Human Evolution: Why are we like we are?

Posted: April 22, 2008 in Atheism, Evolution
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Often I get into discussions with people about how the Human body relates to the Theory of Evolution and how we ourselves can be pretty good evidence that Evolution happened.  I sometimes get told that the human body has almost never changed which, of course, is rather silly.   So just to throw it out there, I found this article which might serve well as a very brief overview of just how the human body has changed over many centuries.

We’re not finished yet

Mankind is no perfect work. As a product of evolution, the design is haphazard, part fish and part monkey.

‘Oh what a piece of work is man,” wrote Shakespeare, long before Darwin suggested just how little work went into us. Somehow, that same process that gave us reason, language and art also left us with hernias, male nipples, impacted wisdom teeth, flatulence and hiccups.One argument scientists often make against so-called intelligent design – the idea that evolution cannot by itself explain life – is that on closer inspection, we look like we’ve been put together by someone who didn’t read the manual, or at least did a somewhat sloppy job of things.

Viewed as products of evolution, however, our anatomical quirks start to make sense, says University of Chicago fossil hunter and anatomy professor Neil Shubin, author of the recent book Your Inner Fish (Pantheon Books). And by focusing on our less lofty traits, evolutionary biology can help dispel one of the most egregious and even tragic fallacies surrounding Darwinian evolution – that it moves toward perfection, with man at the apex of some towering ladder.

That misreading of evolution has been connected to the eugenics movement of the early 20th century, with the Nazis extending the man-as-ideal notion to blue-eyed blond German-man-as-ideal notion.

“Darwin didn’t believe it, but some, who saw it through a more religious light, tended to want to interpret evolution as a steady march toward the pinnacle of humanity,” says Penn ethicist Art Caplan, who has written extensively on the eugenics movement.

By today’s understanding, evolution by natural selection doesn’t march toward anything – it just modifies existing creatures to better compete in ever-shifting environments.

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 and Anthropology.

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.”

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Comments
  1. Carlos says:

    If we are to believe in Darwin’s theory of natural selection, then we need to just let people, plants and animals die off when they are sick. Why are we trying to preserve life at all? If the best of all creatures survive, then the world will be better off without all these lesser beings dragging us down. Don’t let the weak ones reproduce, it’s just making matters worse.

  2. Matt says:

    What you speak of has nothing to do with the Theory of Evolution at all. Instead you refer to a rather bad thing called Eugenics, which really is a horrible practice. Social Darwinism (which actually has nothing to do with Charles Darwin or Evolution despite the name) is also related to this.
    Now would you care to take a guess who was one of the earliest proponents and probably historically the biggest driving force behind it? Thomas Malthus, who lived before Darwin, who was a christian minister.

  3. arthurvandelay says:

    If we are to believe in Darwin’s theory of natural selection, then we need to just let people, plants and animals die off when they are sick. Why are we trying to preserve life at all? If the best of all creatures survive, then the world will be better off without all these lesser beings dragging us down. Don’t let the weak ones reproduce, it’s just making matters worse.

    Is-ought fallacy.

  4. S. Keita says:

    I am trying to find out how the 750 trillion connections in the brain evolved. I.e., how many connections per year.

    Unfortunately, this important question is not dealt with by anyone.

    Could you please help?

  5. AV says:

    I am trying to find out how the 750 trillion connections in the brain evolved. I.e., how many connections per year.

    Unfortunately, this important question is not dealt with by anyone.

    Then you’re not looking hard enough. Try Googling “brain evolution.”

    One of the first results that my search returned:

    Without Miracles: Brain Evolution and Development

  6. AV says:

    See also this entry in the Talk.Origins Index of Creationist Claims.

    My own question: why should the argument from incredulity fallacy be considered a valid heuristic in science?

  7. S Keita says:

    AV. I’d already googled it. Please tell me in which paragraph there is a mention of 750 trillion.

    Wikipedia’s timeline of brain evolution is merely a series of notes, some in Portuguese!

  8. S Keita says:

    AV- your 2d entry contains no reference to 750 trillion connections and is laughably brief.

    Please tell me the answer yourself. All you have to do is divide 750 000 000 000 000 by 600 000 000 .
    I’m too stupid to do it myself.

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