[♪ INTRO].

There's this idea in evolutionary theory, often referred to as Dollo's Law, that once a complex trait disappears, it can't come back. Which makes sense.

Everything about a living creature can be traced back to the activity, or expression of its genes. But once a gene stops being actively used, tiny mutations start to build up. So, over time, it deteriorates.

And soon enough, it is too far gone to function the same way again. I mean, sure, a species could evolve something similar to what it once had. But it cannot go back.

Take all the whales and dolphins in the mammalian order Cetacea, for instance. Like all other mammals, they evolved from a land-dwelling animal that itself descended from a water-dwelling fish. And nowadays, cetaceans do have a few fish-like traits, like tails that propel them through the water.

But they didn't get those by reverting back to the tails of their ancient fish ancestors. New tweaks and adjustments to their genes turned the base of their spine into a totally new kind of tail, which is why those tails are turned 90 degrees relative to the ones that fish have. And if, for some reason, they were to be pushed back towards living on land, they wouldn't be able to just dig around in their genomes to find the genes for legs and switch ‘em on.

Except actually, that does happen. Every once in a while, a dolphin or a whale is born with surprisingly complete hind limbs! Such re-appearances of traits thought to be lost forever are known as atavisms.

These apparent defiances of Dollo's law can teach us all sorts of things, like how limbs actually grow. But perhaps most importantly, they're helping us understand how evolution happens by revealing what genetic information is truly lost, and what is simply tucked away for a rainy millennium. The ancestor of all cetaceans, a four-legged, hippopotamus-like creature, entered the water around 50 million years ago.

And for about 34 million of those years, its descendants have lacked hind limbs. Which is a really long time; long enough that you would think the random accumulation of genetic changes would prevent those limbs from ever coming back. But every once in a while, people find a dolphin or a whale with rear appendages.

And that's probably because the genes for hind limbs never went away. See, for about 15 million years after that hippo-ish creature took to the water, its descendants held onto their hind limbs. But, these legs got smaller.

They kept the same structure, complete with toes, they just shrunk. And that was probably due to a phenomenon called heterochrony: a change in the timing of gene expression. You see, the embryos of whales and dolphins still develop hind limb buds.

Those are structures that grow into limbs. And in all vertebrates, that growth is largely overseen by a gene called Hand2 which, essentially, gives the OK for other genes to do their jobs. So researchers think the gradual shrinking of hind limbs was probably caused by a bunch of little genetic changes that affected the duration and intensity of Hand2 expression in those rear limb buds, especially later on in fetal development.

Then, around 34 million years ago, cetacean hind legs abruptly disappeared. And that's probably because their Hand2 got switched totally off. So instead of heterochrony, it's an example of heterotropy: a change in the location of gene expression.

This was a considerable improvement, as even those mini legs created drag that slowed them down. So getting rid of them entirely made the animals sleeker, faster swimmers. And, all of this explains how this atavism can happen now, even though it's been forever since these animals had real hind limbs.

Cetaceans still use Hand2 to direct the growth of their “arms” into flippers, which help them steer and stop. And that means that to re-evolve legs could be as simple as a mutation that switches this gene on in the back, too. Though, exactly how Hand2 gets reactivated to make mini-legs is still a bit of a mystery.

Studying this particular atavism has given researchers a better understanding of the changes that whales have gone through to become the gentle giants of the sea. And, there are lots of similar examples of one-off atavisms like this, from extra toes in horses to extra nipples in people. But sometimes, atavisms change the course of evolution, leading to the reemergence of traits in an entire species.

Limpets are snails, but they sure don't look like them. Instead of having long, coiled shells, they have short, squat little ones. So they sorta look like they're carrying around teeny shields on their backs.

These non-coiled shells are especially useful for squeezing into tight spaces or hunkering down in the pounding waves of shoreline environments. And scientists estimate that limpets lost their coiling at least 20 million years ago. So, it was thought that their ability to curl up their shells was lost for good.

Some limpets in the Calyptraeid family do have coiled shells, though. Out of the 200 or so species, 10 to 12 have coiled shells as adults. And at first, researchers thought those species just hadn't gotten around to losing their ancestral coils.

But in 2003, scientists realized they'd actually re-evolved them. And when they looked a little closer at one genus in particular, they found out how that could be. See, the snails in the genus Crepidula all have coiled shells as babies, but grow up to be uncoiled adults.

This means that the genes for coiling are still kickin' around and expressing at some point during development. So all a limpet has to do to re-evolve coiling is express those genes for a longer period of time. And that seems to be exactly what's happened in those re-coiled limpet species.

The timing of this gene expression has changed, leading to a coiled shell all the way into adulthood. So, it's another example of heterochrony. Except, this one stands out because it is the only atavism we know of so far that arises solely from a change in developmental timing.

