Damn. Mistaking evolution for a ladder with us on top is something I fully expect from people who don’t study it for a living, but when evolutionary scientists make that mistake, it drives me apeshit. And they do it all the fucking time.
I don’t think most of them are aware of it. You’ve got to be really watching for the trap to have a chance of avoiding it. I slip every now and then, and then I spot it and rage at myself and get deeply philosophical about human nature and such. It’s such an easy and convenient thing to do. (Think of evolution as a ladder, not get philosophical, I mean.) It’s the way we’ve been conditioned to think since the first time we heard about evolution.
For most of the history of biology, no one blinked twice if you talked with culturally sanctioned anthropocentrism about “lower animals” or “higher vertebrates”. Evolution was a highway of progress, and some creatures just got further along than others. Naturally, we were speeding along right at the front.
Nowadays, I think most biologists who have to consider evolution in their work would tell you that evolution doesn’t work like that. The papers I read rarely contain such explicit references to the “march of progress”. (Can I call it the MOP?) However, that doesn’t mean the references are gone. They’ve just become so subtle that, I suspect, not even the people who make them realise they’re there.
It’s “basal lineages”. “Phylogenetically more primitive” creatures. Or “early-branching organisms”. Or “evolutionary old animals”. All of these are real terms used in real papers published this year. They aren’t restricted to bad papers. And if you stop to think about it, none of them make any goddamned sense.
Let’s picture an evolutionary tree first. I can’t really use my usual tree with all its question marks, but the one below, which I nicked from Srivastava et al. (2008), will do:
(The species from top to bottom are: brewer’s yeast, a choanoflagellate, this tentacled little guy, a sea anemone, humans, a limpet, everyone’s favourite fruit fly, the Blob, and a sponge.)
The “base” of the tree is to the left, where animals, Monosiga and fungi have their last common ancestor. (That was a long time ago.) “Basal” means close to the base. The branching point (node) that separates animals from the non-animals at the top is the basalmost node in this tree. The node that separates the sponge from the other animals is also a pretty basal node. The creature that gave rise to both sponges and other animals was a truly basal animal.
Now, which is the basal lineage?
The correct answer is “relative to what?”
Every node divides the tree into two lineages. It doesn’t make any sense to say that one of them is more basal than the other. There’s a basal node in the tree of animals. Sponges are on one side of that, the rest of the animals are on the other. If you take a vertebrate species, sponges are the last animal lineage you’ll encounter if you trace its ancestry back towards the base of the tree. If you take a sponge species, the lineage with vertebrates (and lots of other things) on it will be the last.
“Basal lineage” depends on your point of view.
Maybe actually taking the sponge point of view will help illustrate this. This tree comes from a paper about sponges (Sperling et al., 2010):
Unlike the previous tree, its branches are labelled with larger groups rather than species, but these represent more or less the same range of creatures. Monosiga from tree one is a choanoflagellate. Amphimedon is a haplosclerid demosponge, on the second branch from the bottom. Every other animal from the first tree is compressed down into that one branch labelled “Eumetazoans”. (OK, Trichoplax is not a eumetazoan, but that’s a technicality that doesn’t affect the point.) From this angle, it’s rather harder to see sponges as a basal animal lineage!
Equally, sponges are just as old as non-sponge animals, so calling them “old” is a tad dodgy. Here, you could argue that sponges have been around longer than, say, vertebrates, which is true to the best of our knowledge. In that sense, “sponges” is an older lineage than “vertebrates”. But that only means that “sponges” should be compared to “non-sponges” rather than “vertebrates”, and anyone making such comparisons should be as aware of the diversity lurking within sponges as they are of the diversity of other animals.
The “evolutionary old animals” quote actually comes from a paper that looked at stem cell genes in Hydra to understand the evolution of stem cells in animals. (Hemmrich et al., 2012). It’s not comparing cnidarians (the phylum hydras belong to) to something genuinely younger than them. I can’t resist quoting the whole offending sentenc:
Our observations provided new and comprehensive insight into the complex network that orchestrates patterning and tissue homeostasis in an evolutionary old animal that branched off almost 600 million years ago. (p3277)
Honestly, what does that even mean? Branched off from what?
OK, I know it means from our own ancestors. But my point is that this should not be taken for granted, and if you do take a human-centric point of view, you should bloody well make that explicit. You should not write as though evolution had some sort of “main branch” leading to us from which things split every now and then. Lineages split from each other.
You might think that I’m being pedantic just to have an excuse to rant, but the implicit views underlying examples like the above have real consequences for the study of evolution. Namely, they might lead scientists to assume that representatives of “basal” lineages got stuck in the Precambrian and could just stand in for their distant ancestors. This is dangerous.
Take sponges. Yes, in many respects they probably resemble the first animals more than we do. Chances are those ancient animals didn’t have sophisticated organs and like two hundred different cell types. However, chances also are that they were made of distinct cells rather than huge merged syncytia, and that they didn’t have elaborate skeletons made of some sort of mineral, both of which are properties of many sponges. All animals alive today had exactly the same amount of time to evolve their own quirks since their last common ancestor. We shouldn’t just assume that anything “simple” in an animal we regard as “basal” is inherited straight from that ancestor just because it fits our favourite story.
Case in point: the Amphimedon genome was found to be impoverished in many families of developmentally important “master” genes, and this fit nicely into the prevailing view of the increasing complexity of animals throughout their history (Larroux et al., 2008). But it’s likely that at least some of those genes were actually lost by Amphimedon‘s ancestors and not gained by ours (Mendivil Ramos et al., 2012). Assuming that “basal” (relative to us) means “similar to ancestor X” can very easily lead to unwarranted conclusions, and that can hinder our ability to figure out what really happened. To me, that’s a big deal.
Hemmrich G et al. (2012) Molecular signatures of the three stem cell lineages in Hydra and the emergence of stem cell function at the base of multicellularity. Molecular Biology and Evolution 29:3267-3280
Larroux C et al. (2008) Genesis and expansion of metazoan transcription factor gene classes. Molecular Biology and Evolution 25:980-996
Mendivil Ramos O et al. (2012) Ghost loci imply Hox and ParaHox existence in the last common ancestor of animals. Current Biology 22:1951-1956
Sperling EA et al. (2010) Where’s the glass? Biomarkers, molecular clocks, and microRNAs suggest a 200-Myr missing Precambrian fossil record of siliceous sponge spicules. Geobiology 8:24-36
Srivastava M et al. (2008) The Trichoplax genome and the nature of placozoans. Nature 454:955-960