A new paper has just come out on the functionality of the human appendix, or cecum (caecum in British biology). The authors, following some work done on appendix function in 2006, have said that the caecum in humans has a role in repopulating gut flora. I was going to do an extensive discussion, not so much of the scientific results as of the way in which the paper is framed (Darwin was wrong about the utility of the appendix), but I see PZ Somebody has already done that (here and here). I would, however, like to make a couple of points.
Darwin’s argument in the Descent of Man is this:
With respect to the alimentary canal I have met with an account of only a single rudiment, namely the vermiform appendage of the cæcum. The cæcum is a branch or diverticulum of the intestine, ending in a cul-de-sac, and it is extremely long in many of the lower vegetable-feeding mammals. In the marsupial kaola [sic] it is actually more than thrice as long as the whole body. It is sometimes produced into a long gradually-tapering point, and is sometimes constricted in parts. It appears as if, in consequence of changed diet or habits, the cæcum had become much shortened in various animals, the vermiform appendage being left as a rudiment of the shortened part. That this appendage is a rudiment, we may infer from its small size, and from the evidence which Prof. Canestrini has collected of its variability in man. It is occasionally quite absent, or again is largely developed. The passage is sometimes completely closed for half or two-thirds of its length, with the terminal part consisting of a flattened solid expansion. In the orang this appendage is long and convoluted: in man it arises from the end of the short cæcum, and is commonly from four to five inches in length, being only about the third of an inch in diameter. Not only is it useless, but it is sometimes the cause of death, of which fact I have lately heard two instances: this is due to small hard bodies, such as seeds, entering the passage and causing inflammation. [References omitted]
Now Darwin holds that the cecum is a rudiment of a past organ in other animals. That it causes death occasionally is true. That we can live without it (like the spleen, the tonsils and other organs) is also true. That it varies among humans from large to absent is also true. That it is useless is false. This was first claimed, as the authors observe, over 100 years ago. They and others have shown that it has a utility. Does this undercut Darwin’s argument here?
I think not, and the reason why has to do with the very methodology the authors have employed – phylogenetic reasoning.
Darwin is arguing that the task the cecum has in animals that eat a lot of plant food (“vegetable-feeding mammals”) is no longer a task in humans, and thus the cecum is reduced and optional in humans. He argues this on the grounds of homology: it is the same organ under all modifications. Since it is the same organ, but has a different task, or no task at all, this is evidence of humans sharing common ancestry with mammals that are vegetarians, and that the common ancestor was either a herbivore or had a herbivorous ancestor itself.
Hence, it is a “rudiment”, a vestige. However, as an excellent page at the talkorigins.org website shows, vestigial organs can have present tasks so long as they are vestigial for the task they evolved to deal with. That page needs now to be updated, but the point remains.
The authors have used comparative anatomy to work out the cladogram of the cecum:
The left side represents one analysis, while the right side another (I can’t link to the original paper or figure since it is behind a paywall). The tree is the same on each side, being independently derived. It is obvious that the cecum is not present in many mammal species or groups (the gray lines). However, parsimony suggests, and the authors argue, that the cecum or its precursor evolved before monotremes split off from marsupials and placental mammals. This means that the arrangements of functional cecums (ceca?) are evidence for the history of the relations between groups – all primates have a cecum for example. Also, it is pretty obvious that there has to be some tissue at that point in the gut for all mammals, so it’s unsurprising that it might have evolved into a cecum several times. In such a case it would still be homologous, but not as a cecum, rather as a particular region of the intestine.
It is therefore evidence that humans are primates. Darwin was right. It remains a rudiment of the past, and a vestige, and having a present task is not a contradiction to Darwin’s argument. He was not wrong, just underinformed.
The paper itself is fairly temperate in language; the real harm is done by the use of the old journalist’s trope “authority X has been overturned” in science reporting. Of course, it had the effect, didn’t it? I’m reporting on it because that trope was used. Oh well.
Thanks to Bill Parker, the lead author (scientists use an odd order in papers for their priority of authors’ names – the first author is usually the project leader, and the last author is usually the lab head, and the intermediates are the lab workers and collaborators. I don’t know why) for sending me the paper and having a discussion. I hope that I didn’t make too many misintepretations of his actual work.
Smith, H. F., R. E. Fisher, M. L. Everett, A. D. Thomas, R. Randal Bollinger, and W. Parker. 2009. Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. Journal of Evolutionary Biology (forthcoming).