Evolution is constrained it has to run in certain channels Evolution always has to work with whatever is already in place

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Legacies of Evolution

  1. Evolution is constrained - it has to run in certain channels

  2. Evolution always has to work with whatever is already in place

  3. Ours arms and legs descended from the fins of primitive fish

  1. Fish didn’t evolve fins so we could sing and dance

  2. Fish evolved fins as horizontal stabilizers, to keep them on an even tool

  3. Used their powerful tails for forward thrust

  4. Fins were a preadaptation…

  5. Preadaptations are when evolution takes an existing structure and finds a new use for it

  1. Consider the function of the human lung

  2. Our lungs originated in the swim bladders of bony fishes

  3. Swim bladder is just a gas bag with a rich supply of blood vessels

  1. Swim bladder is used to exchange gases, just like a fancy life vest

  2. Can easily rise or fall in the water column

  1. Hundreds of millions of years ago, primitive lobe-finned fishes poked their heads out of the water, and hauled themselves up on land

  1. They only had to reach the next pool of water

  2. Their swim bladders acted as a rudimentary lung

  3. First amphibians that evolved from them had primitive lungs not much better than swim bladders

  1. Amphibians (frogs, salamanders) have to supplement their weak lungs by breathing through their skin

  2. Makes them really vulnerable to UV radiation, acid rain etc…

  1. So our vertebrate lungs evolved from the swim bladders of bony fishes

  2. They evolved as extensions of the digestive tract

  3. Swim bladders are a preadaptation for the vertebrate lung

  1. Very primitive invertebrates get by without any lungs at all

  2. If you are small enough, you can rely on diffusion to move gases in and out (bacteria)

  3. Same strategy works if you are very thin or flat (flatworms, kelp)

  4. Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration

  5. It is a natural (if slow) consequence of the random collisions that are always occurring as molecules move about

  1. Larger creatures needed extended surfaces to exchange gases, developed highly folded sheets of tissue (gills, lungs)

  2. Needed to suck gas into their lungs through an external tube

  3. Breathing tube evolved as an extension of the digestive tube

  1. Because of this evolutionary sequence, we draw air into our lungs through the same tube we use to send food to the stomach

  2. That poses a bit of a problem…

  3. Because of this legacy of evolution, we risk choking to death every time we eat!

  1. There’s always the chance that the food “will go down the wrong hole”

  2. If food slips into the trachea leading to the lungs, we can quickly suffocate and die

  3. Where’s Mr. Heimlich when you need him…

  1. Evolution often works in this fashion

  2. It solves one problem, and inadvertently creates another

  3. That’s what happened to plants when they had to learn to “breathe” on land

  1. When plants made the same transition to land that primitive fish would later make, they risked drying up

  2. Solution was to evolve a waxy cuticle to keep water inside the plants body

  1. But you can’t easily exchange gases through a waxy coating

  2. Plants had to evolve breathing holes, called stomata, to let them exchange gases without compromising their waterproof outer layer

