There has been a lot of discussion recently about the evidence that we are currently within a period of mass extinction, the kind of event that will show up in the fossil record a few million years from now as a clear discontinuity, a radical change in the diversity of life on the planet. This Holocene extinction (or sixth extinction) seems to have started around 10,000 years ago with the disappearance of mammalian megafauna, and appears to be continuing - or restarting - in modern times, with theoretical estimates of as many as 140,000 species currently going extinct per year.

That this all coincides with the 'rise' of modern humans and our measurable impact on the planet - from rapid climate change, to our severe re-sculpting of the physical, chemical, and biological structure of the planetary surface - strongly suggests that this extinction event is causally related to our activities.

It's sobering, guilt inducing, and rather depressing. But it also brings into focus some rather profound questions about the nature of the survival of a species, and the notion of preserving or maintaining the 'natural' status quo. In particular, when we talk about the long-term future of humans the discussion tends to branch into two directions (neither of which are necessarily actually separate). One is the 'stewardship' route. Here the emphasis is on how we should learn to become good stewards of the planet, not just for our own survival, but also for a rather nebulous greater cause; not upsetting the natural cart, allowing the Earth to maintain a more stable balance in terms of climate and biodiversity. A balance perhaps more representative of the long-term state of the environment without a short-term perturbation like ourselves.

The second route doesn't necessarily obviate the need for home stewardship, but it looks beyond the Earth. One of our biggest talents, and one of our biggest problems as a species, is that we thrive on expansion. We're resource and space hungry. But instead of trying to curtail ourselves, we have the option of spreading beyond, to the vast and untapped wealth of the solar system. Call it the ultimate manifest destiny if you will, except that it also offers the possibility of preserving our homeworld by altering the fundamental equation of our existence, by outsourcing many of our material needs.

Of course, this cosmic pathway could go wrong. We could start altering the environmental state of Mars and mess that up. Or, without care, we could risk destabilizing our global economy and balance of power. After all, we seem to be barely capable of managing 196 recognized countries, adding more offworld states is unlikely to help. But on a grand scale, for the ultimate preservation of the species, the solar system may be our savior. There's only one surefire way to avoid extinction by asteroid impacts or supervolcanoes, or sheer overcrowding. Put some of us somewhere else.

Yet this is also where things get increasingly complicated. If we're genuinely concerned about the very long term survival of our species, and about minimizing our impact on the rest of the planet, we need to ask just what that entails. And that could involve facing up to some disquieting truths about the nature of evolution itself.

The thing about natural selection is that it's a relentless process, it's always happening. This is the nature of the complex phenomenon that is life on Earth. For example, modern humans - Homo sapiens - are merely the survivors from one part of an array of hominidae radiating out from a last common ancestor with the great apes some 14 million years ago. We're a very recent offshoot of the Homo genus, our own speciation occurring some 200,00 years ago. And, unsurprisingly, we continue to evolve.

We can't at present quantify all the factors that have contributed to, and are presently contributing to our evolution, or for that matter the evolution of any species. There are certainly indications that, for us, traits such as agriculture and livestock domestication have been profoundly intertwined with our genetic makeup over the past five or six thousand years - through qualities such as adult lactose tolerance and other digestive capabilities. Resistance to diseases, tolerance for climate extremes, intelligence, and other aptitudes or failings all fold into this hugely complex equation, together with sheer unadulterated chance. The rock that squashed great-great-great-great-great-great-great uncle Charlie? Yep, that probably altered the trajectory of the entire species at some level. As will some rock-squashed descendent yet to be. Natural selection is changing us right now.

What this means is that, like it or not, humans as we presently know them will not, and cannot be around forever. Each new generation is already different, it's just that the direction the changes are taking are hard to spot. Push 100,000 years into the future and it's a fair bet that (barring catastrophic extinction) new speciation will have occurred, no matter how well we steward the Earth or how much we expand into the solar system.

That's the curious thing about life, and the evolutionary process, it can never be static. Certainly, some species change more slowly than others, or the changes are more subtle, hidden in the details of nucleic acid sequences and biochemical function. But every base-pair alteration, every point mutation during reproduction that induces biological change in the next generation has the potential to leave the old behind. In some sense extinction events are happening all the time at a molecular level.

And this is where extrapolations about our own future take us to a grander, cosmic, scale.

Ideas about other life beyond the Earth inevitably have to confront the Fermi Paradox - the conundrum of why, if life is reasonably common throughout the cosmos, it hasn't already spread everywhere and shown up on our doorstep (you can read more details here). Numerous potential 'solutions' exist, ways in which life is prevented from doing this. Here's another one.

Let's assume that a species might begin to spread beyond the confines of its home world, and its home system, driven by some of the same issues forcing us to consider such options. The extraordinary challenges of space travel may ensure that this spread is slow, a generational process. Extrapolating from estimates of how we'd accomplish such exploration it's reasonable to imagine that tens of thousands, or hundreds of thousands of years could easily pass before there is an appreciable occupation across even just a few dozen light years of the Galaxy.

So far so good. It's a lot of time, but that's ok right? Well, perhaps it's not. By the time a species has spread this far it is almost inevitable that it will have evolved into something that is, by our terrestrial standards, different from what it started as. Furthermore, it will have probably diverged within every new environment, every new home world. In other words the original species will be extinct, not because of severe environmental pressure or a catastrophic inability to adapt, but through longer term natural selection and evolution.

Although genetic engineering and control might, in principle, allow an advanced species to prevent that drift, it might not be a good idea to do so. This dynamic process is what helps keep organisms ahead of evolving pathogens. It helps living populations bend and not break with the winds of change. It's not clear that fiddling with this is going to be either practical or desirable. So let's assume such a level of control is absent.

Obviously a technological species will retain memory of what it once was. But will it always feel the same impetus, the same primal urge to survive by expanding as its ancestral species did? To draw a parallel, how likely is it that our motivations are the same as humans from, say, 50,000 years ago? We are not just culturally different, we know we are already biologically different - and that means that some of our fundamental needs and motivations have to be different.

This represents yet another potential answer to the Fermi Paradox. Even if a species goes interstellar, by the time it has spread a little way, it may no longer be the same. That form of organism will have evolved into something new, resetting the clock, changing its priorities. There may be no billion-year-old civilizations, because no species can maintain itself against the might of evolutionary forces for that long. In this case life is perhaps less hemmed in by the technological hurdle of space travel, and more by the nature of life itself - at least as we know life to be. On the truly grand scale of things the existence of a self-aware, technological species could always be a fleeting event, a wisp reaching out from the tumult of molecular evolution, to be inevitably reabsorbed or redirected.

Perhaps all the more reason to drastically alter our present behavior, to better enjoy what little time we have left.