"Humans are not the end result of predictable evolutionary progress, but rather a fortuitous cosmic afterthought, a tiny little twig on the enormously arborescent bush of life, which if replanted from seed, would almost surely not grow this twig again."- Stephen Jay Gould
Evolution is a process of constant branching and expansion. Life began three and a half billion years ago, necessarily about as simple as it could be, because life arose spontaneously from the organic compounds in the primeval oceans. You couldn't begin by precipitating a giraffe out of this primordial soup, so it began the history of life with the simplest possible form of cellular life, namely bacteria. And since there's no way of getting any simpler as life expanded every once in a while you get something more complex because that's the only direction open, but if you look at the full range, rather than falsely, and myopically concentrating on the history of the most complex thing through time.
What you see is that the most outstanding feature of life's history is a constant domination by bacteria. In fact, this is not the age of man as the old textbooks used to say, or the age of mammals, or even the age of insects, which is more correct, if you want to honor multicellular animals. This is the age of bacteria. Bacteria have always been dominant. The bacterial mode, the mode being the most common form of life, is never altered in 3 1/2 billion years. We don't see it that way because bacteria are tiny and they live beneath us, but bacteria live in a wider range of environments. There are more, just E-coli cells, that's only one species in the gut of every human being than there have ever been people on earth, and if this report on Martian life is true, then the bacterial mode is universal. It's not only planetary, and there's no universality for little green men. So this is a bacterial planet. You can't nuke them into oblivion. They've always dominated life on this planet.
As far as they're concerned, we're just little islands of mobile resources which they can exploit for a while. They're happy to let us strut this little hour on the stage because they'll still be here when we're gone. But, you see, you don't see that unless you're willing to look at the history of life as the full range of its variation through time. I mean, it is true the most complex thing has gotten more complex. Once there were only bacteria. Now there are humans, but that's not the result of an intrinsic defining central drive. It's just a kind of random movement away from a necessary beginning at maximal bacterial simplicity. That's all it is.
Stephen Jay Gould