  1. How did animals deal with their breathing problem?

  2. Bodies evolved a reflex mechanism that blocks the air passage when we swallow food (epiglottis)

  3. Once in a while, it doesn’t work fast enough, and we choke

  1. That still left us with another related problem

  2. We evolved from a line of animals that breathed through the same tube they used to eat

  3. We couldn’t eat and breathe at the same time

  4. Solution was a separate opening to the lungs, above the mouth - the nostrils

  5. And this in turn aggravated the earlier problem

  6. Now that we could eat and breathe at the same time, we face an even greater risk of choking!

  1. Many of our common human frailties are a simple consequence of the way evolution works

  2. Like our daily lives, evolution is an endless series of compromises

  3. What can we actually accomplish with what we’ve got on hand

  1. How many of you have had your appendix out, or know someone who did?

  2. I’ve been spared that particular agony, but I’ve watched relatives go under the knife

  1. The appendix is one more piece of evolutionary baggage that we all carry about

  2. It all has to do with diet…

  1. Our distant mammalian ancestors used to eat a whole lot of vegetables

  2. They had a special organ, the caecum, that helped process all the foliage

  3. Our diet has changed radically, but we’ve never completely lost the appendix

  1. The appendix is a vestigial organ

  2. Ironically, the agony of a burst appendix may explain why it never goes away

  3. Smaller appendix is more likely to rupture, more easily sealed off, “festers”

  4. Those with slightly larger appendix have a slightly greater chance to survive

  1. And let’s not forget scurvy…

  2. Scurvy is something we mentally associate with pirates, or Gilbert-and-Sullivan era sailors

  3. Frequently occurred on long ship voyages

  1. Scurvy is a deficiency disease, condition resulting from deprivation of some essential nutrient

  2. Every species has a short list of certain chemicals that their body needs but can’t manufacture

  3. That’s why we take vitamins…

  1. Vitamins are chemicals our bodies need, but cannot synthesize for themselves

  2. We either never knew how to make them, or we knew how but we just forgot…

  3. How could we have been that absent minded?

  4. Natural selection can be a subtle thing

  5. Selection should favor organisms whose metabolic chemistry can produce all the chemical compounds it needs to function

  6. And that is usually the case…

  1. But if everyone is already getting massive doses of that chemical in their diet, the rules are changed

  2. Natural selection no longer favors those who can make that chemical for themselves

  3. Ability to make that chemical might disappear altogether

  1. Which brings us back to scurvy

  2. Scurvy causes anemia, bleeding from the gums, bleeding under the skin

  3. Scurvy is caused by a deficiency of Vitamin C, which our bodies can’t make for themselves

  1. Sailors quickly figured out that occasional doses of fresh fruits and vegetables would keep scurvy at bay

  2. James Lind discovered that citrus fruit was especially good at preventing scurvy

  1. Vitamin C is ascorbic acid, easy to make

  2. How did we forget the recipe for this?

  3. Our primitive diets about 40 mya were rich in fresh fruits and vegetables

  4. No need to make more Vitamin C, we lost the ability to synthesize it (if we ever had it)

  1. Oranges aren’t the only source of vitamin C

  2. Fresh meat is rich in vitamin C

  3. But cooking destroys it, along with other useful vitamins

  4. So should we eat raw meat, like our primate ancestors?

  1. The quest for fire was one of the great accomplishments of the Paleolithic, the Old Stone Age

  2. The Paleolithic began ~ 2.5 mya, long before the dawn of agriculture

  3. It was the era of stone tools

  1. Domestication of fire occurred around 1.5-0.5 mya

  1. In mythology, fire was brought to man as a gift from the gods

  2. Or rather, one minor deity in particular, Prometheus

  3. Prometheus gave fire to man as a consolation prize, felt that the other animals had gotten better gifts than man

  1. And of course, for his generosity he was sentenced to be chained to a rock for all eternity while vultures fed upon his liver, which regenerated every night so he could suffer again the next day

