Shortly after the return of the HMS Beagle to Cornwall, Charles Darwin jotted down the following note:

It is absurd to talk of one animal being higher than another.

In the subsequent twenty years, Darwin either forgot or reconsidered his note to himself.* He shouldn’t have.

The Origin of Species is full of references to higher and lower animals (and sometimes plants), for example (p. 440):

The embryo in the course of development generally rises in organisation: I use this expression, though I am aware that it is hardly possible to define clearly what is meant by the organisation being higher or lower. But no one probably will dispute that the butterfly is higher than the caterpillar.

I dispute it. There is no scala naturae, no universal standard by which organisms are ranked. Biologists and philosophers of biology have generally rejected the idea these last 160 years, but just like Darwin, we keep coming back to it.

Although exceptions certainly exist, the scientific literature has mostly pushed out judgements of which organisms are ‘higher’ or ‘lower’, ‘advanced’ or ‘primitive’, or more or less evolved. But though we change the terminology, we never quite seem to internalize the deeper message that these comparisons are meaningless. We seem to have a deep need to make such judgements, and so we replace the old terms with new ones without much changing their meaning.

The current versions are ‘basal’ and ‘derived’, or occasionally ‘early’ and (I suppose) ‘late branching’. These terms have useful meanings in describing evolutionary relationships, but they are never useful in describing living, or extant, taxa.

Extant taxa can not be basal.

Yet they are often described that way, even in the peer-reviewed, professional literature. The following examples were not chosen because they are particularly egregious, nor because I mean to pick on anyone in particular. They are just the ones I’ve happened across recently (and I hope the authors will remember that if they’re ever on a hiring committee). But they are wrong, all of them.

From Current Biology:

Genomic sequences reveal that the basal lineages Porifera, Placozoa, Ctenophora, and Cnidaria share a large fraction of the genetic toolkits involved in cell fate, patterning, differentiation, and cell-cell communication in bilaterians.

From Nature Protocols:

Among the basal animals, the starlet sea anemone Nematostella vectensis (phylum Cnidaria) has emerged as a leading laboratory model organism…

From Current Opinion in Genetics & Development:

The homeobox and kinase gene families have been further analysed in basal animals…

Not unless they’re sequencing Precambrian DNA. From Campbell Biology, undoubtedly the most common textbook for freshman biology classes:

Sponges are basal animals that lack true tissues.

It even shows up in the Wikipedia page for Volvox:

The cells of colonies in the more basal Euvolvox clade are interconnected by thin strands of cytoplasm, called protoplasmates.

So there’s part one of my argument: even the very current scientific literature contains numerous instances of extant taxa being called basal. This can’t be dismissed as a non-issue on the grounds that ‘nobody does that anyway’. These are just a few of many examples, as a quick Google Scholar search will confirm.

The part I haven’t covered yet is why it’s wrong. Basal, in the context of phylogenetics (the study of evolutionary relationships) means closer to the base of the tree. Closer to the base of the tree means further back in time. Living taxa are, by definition, maximally distant from the base of the tree; they can’t be basal to anything (except future taxa). Several published papers have explained this in greater detail than I plan to do, notably Krell & Cranston, Crisp & Cook, and the excellent “Tree thinking” paper by Omland, Cook, & Crisp (there’s also a nice blog post on the topic at For the Love of Trees).

One of the figures from that last paper is a nice visual representation of a problem with the way we often read phylogenetic trees. On a comb-like phylogeny, we tend to call the first species to diverge from the others basal. For example, in Figure 4A, we might call the marsupial Virginia Opossum basal to the placental mammals:

This is very often how people, even biologists, talk about mammals: extant monotremes and/or marsupials are basal to placental mammals. In some cases, this is accompanied by judgements that they are more primitive or even (worst of all) ancestral to the placentals. It’s true that the basal divergence in Figure 4A is between the opossum and everything else, but that is purely an artifact of taxonomic sampling, as Figure 4B dramatically shows:

Both 4A and 4B accurately reflect evolutionary relationships. But if we consider the basal-most species that which first diverges from the others, 4A shows the opossum as basal to the placentals, while 4B shows humans as basal to the marsupials!

Neither is true. Extant taxa cannot be basal.

Humans (in 4B) diverge basally from the other species, but humans are no closer to the base of the tree than, say, mouse opossums. Humans are not more primitive than mouse opossums, and I’m nearly certain that humans are not ancestral to mouse opossums. Now let’s apply that same thinking to 4A:

Humans Virginia opossums (in 4B 4A) diverge basally from the other species, but humans Virginia opossums are no closer to the base of the tree than, say, mouse opossums ring-tailed lemurs. Humans Virginia opossums are not more primitive than mouse opossums ring-tailed lemurs, and I’m nearly certain that humans Virginia opossums are not ancestral to mouse opossums ring-tailed lemurs.

It’s understandable that biologists keep coming back to calling extant species basal, even though we’ve been told time and again that this is wrong. After all, we need some way to verbally describe the evolutionary relationships that a phylogenetic tree represents. The right way to do this is using cladistic terminology, that is, describing the tree topology in terms of sister group relationships. Sister groups (sister species, sister clades, etc.) are each other’s closest relatives, at least among the taxa represented in the tree. Each node, or branching point, in such a tree leads to two sister groups. For example, in 4B, the most basal divergence is between humans and marsupials. Humans and marsupials are sister groups (again, among the sampled species). Virginia opossums are sister to southern opossums, mouse opossums are sister to southern opossums plus Virginia opossums, and red kangaroos are sister to all the opossums.

It may seem a bit unwieldy at first, but it’s not hard to get used to. The last two sentences of the previous paragraph fully describe the tree in 4B; it contains no other information. Thinking of living species as ‘basal’, ‘primitive’, or even ‘early branching’ is not just wrong, it reinforces habits of thinking that biologists should have left behind before starting graduate school. Modern sponges are not basal animals, sea anemones are not primitive animals, and duck-billed platypuses are not ancestral to placental mammals. Cladistic terminology not only avoids these pitfalls, it also has the advantage of being right, where ‘right’ means accurately reflecting evolutionary relationships. That, after all, is what we care about.

 

*Or maybe he couldn’t read his own handwriting. I mean, damn, it’s worse than mine!

 

Stable links:

Brooke NM, Holland PWH. 2003. The evolution of multicellularity and early animal genomes. Curr. Opin. Genet. Dev. 13:599–603. doi: 10.1016/j.gde.2003.09.002