A couple of years ago, the so called 'Rare Earth hypothesis' was proposed by paleontologist Peter Ward and astronomer Donald Brownlee.
As the name quite clearly implies, this hypothesis states that planets like Earth may be far rarer than previously believed.
Among the things it states is that simple, microbial life may be very common in all of the universe, but that complex, multicellular life, such as for instance plants and animals, on the other hand is extremely rare - this because it took so long for life to evolve from single- to multicelled here on Earth.
It claims further that many of Earths special attributes, such as plate tectonics and a large, stabilizing moon that almost makes the Earth-Moon system into a double planet, may have had a determining effect on the rise and evolution of life on the planet.
These and other attributes of our planet, are - according to Ward and Brownlee - extremely rare other places in the universe, indeed possibly unique to our planet Earth.
Of course this provoking hypothesis has gotten its responses, and the responses are based largely on concrete discoveries made over the latest years, and not so much on speculation as the Rare Earth-hypothesis. Especially the discovery of so called extremophiles in very inhospitable environments here on Earth, have provided science with new knowledge about life's toughness and adaptability. It has, for instance, been discovered that many primitive, formerly unknown organisms thrive in volcanic environtments, both on dry land as well as on the bottom of large oceans.
These and other discoveries have provided many astrobiologists with new faith in the possibility that life can be far more adaptable and rich in diversity, than what hypotheses such as Rare Earth claims.
In this post I will focus on what we actually know about life here on Earth. As you all know, our own planet is currently the only frame of reference at our disposal, but we still have obtained some hard facts during the latest years, such as that the number of stars in the observable universe is approximately 70 000 000 000 000 000 000 000, and that the vast majority of these stars are surrounded by orbiting planets.
I therefore greatly doubt the possibility that Earth is the only place in the universe where life has emerged at all.
I'll now try and explain why I also think there must be other species out there, similar to our own, and not just primitive germ analogues or collections of organic slime and foam:
What is it that defines us humans? What makes us unique in the animal kingdom on Earth? One could point out several things, such as the combination of an obviously high intelligence, great social skills, and the ability to use language, grammar and physical tools.
We modify our natural surroundings and in this way we manage to keep nature's threats at a safe distance. If we still somehow should be confronted with such threats, we have the ability to use weapons and other tools to defend ourselves.
This has made us into a superior species among the mammals on the planet, and little by little we have settled on every continent and have developed a very large population, totally out of proportion compared to the populations of other large, mammalian species.
The way I see it there is however another factor that is crucial to our success, and that is man's instinctual drive for exploration and discoveries. We have a built in hang to go places, to find and conquer new land. Because of this, we have all the way from the beginning of our emergence, wandered from continent to continent, and through trial and error we have crossed vast expanses of oceans and mountain ranges, not knowing what we're looking for, simply to explore new areas and break frontiers.
We are driven by a fundamental curiousity towards all that is unknown, and where does this curiousity come from? It comes from the genes, of course. We are genetically programmed to explore, and this exploration- and curiousity-gene has made us into an extremely successful species on this planet.
One might speculate if a global, technological civilization has any chance to survive for long, in a geological or astronomical time perspective, but the human species as a whole will doubtfully die out even if world society breaks down and a couple of billion human lives are lost in nuclear wars. Human beings, as a species, will persist through the millennia, and new civilizations will see the light of day only to possibly go under at a later time.
Imagine then, a scenario on another planet, a planet of the same size and composition as Earth, that orbits a G2-star like the sun, in the same distance to its star as the one we have to ours. All is set for life as we know it to arise, and therefore that's exactly what it does. When life first arises it is very tough. That's a fact we've learned from the studies of extremophiles here on our own planet. If life first emerges, it's unlikely to go extinct in a forseeable future, even if its conditions change dramatically. It will simply adapt to new conditions. The ones best fit for survival, survives. The ones least fit, goes extinct.
Assume further that the planet has almost exactly the same conditions as ours, it's not unlikely that Earth has such a twin planet somewhere in our own galaxy, considering the fact that there are approximately 400 billion stars here.
Evolution goes at a slow pace in the beginning, life dwells for a long time at the microbial form until the plant kingdom finally emerges and starts to evolve. The plant kingdom then lay the ground for animals, life and evolution experiences an accelarating degree of complexity.
Which of the life forms on such a planet is best fit for success, which genes would such an organism be equipped with? It would of course be equipped with genes that make the species spread over the largest possible area. And how should the species spread over the largest possible area? It would of course have a built in hang to continuous exploration, a genetically programmed curiousity.
In this fashion these genes would eventually spread all across the globe, and if we assume that the well renowned biologist Richard Dawkins is correct, humans, as all other life, are primarily controlled by their genes, and not the other way around.
We are steered by our selfish genes, that all fight a mortal struggle for survival.
What gene is it then, that in the long term will have the greatest ability of survival?
It would of course have to be the gene that conquers the entire planet. And when the planet is conquered, it neccessarily has to look towards new horizons, and where does it look then? It looks to the stars.
Such inheritable traits are not unique to Earth. I do not discount the possibility that man could be the_first_spacefaring species in the universe, but I'm convinced that other species, analoguous with ours, are at least evolving in a large number of planetary systems, through out the entire universe. It is highly unlikely that we are a unique species, in the sense that we're spacefaring, as seen from a cosmic perspective.
The short version of my hypothesis goes like this:
If life emerges on a planet, and it will as long as the conditions for it are met, the genetic material that in the long, evolutionary perspective proves to be the most successful, is the one that manages to spread over the largest possible area.
When the home planet no longer has room for more individuals carrying this genetic material, the individuals are neccessarily forced to find new places to settle, in other words: They're forced to become spacefaring.
The Rare Earth hypothesis does not take into account that evolution always will favour those individuals that are best fit for survival. When simple life first emerges on a planet, something that the Rare Earth hypothesis don't regard as unusual, it is inevitable that this life will increase in complexity as long as it gets enough time, something that the Rare Earth hypothesis on the other hand sees as highly unlikely and improbable. There are enough sun-like stars in the universe, with life spans of 10 billion years, so that there're a large number planets where conditions are met for life, is not to be doubted with our present knowledge.
The genes that have the biggest potential for leng term survival in a biosphere of complex lifeforms, are - as previously stated - the ones that spread over the largest area, and these genes will therefore have to create a high level of intelligence and an instinctual curiousity in the individuals that carry them, like us humans ourselves are a good example of.
I therefore see no reason to believe that man is the only spacefaring species in the universe.