  2. Moral of the story - don’t tick off the powers that be

  1. Fire forever altered the human diet

  2. Improved the taste and edibility of many foods, like rice, potatoes, etc…

  1. Also made some foods much safer, destroyed tapeworms, Trichinella and other nematodes etc..

  2. And it was cool…

  1. Fire also helped create nutritional deficiencies

  2. Fire destroys or alters many chemical compounds in fresh meat, including Vitamin C

  3. We switched to a diet high in meat and then started cooking it, destroyed the Vitamin C

  1. Traditional Inuit people almost never eat fresh fruit or vegetables, but they never get scurvy

  2. They eat a lot of raw polar bear and arctic fox, both high in Vitamin C

  3. Also frequently get trichinosis - pass me that char-broiled burger, please!!

  1. Typical primate diet varies - fruit, leaves, flowers, seeds, also some animal protein, insects, meat (opportunistic)

  2. Our descent from the trees marked the beginning of a gradual change in diet

  3. Dietary changes ended up having significant health effects

  1. Hominids are more carnivorous than their primate forbearers

  2. Better able to organize themselves to hunt for food, like wolves

  3. Fire was probably first used in this fashion, as it still is in many parts of the world

  1. Coming down out of the trees not only changed our diet, it dramatically reshaped our bodies as well

  2. Upright posture required substantial changes in primate anatomy

  1. Consider the anatomy of the femur (thighbone)

  2. Human femur is lighter, longer than ape femurs

  1. Walking generates uniform forces, femur can be smaller and still work fine

  2. Swinging through the trees is a heavier, more variable load, need heavier femur

  1. Human femur became a weak point, especially in the elderly, when bones are weakened with age

  2. Head of the femur frequently snaps

  1. Our anatomy is substantially different from that of the great apes

  2. Reduced tail (dang…)

  3. Torso is flattened dorso-ventrally, not laterally

  4. Lower back (lumbar region) is shorter, curved differently

  5. Longer, more flexible arms

  1. Shift to upright posture required substantial remodeling

  2. Changes in legs, pelvic structure

  3. Changes in feet, hands

  4. Changes in the lower back

  5. Many of these design changes spell bad news for modern man

  1. When we stand upright, gravity compresses the bones and delicate joints of the back in an unnatural way

  2. Spine of our distant ancestors, on all fours, was like a big “C”, propped up on the legs like a bridge on two support pilings

  1. Our spine is like an S, a second “inverted C” curve in the lower spine allows us to stand erect for long periods of time

  2. Second “C” curve develops in infants, first years of life before they start to walk

  1. Some of the many problems thought to be associated with bipedal posture:

  2. Sinus infections, dental malocclusion, impacted wisdom teeth, neck pain, back pain, hip pain, knee problems, varicose veins, broken ankles, hemorrhoids, fallen arches, bunions, corns, lumbago and sciatica

  3. Ouch...

  1. Evolution can be a real pain…

  2. Like the pain in my lower back as I wrote this lecture (and the one before that, and the one before that, and the one before that…)

  3. At any point in time 18-26% of the population of the industrialized nations is experiencing lower back pain (LBP)

  4. From 50-80% of the population will have at least one episode of LBP in their lives that will lay them low for a month or more

  5. Treatments are legion - muscle relaxants, pain pills, massage therapy, chiropractors, steroids, acupuncture, hot packs, cold packs, biofeedback, physical therapy….

  1. Our spines evolved for actively moving animals

  2. Unfortunately, we’re not so active anymore

  3. Mostly we just sit around - in class, at lunch, in front of the TV, or the Game Cube, or the computer monitor

  1. When you couple excess sitting with the bad posture that generally goes with it, you have the ingredients for chronic spinal problems

  1. When did people start sitting more and walking less?

  2. Probably around 5,000 years ago, when urban societies began to grow

  3. But the roots of our problem go back much farther than that...

  1. It started when we first began to stand erect for long periods, millions of years ago

  2. Our spinal structure evolved to fit a very different kind of animal - a quadruped

  1. And finally, let’s consider one of the cruelest tricks Mother Nature has played on the human race - our inverted eyeballs...

  1. In some respects, the human eye is truly a marvel of bioengineering

  2. Exquisitely shaped sense organ that can perceive the tiniest details, or marvel at the panorama of the night sky

  3. In other respects, it seems to have been designed by monkeys

  1. Ours is not the only such instrument in the animal world

  2. The eyeball of the cephalopod mollusks is uncannily like our own

  3. Cephalopod eye is extremely sophisticated, looks and works like the vertebrate eye, (cornea, lens, iris, and retina)

  1. Marvelous example of convergent evolution

  2. Evolution shapes unrelated species or their structures to look alike, because that’s the best way to be in that environment

  3. Evolution of unrelated species converges on a common solution to a common problem

  1. In one important sense, however, the eye of the octopus is fundamentally different from our own

  2. It is correctly put together

  1. The lens of the eye contracts to focus the image on the retina

  2. The retina is a sheet of light-sensitive cells, rods and cones - rods provide b/w vision, cones provide high resolution color

