The Scala Naturae and Evolution

People often have misconceptions about the process of evolutionary change. I find that one of them is often that evolution is sort of a giant ladder. Life is arranged on this ladder in order of primitive to advanced, with each organism in its place. As evolution progresses, organisms can step up, reaching greater heights of perfection and complexity, climbing until at last, the pinnacle of human perfection is reached. This idea of evolutionary change actually has very deep roots, back to at least the Middle Ages.

The great chain of being, or scala naturae, was a medieval philosophical concept that was the basis of thinking about the order in the world for much of western history. In the great chain of being, all living things are organized from most perfect, at the top, to least perfect, at the bottom, in a continual series of gradations. Of course, God is above everything. Below God is the King, below the King are the lords, and so on. Eventually the chain reaches the last serf, and starts down into the animals. Every animal is ranked according to medieval man’s idea of its nobility, its complexity, and its usefulness to man. Lions and tigers and the like are at the top, ladybugs are above flies, oaks are above the demonic yews, and snakes are at the very bottom of all animals. All of these are then in turn above the minerals. Once microorganisms (animalcules) were discovered by Antony van Leeowenhoek in 1683 (in the plaque of an old man’s teeth), they fit perfectly in that gap between animals and minerals.

Once evolution was accepted by the mainstream scientific community, these ingrained ideas of “the order of nature” became the unconscious basis for the evolutionary ordering of life. The scala naturae was originally meant to be a fixed chain, with each thing’s place immovable, but with minor adjustments the same idea could be read as a ladder instead, a progression through time. And of course, who is at top, but man. Sean Nee has a great article about this in the journal Nature [435: 429] from 2005. He writes how any published phylogeny that includes humans inevitably will place humans at the top of the tree, even when they could correctly be placed in some other ordering. This isn’t coincidental. If you still have your high school or middle school biology textbooks lying around, take a look at them. You’ll notice how they will start out their discussions of living things with viruses and bacteria and other single-celled things, progress through plants perhaps, into insects and squirmy things, through reptiles and birds, and finally into the mammals, with the very last section being about human evolution: the pinnacle, the peak of God’s creation evolutionary history. It’s very deeply engrained in our cultural psyche.

This view of evolution and nature, obviously, is flawed. With this sort of idea it’s natural to visualize a whole-organism progression; a collective complexifying of all the organism’s parts simultaneously. However, organisms are mosaics of derived and primitive features [spoiler warning, see next post!]; our own genome contains remnants of retroviruses; our mitochondria were originally free-living bacteria. Furthermore, plenty of “simple” organisms have immense super-powers that we, the apex organisms, can only dream about, like the ability to survive in environments with and without oxygen, like facultatively anaerobic bacteria. The erroneous view of a progressive ladder implies that “simple” modern organisms are somehow ancestral organisms simultaneously, leading to the “men from monkeys” fallacy of human evolution and the recent portrayal of the platypus genome as “a cross between a reptile's and a mammal’s.”

A much better view of evolutionary “progress,” one that at least gives us an idea of our true place in the universe, is one like this one. Notice the small insignificant little “Homo” down there on the bottom left of this branching bush. (Compare that tree to this one though, to see how the same sort of information can be flipped around to feed that old human ego some more. Why are animals at the top? Bacteria are the ecologically dominant life-form, after all.) Or perhaps a web would be better, to show the interlinking of life: fungi and algae, joining to form lichen; retroviruses and humans, our genomes inextricably intertwined. Or perhaps the map could be of just one single organism, color-coded, labeled, showing the differential evolution of all the various parts—the parts that have slowed down their development, the parts that have gone off into something totally novel, the parts that happen to be very similar to its relatives.

Perhaps the reason why so many people have trouble accepting evolution is that they still have these ingrained ideas about it, based on the centuries old scala naturae. If you really start to look around at the world, nature doesn’t seem to fit such a rigid view, such a strict, ordered progression. If this is how people think of evolution, no wonder they reject it. If we are able to recognize these deeply embedded cultural preconceptions in our own minds, it would go a long way in helping us embrace the odd complexity of the world, the riotous transformations over evolutionary time, and our own place—a small side branch, nestled among the apes and the protists and the dandelions, a great vantage point from which to watch evolution unfold around us.