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Everything living on Earth is related. We're all a part of a giant family tree because we all descended from a common ancestor that lived some 3.5 billion years ago.

But nowadays, some branches of this tree are kind of sparse. And when there's only one species on a branch, we call it a monospecific taxon, which is just fancy biologist-speak for ‘single-species group'. As you trace back from the tips of our family tree, so, from smaller to larger taxonomic groups, or from species to genera to families, orders, and so on, monospecific taxa become rarer and rarer.

Understanding how each of these evolutionary loners ended up that way can teach us a lot about the history of life. Up first we have us! Because we Homo sapiens are the only living species of our genus.

Which goes to show that evolutionary loners aren't always rare or endangered. On the contrary, we're quite abundant and widespread, with over 7.5 billion of us worldwide. But in an evolutionary sense, we're pretty isolated.

We weren't always so alone. At least half a dozen other species of Homo once roamed the planet, including Neanderthals. And like us, those species stand out from other primates because they also walked on two legs and had relatively big brains.

These larger brains enabled advanced tool use and social cooperation, and ultimately allowed us to come up with the creative solutions that let us survive in all sorts of environments. But our big brains were also a big part of why we're alone on this evolutionary tree branch. See, early humans were skilled hunters.

Our advanced weapons coupled with cooperative strategies proved a deadly combination for prey animals, and tribes of other Homo species. Though our interactions with them weren't always bloody. Some were more intimate.

And whether it was because we interbred with them, outcompeted them for food, or just directly attacked them, all our sister species eventually died out. This evolutionary pruning hasn't ended with just our genus. We're the architects of the current mass extinction event, which spans the last 40,000 years and has led to the demise of many of our close primate relatives.

In fact, all of the other great apes in our family are endangered, largely because of human activity. Luckily, our big brains also allow us to understand this is happening, and they can probably figure out how to keep ourselves from becoming truly alone on this limb. Welwitschia mirabilis is a long-lived plant native to the deserts of Namibia and Angola.

It's the only living species in its family, and it's kind of a loner in the larger group of plants it belongs to, the gymnosperms, because it has some weird traits. Like, it has tube-like structures called vessels in its water-conducting tissues. These are normally lacking in gymnosperms, but are found in their flower-producing relatives, the angiosperms.

Also weird: while it's sometimes called ‘tree tumbo', it looks more like a heaping pile of seaweed than a tree. And that tangled mass is actually just two leaves, each of which can be several meters long. Botanists used to think those leaves indicated it was stuck in baby plant mode, a phenomenon called neoteny.

That's because many seedlings start with two leaves, so scientists assumed the whole species evolved to just never grow up. However, it turns out that Welwitschia isn't a life-long juvenile. It's just done away with its stem, or, the top of its stem, to be more precise.

That's the part where growth happens to make a plant taller. And just to put into perspective just how strange it is to lack that part: one botanist wrote that it's the botanical equivalent of an animal species losing its head. Because of its lack of stem, Welwitschia can't make space for more leaves.

And that means, it just keeps extending the two leaves it's got continuously, for hundreds to thousands of years! But Welwitschia didn't used to be so unique. For hundreds of millions of years, Welwitschia‐like plants and other non-flowering groups flourished on Earth.

But then, about 125 million years ago in the Cretaceous Period, flowering plants emerged. And now, these photosynthetic powerhouses make up the vast majority of plant life on land. It's easy to assume that their success was due, in part, to flowers, since those are most notably what sets them apart.

But the efficient architecture of their veiny leaves was probably what actually did the trick. So, instead of asking what happened to its kin, a better question might be how Welwitschia held on. And part of the answer probably lies in those weird leaves, see, they help them live where most flowering plants can't.

In addition to having those vessels which help transport water efficiently, the surface of each leaf is covered in wavy, waxy grooves which collect water from rain, dew, fog and is directed down to the plant's roots. So they, along with other adaptations to a desert lifestyle, allowed Welwitschia to persist as parts of Africa dried up. Amborella trichopoda is a species of small shrub-like tree that only grows in New Caledonia.

And it's considered the most basal living angiosperm. So of all flowering plants alive today, its ancestors were the first to break off and form their own evolutionary branch some 200 million years ago, which is why the species is placed in its own family. One of the ways it differs from other angiosperms is that it lacks the water-moving vessels Welwitschia oddly has.

You know, just to make everything more confusing. Anyhow, because of its unique position on the tree of life,. Amborella can help botanists understand the sudden and impressive evolutionary success of angiosperms.

But without clear fossils of its ancestral kin, it's a little difficult to say why Amborella itself has no close living relatives. We know from looking at its genome that it missed out on some big genetic events that probably helped other lineages of angiosperm become so diverse, like, whole genome duplications that gave them lots of genetic material for natural selection to tinker with. Though, it does have a bunch of extra genetic material in its mitochondria.

