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Understanding something as seemingly trivial as the evolution of hiccups can help clear up some profound misperceptions on the nature of life and humanity.
The sound of a hiccup echoes back to our very distant past as fish and amphibians some 375 million years ago, says Shubin. It’s really just a spasm that causes a sharp intake of breath followed by a quick partial closing of our upper airway with that flap of skin known as the glottis. It’s best if you can nip it in the first couple of hics, he says.
It’s much harder to stop once you’ve let yourself get up to 10. By that point you’ve reverted to an ancient breathing pattern orchestrated by the brain stem that once helped amphibians breath, letting water pass the gills without leaking into the lungs. “Tadpoles normally breathe with something like a hiccup,” Shubin says.
The theme of his book is that we owe much of our anatomy to our animal ancestors. “Parts that evolved in one setting are now jury-rigged to work in another,” he says. “When you look at the human body, you see layer after layer of history inside of us.” The first layer is what we share with chimpanzees and gorillas. The next goes back to mice and cows, while further down, you get to the relatively underappreciated layers we share with fish – which include the backbone and basic layout of the body.
Our descent from fish explains why men are so much more prone to hernias than women. In fish, Shubin explains, the testicles lie up near the heart. (Had they remained there, he said, it would give a whole new meaning to the pledge of allegiance.)
The budding gonads still form up high in a human embryo, but male mammals reproduce better with their sperm kept a bit cooler than body temperature. And so during gestation, human testicles take an incredible journey down through the body to their destination in the scrotum. The trip downward puts a loop in the cord that connects the testes to the penis, leaving a weakness in the body wall where the cord attaches that never quite repairs itself.
Hence the trouble with hernias down the road.
Biologist Sean B. Carroll of the University of Wisconsin-Madison, author of The Making of the Fittest (W.W. Norton & Co.) and other popular books on evolution, says evolutionary leftovers are born of a “use it or lose it” system.
For example, he says, we carry damaged versions of genes for dozens of smell receptors that give mice and other mammals far sharper noses. “Our repertoire of smell-receptor genes has gone to pot,” Carroll says.
Why couldn’t we keep our ancestors’ scent-tracking ability and lose, say, male nipples and wisdom teeth? The nipple issue is complicated, say biologists, by the fact that females need them to reproduce – or at least did for most of our existence. It may be hard to erase the trait in males without compromising it in females, especially since nipples form very early in a human embryo.
But at least male nipples don’t cause men any major pain, unlike wisdom teeth, which can get impacted and then infected. That’s a puzzle addressed in an exhibit called “Surviving: The Body of Evidence,” which opened last weekend at the University of Pennsylvania Museum of Archaeology
Fossils show that our ancestors had bigger jaws than we do, to fit all those teeth that they may have needed to chew uncooked meat and plants. But as our diet changed, our jaws got smaller.
Why did our jaws get smaller at the expense of our dental health? Several years ago, Penn medical researcher Hansell Stedman proposed a genetic explanation. He found that all humans share a mutation in a gene that remains in working order in our ape relatives. That mutation caused a degeneration of our jaws, rendering them much less powerful. It also allowed more room in our heads for the brain.
Using a technique known as a molecular clock, he counted the number of random changes in the gene to estimate that this new mutation took over about 2.4 million years ago, just as our ancestors were revolutionizing the use of stone tools.
We humans have the capacity to evolve away our wisdom teeth, according to geneticist Pragna Patel of the University of Southern California. As many as 25 percent of us are lucky enough to be missing these teeth, also known as third molars. A very few have mutations in a gene called Pax-9 which leads to other missing teeth.
No good story about human design flaws can pass up a discussion of flatulence – and science has addressed the kind that would occur if everyone in the world drank a tall glass of milk at the same time.
Patel said one of her favorite examples of evolution in progress involves the gene that determines who can digest the sugars in milk and who cannot. From genetic studies it appears that so-called
lactose intolerance was our ancestral state.
A few people, however, were genetically gifted with an enzyme called lactase, which breaks down lactose, and in groups that started drinking lots of milk around 10,000 years ago, that version of the gene started to take over.
Scientists recently sequenced the lactase gene and found 43 different variations that allow adults to drink the milk of other animals. “It’s the first clear evidence of convergent evolution,” Patel said, though it’s not known whether those lacking this innovation failed to pass on their genes because they suffered from lack of nutrition or just didn’t get invited to any parties.
As for design, intelligent or otherwise, Shubin says the body only makes sense if viewed as a product of evolution. If it was designed, the designer could have done away with some of our relics of the past.
“This designer, if there was one, liked history, and he really liked fish.”