Since Aristotle, students of nature have been tempted to rank some organisms as somehow “better” than others. Aristotle ranked all organisms from most simple to most complex. In Medieval Europe, his ideas were taken up and incorporated into a grand scala naturae or ladder of life, with lowly worms at the bottom, humans at the top of the mortal beings, and angels above us.
Evolutionary theory has had its share of attempts to understand the scala naturae, usually with time playing the role of the force that makes some organisms more “evolved” than others. Lamarck posited multiple origins of life over the ages and suggested that the lowliest species are newcomers on the world’s stage, whereas loftier species had been around for longer and attained greater heights. Hints of this view still resonate in popular misconceptions about evolution.
Darwin eschewed concepts of better or worse, but his concept of fitness was soon pressed into service (by others) to support a ladder-like conception of nature. According to views of this sort, some organisms are more evolved or fit than others, and the assumption is that fitness increases over time because of natural selection.
By this reasoning, today’s organisms are obviously superior, in the sense of being better adapted, relative those in the past. The present is the crowning achievement of evolutionary history.
Modern evolutionary biologists don’t generally think this way, and paleontologists in particular – because they constantly consider the lives of extinct organisms on their own terms – work hard to avoid such assumptions.
What we are left with is a curious sort of time-neutrality. The present is not necessarily different from the past (or the future) in terms of total fitness. Indeed, fitness is a relative term, and only implies reproductive success in a certain environment. If we look at any chapter in life’s history, we must assume that the creatures alive at that time were well adapted to their environment – at least as well adapted as creatures are today. Conditions differed, and a modern species sent back in time would not automatically dominate over species that are now extinct. Species are not intrinsically “primitive” or “advanced”.
Similarly, we try to to avoid the idea that the past prefigures the present. Evolutionary biologists once spoke of “pre-adaptation” as a means of success for a new or evolving species. For example, If the climate of an area grew more dry and hot, the species that could live with less water were called pre-adapted to drought, as if they had been sitting around in the nice wet jungle, purposefully harboring a super-efficient kidney just in case the world should run low on water.
Of course, if the past is no different from the future in terms of total fitness, we are left with an important question: how do things ever manage to evolve?
Obviously, that question can’t be answered in one small essay; all of evolutionary science is bent on finding better and better answers to that very question. But perhaps I can borrow a helpful idea to get us out of the quandry.
In physics and chemistry, we are taught about equilibrium. Equilibrium is the state of a system of matter in which although individual particles are moving or changing, the direction of change is varied in such a way that all the motion cancels out. For every atom zipping to the left, we have an atom zipping an equal distance to the right. As a whole, the material system seems to be at rest. Change happens when something knocks the system out of equilibrium. For example, you could tilt the system on its side, or accelerate it, or heat it up. Then, all the particles will go zipping around in response to this change – and mostly in the same direction – until they reach a new equilibrium. For example, take a glass of salt water. For every ion of sodium that wanders to the left, there is another wandering to the right, so the total concentration of salt doesn’t get higher on one side and lower on the other. But, if you put a drop of red food coloring on the top of the solution, the red dye molecules have only one way to move: down. So, they will spread out until they have become as uniformly distributed as the salt was, and therefore reach a new state of equilibrium.
Fitness could be like that. If the conditions on the earth remain similar for a long period of time, we expect species to reach a sort of equilibrium. Minor differences from generation to generation will tend to cancel out, and the species as a whole remains the same. But change the environment or add a new species to it, and something gets out of whack. The species will then change toward a new equilibrium state, which we see as evolution.
When we say that the past is no better or worse than the future, we are really saying that in any given slice of time, we can expect some species to be in equilibrium and some to be seeking a new equilibrium. Species don’t go extinct because they are primitive. They just die out if they can’t find a different equilibrium before their birth rate drops to zero.
This whole discussion has one last flaw, however. Life has in fact evolved from simple to complex; we can see it in the ancient fossil record. The earliest organisms were simple single-cellular creatures. Complex (eukaryotic) cells did not evolve until about 2 billion years had passed and multicellular life didn’t appear until at least another 500 million years after that. Similarly, we know that the first multicellular animals did not have eight jointed arms and a half-dozen different organs and a complex nervous system and so forth. They were probably balls or sheets of cells with some regional specialization.
Doesn’t this violate our time-neutral assumption? Yes and no. Yes, things have gotten more complex, but that doesn’t mean they are better or more fit. For one thing, we sometimes see examples of species getting less complex over time. Blind cave fish, for example. For another thing, we sometimes see very simple-seeming organisms with large amounts of duplicated DNA For example, plants sometimes become “tetraploid”, which means that they fuse two gametes without first reducing the number of chromosomes in each. The new plant (or plant species) may fare no better or worse than its parents, despite having twice as much DNA. A more complex solution isn’t always better.
There is also a consideration of time-scale. The time between the first life and the modern day is far longer than the time between the last dinosaurs and ourselves. The first cells were much closer in time to the original state of zero complexity, so any change would seem more significant. The change from single-cellular life to multicellular life is much bigger than the change from dinosaur to chicken, or any change that we are likely to see in the near future. We might even be reaching a sort of limit on how complex life can get. In that case, perhaps we are approaching a sort of grand life-wide equilibrium, and just maybe we have been in such a state for the last eon. Some species get more complex, and some get less, but on the whole there is no more over-all “advancement”.