxy. Not only was the acquisition of mitochondria so early fortunate, the appearance of land-based life was greatly accelerated by having such a large moon to create large tides. We could well be the ‘elder race’ of the galaxy.

Although intelligent life could be ubiquitous in our galaxy in a couple of million years if we get out there, and speciation would be inevitable. Then there will be aliens, but they’ll be a lot like us.

January 31, 2013 at 10:59 pm
(2) Peter Kinnon says:

While it is still fashionable to treat the advent of the eukaryote as an extremely improbable event there is increasingly a body of thought that considers the acquisition of symbionts to perhaps have been a virtually inevitable stepwise process in keeping with the general mechanisms of biological evolution.

The recent discovery of what appears to be an intermediate organism, Parakaryon myojinensis lends weight to such speculation.

This has the nucleus characteristic of eukaryotes but it is bounded by a single rather than double nuclear membrane and endosymbionts that resembled bacteria not yet differentiated into mitochondria.

This suggests the possibility of a gradualistic evolutionary pathway for the emergence of the eukaryote. This organism was found rather serendipitously in a study of a type of sand worm.

Intriguingly, this is found in the vicinity of the very hydrothermal vents which are implicated in the most plausible scenario for abiogenesis to date.

Furthermore, there is actually a good case to be made for extending the concept of “life” beyond biology.

There is a further case to be made for a far broader evolutionary model that can be traced back at least as far as the formation of the chemical elements in stars and supernovae that extends through to the autonomous evolution of technology within the collective imagination of our species.

While the mechanisms of the various phases of this evolutionary continuum are, of course, quite different, there are some commonalities.

Moreover, a reasonable prediction can be made as to short term future outcomes of this process- the emergence of a new non-biological cognitive entity on this planet.

This evolutionary continuum model is outline (very informally) in “The Goldilocks Effect: What Has Serendipity Ever Done For Us?” , a free download in e-book formats from the “Unusual Perspectives” website.

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