After a century of denigration, the appendix has been redeemed.
Two years after proposing that the organ serves as a "safe-house" for beneficial gut bacteria, Duke researchers and collaborators from Midwestern University and Arizona State University have discovered that the appendix is not an evolutionary remnant, as Charles Darwin once concluded. The findings will appear in the next issue of the Journal of Evolutionary Biology.
"We know Darwin was right about a lot of things-the whole field [of evolutionary biology] was built on his work-but we found that his idea of the appendix is wrong," said William Parker, assistant professor in the department of surgery. "The widely accepted idea was that the appendix was a vestigial organ that was not as important as it had been."
Darwin concluded that the appendix likely served a purpose in leaf-eating primate ancestors, but became reduced in humans due to a change in diet, said Heather Smith, assistant professor in the department of anatomy at Midwestern University.
But the appendix has evolved independently at least twice, according to the study. Furthermore, the organ has been maintained in mammalian evolution for at least 80 million years.
When the intestine becomes infected, the appendix houses good bacteria that quickly reestablish normal gut flora, Parker said. This defense response is particularly critical in the absence of modern health care.
"If you're a teenager in the U.S., you probably don't need it," Parker said. "But if you don't have access to clean water and other resources, the appendix is a different matter."
The research team used a modern approach called cladistics, compiling data on the distribution of traits across a wide range of species. A complex computer algorithm then calculated the most likely explanation given several important evolutionary principles, Smith said.
"The [computer] program gives you an output indicating the more likely evolutionary pattern, including where on your phylogenetic tree the trait first evolved and how many times it has changed and appeared or disappeared," she said.
Darwin lacked these modern tools 150 years ago, Smith noted, adding that Darwin's conclusion that the appendix served no function was a "very valid hypothesis given the data available to him."
"Darwin didn't have the advantage of having histological data on the appendix, so there is no way he could have known about the biofilm that indicates a layer of beneficial bacteria housed in the appendix," she said.
Furthermore, Smith advised caution in comparing the results of the study with Darwin's ideas.
"In my opinion, and that of most evolutionary biologists, it's not really fair of us to say 'Darwin is wrong,'" she said. "He made incredibly keen observations about the natural world, and revolutionized the way that biology was considered."
Smith added that scientists typically "recoil" from comparisons of recent research with work done 150 years ago. She advised scientists to evaluate their results in light of newer findings on the topic.
"It's part of the scientific process that future research always reassesses previous claims once new evidence is available," she said. "It's better to appreciate what we learned from the previous studies-they did the best they could, basing their conclusions on the data that was available to them at the time-than to point out their shortcomings."
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