And it reveals just how powerful small tweaks to timing can be. If you looked super closely at the dirt in your garden or a nearby forest, you'd probably find what appear to be teeny tiny beetles. But they're actually soil mites.

And the odds are pretty high that the ones you find are female, because long ago, entire lineages of these mites lost their ability to have sex. They reproduce through parthenogenesis instead: a type of asexual reproduction where embryos develop from unfertilized eggs. That often leads to entire species that are 99 percent female.

And nearly 10 percent of all soil mite species are parthenogenic, including hundreds of species in the Desmonomata suborder, the largest known grouping of animals that reproduces this way. But weirdly, this group also contains the family Crotoniidae, soil mites which seem to have gone from having sex, to reproducing parthenogenically, to having sex again. That may be because they don't really act like soil mites anymore.

See, these mites spend their lives in tree bark instead. In fact, most sexually reproducing mites are found in tree barks and mosses rather than in soil. There are a number of hypotheses about why sex is more prevalent in these habitats.

Like there might be more predators and parasites there, so the species need the increased genetic diversity that comes with having sex. But it's extremely unusual for a species to regain the ability to have sex after being asexual for so long. Somehow, despite not making any males for millions of years, they held onto the genes needed to make them.

And at this point, we're not totally sure how they did that. But researchers are eager to learn, because hanging on to inactive sex genes seems like a waste of genetic space. It's possible, for instance, that these genes served some other important purpose for the mites survival.

And if that's true, then figuring out why they'd keep these genes around for so long could teach us a lot about what else they do. Osedax worms live in the depths of the oceans where they eat bones for a living, hence, some have taken to calling them bone or zombie worms. And their diet isn't their only radical feature.

They also exhibit some pretty extreme sexual size dimorphism, which is to say, the females are a lot bigger than the males. In fact, male bone worms are so small that they live in harems inside the females' body. And not just like, two or three males.

Scientists have counted as many as 607 males living inside a single female! That's right. A very specific number.

That's how you know they did their homework. Except for Osedax priapus, that is. In this one species, males are up to one-third the size of females, and they live their own, independent lives.

Researchers are pretty sure that the species came from ancestors that had miniature, internalized males, like other bone worms. And it's thought, in general, these worms evolved such differently-sized sexes because they feed off of the bones of dead animals in the deep sea. So food is somewhat scarce.

Females can't get around the fact that they need to eat a lot to make baby worms, and the bigger they are, the more they can give to their offspring. But male worms can be small and still make plenty of sperm. And the smaller they are, the less space and food they use up, leaving more for the females they want to inseminate.

So it's not hard to imagine how females might have gotten bigger and bigger while males got smaller and smaller, until eventually, the males were small enough to fit inside their mates. Now, their sole purpose in life is to produce sperm. Dwarf males don't even eat!

They just digest the yolk they got as eggs and then pump out sperm until that energy resource dries up. The question becomes why, then, the males of this one species would go back to eating and living on their own. Most hypotheses center around the fact that the females of this species are particularly small themselves, when compared with other bone worms, anyway.

Small females produce smaller yolks, which don't provide the energy for males to produce oodles of sperm. So, the species may have evolved larger males to shirk the requirement for maternal yolk altogether. Also, smaller females don't take up as much bone space.

So maybe there was simply room for the males to go out on their own, and by actually eating, they could produce more sperm. Plus, males that don't live inside a female can reach, get this, other females, which helps mix up the genes in a population more effectively. So perhaps there was something which led the species to crave more genetic diversity.

It's also possible that a perfect storm of all of these hypotheses ended up creating the unique evolutionary conditions which drove this species back towards larger, independent males. Biologists will probably get a little more insight into this mystery by determining how this atavism occurred. Because right now, they're not even sure how a worm becomes male or female, let alone what genes govern things like body size.

And they're interested in the specifics here, because as far as we know, this atavism is the only example of the loss of dwarfism in an animal. So already it's shakin' up our understanding of what kinds of evolutionary shifts are possible. The more we study the living creatures around us, the more we're realizing that we've made a lot of assumptions about how evolution works.

And that's why atavisms like the four we discussed today are always thrilling for researchers to discover, whether they occur in an entire species or are a fluke appearance of a long-lost trait. Atavisms give us insight into the ancestral lineages of organisms, and help prove, or disprove, what we think we know about evolution. Thanks for watching this episode of SciShow!

And a double thank you to all of our supporters, like, all you awesome people who have signed on as channel members. The money you pledge each month as a channel member helps us keep making fun science videos like this one here on YouTube where they can be watched for free. So thanks, channel members!

And if supporting free, educational science content sounds like something you might be interested in, then I encourage you to click the Join button below and learn more about becoming a member. [♪ OUTRO].