One of the things that interest me most in education today, and specifically about education in evolutionary biology, is trying to diminish or even to abolish the adverse effects of this terrible and nefarious conception called scala naturae,both in our culture and in our way of thinking. You will see me constantly dealing with this topic here in the blog, the infamous scala, “the great chain of being”.
But what is the scala naturae? It’s Latin for “nature’s ladder”. Roughly saying, it is a classical concept, which achieved his most prominent status in the middle ages, that states that all matter and all life on Earth is hierarchically organized, one being upon other, in a huge single line that goes from bottom to top, from inferior to superior, from less developed to highly developed. One can suppose that this medieval concept had died in the middle of 19th century, when scientific evolutionary biology aroused, or in the middle of 20th century, when phylogenetic systematics aroused. We’ll see, however, that this absurd and pernicious concept is still widely used and well alive today, not only among laypeople, but among scientists as well.
One of these days I was teaching a very commonplace and repetitive, to say so, class on chordates. Nothing complex, just the basics. I was using a Powerpoint presentation that in fact I created a long time ago, in which I add a new slide from time to time. When I came to discuss the evolutionary emergence of the first tetrapods, I moved to a slide that, year after year, always bothered me. And I, always indolent, year after year have forgotten to change it. The slide is this:
This is an image taken from Peter Raven’s textbook on general biology (“Biology”, Raven and Johnson, 6th ed., Mcgraw-Hill). I must emphasize here that Raven is an excellent author, and I don’t mean to criticize his book: the problem I am about to discuss happens in almost all the textbooks…
What the image shows us is that the first tetrapods that left their footprints on the dry land were amphibians (in the picture above, this animal is represented by the letter b, early amphibian). We are used to this kind of description, which actually permeates almost all areas of zoology and botany: the first animals to have nerve tissue were the cnidarians, the first plants to have seeds were the gymnosperms, the first bilateral animals were flatworms, the first amniotes were reptiles, and so on (note that in these preceding examples there are both monophyletic and merophyletic groups, causing even greater confusion). The problem is that this way of thinking about the natural history of living beings are not only wrong, but can as well impair one’s ability to understand evolution and evolutionary biology. Why?
The main problem is the maintenance, in 21st century, of this medieval thought, namely the scala naturae, in which extant species are ancestors of extant species. Let’s see: It is normal to think that the fishes originated the amphibians, the amphibians originated the reptiles, the reptiles originated the mammals, etc. We therefore have a sequence summarized as follows: fishes → amphibians → reptiles → mammals. The scala naturae, without any contestation, in broad daylight.
But consider this: what does it mean when one states that the first tetrapods were amphibians? And when one defines that the reptiles came “from amphibians”? In practice, the extant reptiles are not descended from extant amphibians and, that’s what I like to say, extant amphibians are not descended from extant reptiles either. Using the same reasoning, we can’t say that man descended from chimp, or from any other ape, and much less that chimps descended from man. What these two groups share is simply a common ancestor.
Also, saying that the ancestral tetrapod would be today classified as an amphibian doesn’t help at all. Regarding certain characters (structures), its “reptile” descendant may be much more similar to it than its “amphibian” descendant, because apomorphies occurred both in the branch leading to reptiles as in the branch leading to extant amphibians (of course, one does not expect that extant amphibians are morphologically and physiologically equal to those “early amphibians”). And we should not, humans, introduce moral judgements about this or to that characteristic, defining arbitrarily which one is more important: it would be like saying “if that ancestral tetrapod lacks an amnion, as the amphibians do, while reptiles are amniotes, then this ancestral tetrapod was an amphibian”…
It would be more correct, therefore, to say that the animal represented on the slide was the ancestor of extant tetrapods, i.e., it was a component of the group from which both reptiles and amphibians emerged, both with new features, features not present in the animal depicted in the slide.
It is very difficult to get rid of an iconic structure that has permeated the human intellect for over twenty-five centuries. Nevertheless, we must try…