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  • Guys, have you ever looked at your hands, and you're like,

  • why do I have these?

  • Hey, peeps, this is D News.

  • Thanks for watching.

  • Sometimes when you're sitting around

  • in your friend's basement and you're all zoned out,

  • and you look down at your heavy limbs and you start pondering,

  • why do we have four of these, man?

  • Bats got four, and monkeys and lizards and frogs and birds

  • and drag-- you get the idea.

  • I'm not saying that's what happened.

  • But a team of scientists from the University of Vienna

  • wanted to know exactly why limbs always come in pairs,

  • and published their model in the journal

  • Evolution and Development.

  • By the way, it has to deal with our tummy.

  • I like to point out that this is the hypothesis.

  • The researchers have created a model.

  • And now that it's published, other researchers

  • can begin to study it.

  • They will pick it apart for accuracy

  • and tweak it over time.

  • This is science in action.

  • Of all the known species on the planet,

  • both living and extinct, those with both a backbone and a jaw

  • all have paired appendages .

  • Using information from molecular embryology, paleontology,

  • and classical morphology, the researchers

  • examined all these jawed vertebrates

  • known as gnathostomes in as basic a state as we can,

  • by looking at their gene expression

  • and embryonic development.

  • Their model explains the reason we

  • have paired appendages is because of the three

  • types of basic tissues in a developing embryo.

  • The ectoderm becomes the skin and nervous system,

  • the mesoderm, which forms muscles, bones, and organs,

  • and the endoderm, which forms the digestive tract.

  • Usually the mesoderm splits into two layers

  • to line the body cavity and the gut.

  • And if those layers are sufficiently separated,

  • they'll form into limbs right below the gut.

  • Why?

  • Because the developing gut keeps the layers nearby

  • too close together to form those appendages.

  • As the body forms, the end of the digestive tract

  • fits between the legs, and everything else

  • just kind of closes up down there.

  • Above the gut, the mesoderm and ectoderm are messing around.

  • And they form another set of limbs

  • with the separated tissue.

  • Two sets of opposing appendage pairs, all

  • because that digestive tissue sits in the middle.

  • Thanks, belly.

  • If this works out, we'll have a great theory

  • on why all the gnathostomes have two pairs of limbs.

  • We'll have to wait and see if the science pans out.

  • Would you rather have more arms?

  • Maybe another set, like down here or something?

  • Where?

  • How?

  • Grab your keyboards and head down to the comments

  • to share your [INAUDIBLE] dreams.

  • Thanks a lot for watching D News, everybody.

Guys, have you ever looked at your hands, and you're like,

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B1 gut digestive paired tract model tissue

Why Do Limbs Come In Pairs?

  • 158 21
    Paul Huang posted on 2014/02/06
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