Sunday, August 23, 2009

Science knows it doesn't know everything... and neither did Darwin

A couple of interesting examples of the self-correcting nature of Science today:



ResearchBlogging.org1. It turns out that good ol' Charlie Darwin was wrong about the human appendix! Bollinger and colleagues reported several years ago that this sometime exemplar of vestigial organs is not so useless after all:


The human vermiform (“worm-like”) appendix is a 5–10 cm long and 0.5–1 cm wide pouch that extends from the cecum of the large bowel. The architecture of the human appendix is unique among mammals, and few mammals other than humans have an appendix at all. The function of the human appendix has long been a matter of debate, with the structure often considered to be a vestige of evolutionary development despite evidence to the contrary based on comparative primate anatomy. The appendix is thought to have some immune function based on its association with substantial lymphatic tissue, although the specific nature of that putative function is unknown. Based (a) on a recently acquired understanding of immune-mediated biofilm formation by commensal bacteria in the mammalian gut, (b) on biofilm distribution in the large bowel, (c) the association of lymphoid tissue with the appendix, (d) the potential for biofilms to protect and support colonization by commensal bacteria, and (e) on the architecture of the human bowel, we propose that the human appendix is well suited as a “safe house” for commensal bacteria, providing support for bacterial growth and potentially facilitating re-inoculation of the colon in the event that the contents of the intestinal tract are purged following exposure to a pathogen. (emphasis added)


Now we have a broader comparative phylogenetic analysis by Smith et al (incl. two of the authors of the 2007 paper) where they looked at the distribution of similar structures across the mammalian phylogeny to report:


A recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix, and provides an impetus for reassessment of the evolution of the appendix. A comparative anatomical approach reveals three apparent morphotypes of the cecal appendix, as well as appendix-like structures in some species that lack a true cecal appendix. Cladistic analyses indicate that the appendix has evolved independently at least twice (at least once in diprotodont marsupials and at least once in Euarchontoglires), shows a highly significant (P < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer.


How exciting: the appendix has evolved and been maintained multiple times among different mammal lineages likely because if confers some selective advantage; and it continues to serve a useful function in the human body too! Tell that to the surgeon who offers to remove yours as an elective procedure while he has you opened up for some other reason (e.g. a cesarean section)!


So Darwin, who didn't know about appendix like structures in other mammals, and didn't have access to phylogenetic analyses of the sort that is routine these days, wrote that the appendix was useless, and even a cause of death (which it still is sometimes, when infected). A classic "vestigial" organ. Now we know he was wrong - as he was about some other things too; but not about his theory of evolution by natural selection. As I try to impress upon my students every year, scientists have been trying to prove Darwin wrong for 150 years, but have only ended up strengthening the evidence for his theory of evolution. So it is only natural that when someone finds something, anything, in the details that Darwin was wrong about, there is some excitement. Understandable, really, for this is how science works - we don't venerate our authority figures, but try to prove them wrong, and respect only those theories that can withstand such relentless questioning. What I don't understand, however, is PZ's reaction over on Pharyngula, where he's taken the authors of these studies to task for suggesting Darwin was wrong! Surely PZ knows better - even Darwin would have reveled in the evidence we now have, and gladly accepted his error! So I share my bafflement over PZ's rather odd interpretation of what is "vestigial" with Bjørn Østman of Pleiotropy. If an organ has a function that contributes to current fitness, its hardly "vestigial" now, is it?.



And those that berate us as adherents of "scientism" or "Darwinism", take note: we are quite happy to prove even Darwin wrong, and not afraid to stand up to the fierce atheist PZ neither! (and may PZ's octopod friends and minions never find this obscure little blog...)

2. The other example of science's self correcting ways comes in the form of a mea culpa by L. David Mech, the leading wolf expert who once suggested that wolf packs are organized in dominance hierarchies with "alpha males" leading the pack - except that most natural wolf packs aren't like that at all! Having continued studying wolves for some 4 decades since he suggested the "alpha male" notion, and increasingly concerned about the population status of wolves in the wild, Mech now admits he was wrong about their social organization, and would like to undo the damage his initial descriptions may have caused to the wolf's reputation.










His initial conclusions were biased by studies of artificial wolf groups in captivity, where such dominance hierarchies formed; in natural packs, familial relationships and lineage play a bigger role than combat for leadership (sound familiar?). Mech, therefore, would like us to stop using the "alpha male" terminology which has taken a metaphoric life of its own in popular culture.



And rather unlike its depictions in said popular culture, this is exactly how science is supposed to work: through constant self-examination and self-correction! So let me leave you with another video, from a British comedian, who puts it best: "Science Knows It Doesn't Know Everything, Otherwise It Would Stop". Let that be the take-home message this Sunday; and don't stop the questioning!





References:


Randal Bollinger, R., Barbas, A., Bush, E., Lin, S., & Parker, W. (2007). Biofilms in the large bowel suggest an apparent function of the human vermiform appendix Journal of Theoretical Biology, 249 (4), 826-831 DOI: 10.1016/j.jtbi.2007.08.032



SMITH, H., FISHER, R., EVERETT, M., THOMAS, A., RANDAL BOLLINGER, R., & PARKER, W. (2009). Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2009.01809.x




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