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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 this — in 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-called “third 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, presumably — gradually 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' “normal” eyes 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.

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