http://en.wikipedia.org/wiki/Power,_Sex,_Suicide
It is a possibility that there is a relatively small window where single celled life has the opportunity to become multicellular life. As one amazon review summed it:
'After the origin of life, the next big step on the way to us was the origin of eukaryotes. These are all the organisms - including people, trees, mushrooms, and slime molds - who package most of our DNA into chromosomes in cell nuclei. Mitochondria, the "powerhouses" of eukaryotes, are descended from bacteria which took to living in a very close relationship with another type of one-celled organism; in fact they came to live inside the other. Nick Lane argues that this merger must have preceded the formation of the nuclear membrane.' *
The great filter is stronger and weaker in argument depending on where you place the filter, but the idea in general remains the same.
*more info here but less than the book, via an article in new scientist by the author: http://ronbarak.tumblr.com/post/25996121029/life-is-it-inevi...
Regarding how common life is in the universe, I wasn't talking about non-intelligent life. I fully expect that we will find signs of such life elsewhere in the cosmos in the next hundred years, either on Mars, Europa, Titan, Enceladus, or perhaps in the atmospheric spectra of some extra-solar planet.
No, I was talking about intelligent life, which if iot exists has a very short gestational period before transforming into lasting intelligence(s) expanding into the universe at close to physical limits. In particular the window between development of technology detectable from distant observation (e.g. radio) and a runaway singularity pushing that civilization's expansion rate to light speed limits is so small that regardless of the prevalence of intelligent life in the universe, we should expect to see an empty sky. Why? Because for most of the history of the cosmos we see an empty sky in our light cone. Then suddenly the most distant stars start to go dim with strangely shifted spectra, in an expanding wave that flows through visible star systems at >0.9c until it hits us, and ... well who knows what happens then.
The Fermi paradox is: "if there are so many intelligences, why can't we see them?" The transhumanist response is: "if we could see them, we'd be dead." By the anthropic principle we can only expect to exist in the time period where we are alive, not dead, so we should expect with very high probability to see an empty sky, and with very low probability the coming onslaught of darkness.
"Power, Sex, and Suicide" argues that eukaryotic (and thus multicellular) life is basically a fluke—it is the great filter. The tumblr link Lost_BiomedE gave takes a while to get to that punchline but it's worth a read.
