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  • A common misconception about natural selection, and evolution as a whole, is that it's essentially

  • a long chain of progress, adding on new features over and over again.

  • But evolution doesn't always add stuff.

  • Sometimes it takes away traits too.

  • That's because natural selection only favors traits that help an animal reproduce, or survive

  • until it can reproduce.

  • If a trait doesn't do that, it's not selected for.

  • In fact, it might start to disappear thanks to mutations piling up.

  • That's known as relaxed selection, because natural selection no longer maintains the trait.

  • If the trait takes a lot of energy for the organism to maintain, there can even be a

  • selective pressure to lose it.

  • Human evolution contains plenty of examples of thisin fact, there's a whole host

  • of things our distant ancestors could do that we just can't anymore.

  • From pheromone signaling to detecting electricity, here are some weird abilities evolution has denied us.

  • First up: the so-calledthird eye.”

  • If you take a look at some lizards or frogs, you might see something that looks like a

  • small, grayish dot on their forehead.

  • This isn't a scale.

  • It's actually an organ called a pineal or parietal eye.

  • It detects light, and scientists think it functions as a kind of daylight sensor — a

  • way to keep track of the seasons and how long days are.

  • It also synthesizes melatonin, a hormone that helps regulate biological cycles related to

  • sleep, reproduction, and body temperature.

  • Parietal eyes are likely very old.

  • The evidence can be seen in the fossil record, including the skulls of the long-dead precursors of mammals.

  • It's an opening in the skull called the parietal foramen, and in life it accommodates

  • the parietal eye and its associated nerves.

  • But a 2016 study found something interesting.

  • Looking at the skulls of mammal ancestors, the researchers observed that the foramen

  • and the eye, presumablygradually became both smaller and less common around

  • 245 to 260 million years ago.

  • The researchers suggest its disappearance is evidence of one of two things.

  • One, cells in the animals' “normaleyes might have essentially taken over the

  • pineal eye's duty of sensing daylight length and seasonal change.

  • Or two, the loss of the pineal eye might be evidence that the animals were becoming warm-blooded

  • and better able to regulate their body temperature.

  • Thus, being able to sense how long the nice warm sun was out through the top of their

  • head became less useful.

  • Either way, the organ wasn't as adaptive as it had been, and the selection pressure to keep it waned.

  • Today, we've kept part of the organ around in the form of the pineal gland in our brains,

  • which still synthesizes melatonin.

  • But we no longer need an extra eye in the middle of our forehead for it to work.

  • Another ability we lost during evolution is electroreception.

  • This is the ability of some fish and amphibians to detect weak electric fields.

  • The electric eel, for example, can use it to navigate or to detect prey in their sometimes murky river habitat.

  • You can even see the organs responsible for it in the lateral line on some sharks and fish.

  • While not super common today, it's found in a wide array of different lineages, suggesting

  • it might have also been a trait found in very early vertebrates.

  • However, our ancestors seem to have lost the system with the transition to living on land.

  • Why?

  • It might have just not really worked as well in air compared to water.

  • And, again, traits that don't confer selective advantages are apt to disappear.

  • Funnily enough, later on, some mammals, such as platypuses, echidnas, certain dolphins,

  • and maybe others did end up evolving electroreception again, but through completely different mechanisms.

  • And we're not totally sure what for.

  • Finally, let's talk about the Jacobsen's Organ, aka the vomeronasal organ.

  • This structure is found inside the nose, and it's used to sense odors and pheromones.

  • When a snake flicks its tongue in and out, it's using its Jacobsen's organ.

  • That weird face horses and cats make sometimes?

  • Same thing -- it's called the flehmen response.

  • We have a vomeronasal organ too, but it doesn't really seem to work.

  • The topic of whether humans can sense pheromones at all is kind of contentious.

  • But by the time we're adults, our Jacobsen's organ has no sensory neurons connected to

  • it, and most researchers believe it doesn't send our brains any information.

  • And we have a handful of genes that, in other animals, make the vomeronasal organ work.

  • But in us, those genes are non-functional.

  • As for when we lost it, a study from 2003 dated the inactivation of those genes to around

  • 23 million years ago, about when the great apes split from monkeys.

  • That might correspond to apes' visual systems becoming more advanced, and visual cues becoming

  • more important than scents during social and reproductive activities.

  • In fact, we might still be in the process of losing the genes associated with our Jacobsen's

  • organ -- totally at random.

  • Mutations happen all the time in our genome, altering or deactivating genes, and without

  • selective pressure to stop those mutations from building up, eventually the trait those

  • genes contribute to can disappear.

  • Natural selection isn't just a matter of continually adding on new features.

  • If something isn't helping an organism produce offspring, that feature is likely to get left

  • behind in the long run.

  • The reason we don't have third eyes or any of these other neat traits is that, in the

  • end, we ended up getting along fine without them.

  • Thanks for watching this episode of SciShow.

  • If you like learning about why we don't have weird electricity-sensing superpowers

  • and want more videos like this one, consider supporting us on Patreon!

  • You can get started at patreon.com/scishow.

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進化が奪ったアビリティ (Abilities Evolution Took From Us)

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    林宜悉 に公開 2021 年 01 月 14 日
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