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?
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.