The university of life

A scientific correspondence on the belly button

This is a response to the bellybutton lecture.

Dear Spurious Monkey, I was disappointed by the totally selectionist account of the organogenesis of the belly button. A more proper evo-devo study would include proximate causation. Such a study can be found at http://zygote.swarthmore.edu/April1.potato.html . The abstract reads: Anyone who has ever seen human beings on a beach knows that our species comes in two distinct morphotypes--innies and outies. With reference to umbilical anatomy, this appears to be a random event, although it is said that innies run in families. Panganiban and colleagues (1997) demonstrated that a Distal-less protein (Dll in Drosophila, Dlx1-3 in mammals) is expressed in the distal tip of those regions destined to stick out from the main body axis. To determine if Dlx expression provided a genetic basis for this morphological variation, Polly and Spiro Keats probed human embryos with antibodies against the Dlx proteins. They found Dlx3 expression in the umbilical rudiment. They showed that as early as the ninth week of gestation, Dlx3 was found in the tips of those umbilical rudiments destined to become outies. Such expression was not found embryos in those whose umbilical anatomies proved to be innies.

Scott Gilbert


Professor of Biology

Dear Scott,

Here at the University of Life we are well aware of this study. And although we do not question the data, we do question the conclusion reached in this particular study. We would interpret the presence of Dlx3 expression in the umbilical rudiment as the primitive state of the developmental program of the belly button. It is a remnant of the ancestral function of the belly button, to provide nutrition and oxygen to the fetus via the umbilical cord. To produce a functional belly button (one who gathers navel fluff) evolution had to alter the developmental program of the umbilical rudiment. One of these changes is the downregulation of Dlx3. I hope that this resolves the controversy surrounding this fascinating topic.

It is a mystery why there are still outies, since these belly buttons clearly are not suited for gathering navel fluff. We propose that the evolution of the belly button and the enlargement of the brain coincided to some degree. Although it was a clear evolutionary advantage to have a fluff gathering navel at earlier times, the evolution of a large brain removed some of the selective pressure for this trait. The large human brain made it possible for humans to figure out that large concentrations of unwanted fluff all over the abdomen and thorax is an undesirable state. Creative solutions were put forward by the human brains to deal with this problem, for instance the habit of washing and the invention of primitive soaps. Unfortunately there is no physical evidence for this theory. Plastic soap containers were not invented until recently and hence no physical evidence has ever been found near human fossils.


Professor at the UOL

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