  1. Pattern of firing of rods and cones conveys a mosaic of electrical signals to the brain

  2. Brain reassembles the information into a coherent image

  3. Amazing that it works at all..

  1. The living surface of the retina requires constant nurturing

  2. Plenty of blood vessels, nerves, supporting tissue

  3. In the cephalopod eye, the supply systems are stored beneath the visual cells

  1. In our own eye, the supply system runs across the top of the retina!

  2. Blocks the light, nerves and blood vessel “cables” cast a network of shadowed areas across the retina

  1. Eye has to compensate by constantly and rapidly twitching back and forth

  2. Like moving your head back and forth behind a bush to better see what’s on the other side

  3. Who thought up this brilliant arrangement??

  1. Our eyes are inside-out…

  2. And if that weren't bad enough, this awkward arrangement causes yet another annoying little problem

  3. We have a spot in each eye where we are totally blind

  1. Generally, the constant motion of the eye trying to peer through the web of blood vessels also compensates for the blind spot

  2. But in high-speed traffic, the human eye cannot always compensate in time

  3. Many accidents are caused by vehicles in our blind spot

  1. The blind spot marks the point where all the nerve endings from the rods and cones come together into the optic nerve

  2. Optic nerve has to feed back to the brain

  3. Only way to get there is go right through the retina - what a mess…

  1. Our blind spots are about 20 degrees left or right of the midline of our vision

  2. Try focusing straight ahead, hold a pencil in front of you and move it to either side

  3. Don’t focus on the pencil, you may see the eraser disappear as the pencil enters your blind spot

  1. It is an evolutionary legacy, a design constraint that evolved hundreds of millions of years ago

  2. Ancestral chordates were small, translucent aquatic worms

  1. Light easily found the light sensitive cells that lined the tiny cups forming the first crude visual organs - eyespots

  2. From the standpoint of natural selection, the orientation of the cells was probably irrelevant

  1. One of the many problems this peculiar arrangement causes us is a detached retina

  2. The tissue supporting the rods and cones is not securely fastened down, can come loose

  3. Retina detaches from the back of the eye

  1. Can cause blindness if not treated in time

  2. Often happens in middle age, first symptom is often a sharp increase in the number of “floaters” you see

  1. Another legacy of our cultural evolution results from the endless hours we spend in repetitive motion

  2. Agricultural work, industrial work, endless hours of typing PowerPoint slides or playing Super Mario Brothers…

  3. Our bodies weren’t designed for this…

  1. Arthritis is much more common in farming communities than it was in hunter-gatherer societies, where it mostly affected elderly

  2. Our recent increase in carpal-tunnel syndrome and other repetitive stress disorders is mostly due to the invention of the typewriter

  1. We’ve come a long way, since we climbed down from the trees on the African plains

  2. We’ve carried along many curious legacies of our evolutionary past

  1. Along the way we’ve changed our diet, our posture, our very way of life

  2. Many of those changes have created new health problems, diseases of civilization

Directory: ~bfleury -> darwinmed -> darwinmedlectures
darwinmedlectures -> Although our evolutionary heritage has saddled us with some significant health problems, in the long run it has served us well…
darwinmedlectures -> The 1918 Flu 1 – a conspiracy of Silence
darwinmedlectures -> Germ Theory
darwinmedlectures -> Although our evolutionary heritage has saddled us with some significant health problems, in the long run it has served us well…
darwinmedlectures -> In our last lecture, we looked at the ways that trade, travel, technology and agriculture can provide new habitats and new dispersal routes for microbes
darwinmedlectures -> The Evolutionary Arms Race Remember the Red Queen in Alice in Wonderland?
darwinmedlectures -> The 1918 Flu 2 The Philadelphia Story
darwinmedlectures -> The 1918 Flu 1 – a conspiracy of Silence
darwinmedlectures -> So say the Laws of Manu, an ancient Brahmin text on moral conduct Man is a moral animal…
darwinmedlectures -> Pandora's Box Remember the legend of Pandora's Box?

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