Our perceptions of how the natural world is organised haven’t really changed for hundreds of years, despite biology moving in a very long way. Why is this?
Most people, when asked what a ‘reptile’ is, tend to pick a few scaly options – turtles, crocodiles, lizards, snakes, maybe the odd skink or caiman. And they wouldn’t be wrong- all of these things are in the class ‘Reptilia’. They all share a suite of characteristic physical traits: scales, egg laying (for the most part), ectothermic body temperatures (also for the most part, nature love’s exceptions). But there would be a prominent omission from the list – birds.
‘Reptiles’ in the biological sense are made up of three main extant groups: the Lepidosaurs (snakes, lizards and the tuatara); the turtles; and the Archosaurs (including crocodiles, extinct dinosaurs and… birds). These last don’t fit the common idea of what a reptile is at all. They are warm blooded, feathered, fast moving and often intelligent, in many ways more like mammals than tortoises or crocodiles.
Many of the ‘dinosaurs’ probably didn’t fit our idea of what a reptile is either. Images of the dinosaurs have undergone miraculous changes over the past 100 years, from Victorian stumpy-legged plodders to terrifying but stupid toothy carnivores and mountainous herbivores, to the brightly feathered and potentially whip-smart creatures emerging more recently. The fact is, the ‘dinosaurs’ as we imagine them weren’t one group, but are now split into four phyla: the Hererasaurids, Sauropods, Theropods and Ornithiscians. The word ‘dinosaur’ itself is a Victorian mirage, a complilation of the Greek words ‘deinos’ and ‘sauros’ making ‘terrible-lizard’. As if everything ancient, extinct, scaly and big could be stuffed easily into one category.
So why the omission of birds from ‘reptiles’ or the lumping of disparate species into a convenient category for children’s toys? The name for these kinds of groupings is ‘paraphyletic’ meaning ‘cross-race’: these groups cross biological evolutionary boundaries. They happen when there is a conflict between the ways that biologists have structured the products of evolutionary processes, according to a family tree, and the intuitive categories we use to structure the natural world. Intuitively, it makes sense that all scaly extant egg-layers are reptiles, and all big ancient scaly things are dinosaurs. We can just about accept that ‘crocodiles are dinosaurs’, but the idea that birds are too, is kinda pushing it.
Let’s take a look at how the natural world appeared before genetics and evolutionary theory were on the cards. By far the dominant model of the world through Western history has been the Great Chain of Being: an all-encompassing heirarchy of natural things, underpinned by rocks and inanimates at the bottom, rising through the squirmy and wriggly things – ‘insects’ (including amphibians); the swimming things- ‘fish’ (including cetaceans); the legged walking things; flying things – birds (including bats); with glorious mankind sitting convenienty just below the angels and God himself. We’ll ignore the demon-infested fiery pits of hell right at the bottom for now because biology doesn’t have much to say about that in a literal sense.
Some more recalcitrant beasts seemed to bridge these categories, and there were more than a few inconsistencies, but by and large this structure was satisfyingly coherent with both naturalist’s perceptions and religious doctrine. In fact, for much of history these two have been inextricable. Importantly, the Great Chain allowed for infinite divisions between links, every possible form that could exist could be incorporate: infinite, continuous plenitude. Nature was boundless in the forms she could produce.
The structure relied heavily on Aristotelian categories for natural kinds, based on the morphological and ecological divisions that he carefully delineated and caveated. For example, that birds had two legs, two wings, could generally fly and laid eggs. Though, there were flightless species such as ostriches that were more like the quadrupeds in some ways, with hairs, heavy bodies and stunted wings. Fish had no legs and lived in the water, but some, like the seal, were also more like the mammals, with fur and warm blood.
We haven’t really moved on very far. Aristotelian categories and the heirarchical structure of the Great Chain are still powerful images. They can even be conflated with evolutionary processes: the idea that evolution is always a process of ‘progress to higher forms’ is one that is difficult to shake. Even those that ridicule Intelligent design theorists will themselves use language of creatures being ‘designed’ for purpose – perhaps it is very human to attribute intentionality to things that seem to too good to be the products of chance. Actually, natural selection can cause complexity, loss of complexity, or just staying right where you are (if it ain’t broke, don’t fix it). Ever-increasing complexity only seems aspirational from our perspective, plenty of things have over-specialised and gone extinct as a result.
Evolution also couldn’t give two figs for the neat categorisations either. If the genetic machinery exists to bring useful ancestral traits back out of the phenotypic closet, or new traits spring up through random mutation that are incongruous with the that particular creature’s close family, they will likely be used regardless. We might think platypus, with a ‘duck’s’ bill, webbed feet, ‘beaver’s’ tail, venom, fur and terrible penchent for oviparity might be some kind of taxonomic monster, but it seems to have been doing just fine in evolutionary terms for the past 160 million years or so.
The dinosaur that was characterised a month ago, Chilesaurus, the flat-toothed herbivore amongst a group of meat eaters, was likewise portrayed as a monstrous creation in the media. It seemed to have been pieced together from various groups, a mismatch of bodily traits. Just as we have a bad habit of attributing intentionality to the statistical processes of evolution in one direction, we perceive a Frankenstein-like retrograde assembly process when something doesn’t seem to fit the taxonomic story.
What we are forgetting is that these rules are models, derivations of our observations of nature, not the structures within which we should try to fit it. It is no surpise that the structures underpinning popular perceptions of the world haven’t changed much: we like to arrange things according to phenomena we can personally perceive, and we like to feel that the world is clearly structured. Most importantly, we like feel like we are the centre of the natural world, and that probably won’t change.