The problem with describing organisms as “higher” or “lower”

In my most recent publication, I was not happy with the use of the terms “higher” and “lower” when referring to various organisms but, despite my objections during draft editing, they were retained. However, I want to take this opportunity to state that these terms are the remnants of outdated beliefs, can lead to a poor understanding of biology and do not belong in scientific text. This issue has been addressed in blogs, online fora and the scientific literature (see references at the end).

Ramon Lull’s Ladder of Ascent and Descent of the Mind, first printed in 1305

Terms like “higher” and “lower” stem from the idea of a great chain of being (or scala naturae) originating all the way back with Aristotle. This was later integrated with European religious beliefs took a form that went from god through angels, humans, animals and plants to minerals. Obviously, this a pre-scientific view that has nothing to do with reality and is not borne out by modern biology. The idea of any organism being “higher” or “lower” than another should have disappeared after Darwin published his theory of evolution. In fact, Darwin himself wrote, “It is absurd to talk of one animal being higher than another.”

Evolutionary biology does not support the concept of higher and lower. We should recognise that all living organisms are just as evolved as any other but through different selection pressures. If we look at the phylogenetic tree below, it shows different animal groups with a picture of one of the members and how they are related to one another. We are part of the chordates. Now while some might look at the groups and say those that are higher in the image are “more evolved” this would be wrong. The only information in this image is how the branches of the tree are connected, any node can be rotated without changing the structure. A simple example, rotating at the point labelled 3 will put Echinodermata at the top but not change the structure of the tree.

A simplified phylogenetic tree of the kingdom Animalia. (from Omond et al. 2017)

While it is true, that there are real differences between those groups, it would still be wrong to say interpret “higher” or “lower” in terms of some sort of ranking of complexity. We are bigger and smarter than arthropods (insects, arachnids and others) but this will not be the case for all measures, such as genome size. Rather than using nebulous and poorly defined terms like “higher” and “lower,” we should be specific about what we mean, i.e. by saying chordates instead of higher animals.

Although “higher” and “lower” have no real meaning, it may sometimes be obvious what people intend. Even in such a case, the terms should be avoided as they imply some sort of ranking or superiority among organisms which just does not exist. Evolution is not about progress towards to a single, ultimate goal. This confusion will only serve to misguide and cloud the way people think about biology. In short, unless the situation is similar to a dog on the grass and a cat in the branches of a tree, we should avoid using “higher” and “lower” in reference to biological groups.

References

Katz PS. 2019. The conservative bias of life scientists. Current Biology 29:R666–R667. doi:10.1016/j.cub.2019.05.066

Moore A. 2013. What to do about “higher” and “lower” organisms? Some suggestions. BioEssays 35:759–759. doi:10.1002/bies.201370093

Rigato E, Minelli A. 2013. The great chain of being is still here. Evo Edu Outreach 6:18. doi:10.1186/1936-6434-6-18

Zachos FE. 2016. Tree thinking and species delimitation: Guidelines for taxonomy and phylogenetic terminology. Mammalian Biology 81:185–188. doi:10.1016/j.mambio.2015.10.002

2 thoughts on “The problem with describing organisms as “higher” or “lower”

  1. I guess “higher” and “lower” aren’t the best terms, but I still see a hierarchy of complexity, starting already with non living substances. It seems to me “life” is basically complex matter, and some are more complex than others. Any thoughts?

  2. The first issue that would come up with that is defining what you mean by complexity. There’s a bit of discussion at the “such as genome size” link. Is it just number of different cell types or organs? I would think most mammals are quite similar there. Or could it be complexity of life stages? Humans just get bigger as they grow up, frogs and butterflies completely change their body structure. Perhaps they are more complex?

    We could probably come to an agreed definition of complexity but would that be meaningful? Evolution is not a process converging to any sort of goal and certainly not a process that increases complexity. Evolution only tries to increase survival and there are ways of living, such as parasitism, which result in a reduction of complexity (however it is defined). As it’s unguided, complexity will not be concentrated in one specific place. For example, maybe we want to say intelligence is complexity, so we might say humans, dolphins and octopuses are “higher” animals. That’s cool but they are three species on separate lineages with all sorts of other things between them. So it is not a particularly useful or informative statement because its not a natural grouping.

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