Bizarrely, the Amborella mitochondrial genome contains four almost complete genomes from other plants and a total of six genome-equivalents of foreign DNA. The transferred genes come from all manner of photosynthetic species, including algae, mosses, and other angiosperms. And to be frank, we have no idea what, if anything, those extra genomes do.

But if we figure that out, maybe we'll also get more insight into why this plant is such an evolutionary loner. This might look like a lizard, but it's not. It's a tuatara, the sole surviving member of a separate reptilian order that diverged from the ancestor of true lizards and snakes some 250 million years ago.

One way you can tell is by its skull. Tuatara have a pair of holes on both sides of their skulls, just behind the eye socket. One or both of these have been lost in modern squamates, the order of reptiles that includes lizards and snakes.

So these holes make tuatara skulls much more like those of older reptile groups, like ichthyosaurs and plesiosaurs. And that's not the only ancient trait they have. On top of their heads sits a third eye called a parietal eye.

It's built much like other eyes; it has a transparent outer cornea, a flexible lens, and a light-sensing retina. But they don't really see with it, because it's covered by a scale. Instead, researchers believe it's used to perceive the time of day or what season it is.

This eye is lacking from snakes and some lizards, but all of the tuatara's relatives in that order, Rhynchocephalia had one. And back in the Jurassic and Triassic periods, they were one of the most dominant reptile groups. But then in the Cretaceous, those squamates underwent a burst of diversity.

And kind of like how flowering plants quickly outcompeted other land plants, snakes and lizards just kind of took over. The tuatara's survival through all of this was probably luck more than anything else. It just so happened that almost no squamate species were on the islands of New Zealand when they separated from the larger land mass.

So it got to keep on tuatara-ing in peace, at least, until we showed up. Aardvarks are notable for a number of reasons, but what landed them on this list is the fact that they're the only mammal alone in its order. They kind of look like a bunch of other mammals, though.

And that's because they're a great example of convergent evolution: where different species independently evolve similar traits. If you had to guess their closest living kin, you might understandably say pigs or anteaters. But in fact, they're more closely related to elephants!

Aardvarks look more like those other animals because they're highly specialized ant-eaters. Those pig-like noses help keep their nostrils free of dirt while they root around to find food. And they share several traits with true anteaters, like long tongues and reduced teeth because, well, both eat insects that don't really require chewing.

Historically speaking, it's unclear what happened to the aardvarks' closest relatives, mostly because, well, we don't know who their closest relatives were. The handful of related extinct fossil species from a few million years ago look too much like modern aardvarks to provide any insights. And no one has found transitional fossils that reveal who they're related to.

So all we know is that the aardvark's line appears to be an early offshoot of a very ancient group of ungulates, or hoofed animals, which is at least 20 million years old. Of course, other ungulates have done quite well in Africa. Like, zebras, gazelles, hippos, rhinos, warthogs, giraffes, just to name a few.

So, our best guess is that all the other members of the aardvark order were outcompeted by those many, super-successful ungulate lineages. Limnognathia maerski is one of the tiniest animals on the planet, and it makes up its very own phylum. Well, maybe.

Some taxonomists say it's in a class by themselves or a subphylum instead. The lack of certainty stems from the fact that we haven't known about these critters for very long, so they're still a bit of a puzzle in general. They were originally discovered in freshwater springs in Greenland, and first scientifically described in the year 2000.

Since then, they've been found in a few other arctic lakes and in subantarctic waters. And no, we have no idea how they wound up on opposite sides of the planet. Adding to the mystery: all individuals found so far have been female.

Scientists wonder if they've somehow missed the males, because they're even tinier. Keep in mind these animals are only 80 to 150 micrometers long as adults! Or, perhaps they start their lives as male and then switch to being female as they grow larger, a phenomenon called sequential hermaphroditism.

Or maybe they're just parthenogenic, and females generate clone offspring. Whatever is going on, what sets them apart from other tiny aquatic animals is their serious, scary set of chompers. Their jaws contain fifteen moving, gnashing, microscopic parts.

And packed into those tiny bodies and complex jaws are clues to their possible close relatives. The overall structure of the jaws suggests a close relationship to rotifers, a much more numerous and wide-spread phylum of microscopic animals. So, it is possible the few isolated populations of Limnognathia we see today are the last survivors of millenia of battles with rotifers.

Or, perhaps, further research will relocate them to a closer, and fuller, branch of the animal family tree, and point to some other explanation. Comparing these diverse species shows us that there isn't just one reason why an organism ends up an evolutionary loner. Sometimes they have a trait that allows them to persist through adversity, like the drought tolerance of Welwitschia.

Or maybe, like the tuatara, they just find themselves in the right place at the right time. Piecing together the prehistoric past can be tricky, but studying these special species can help us better understand all life on this planet. We hope you enjoyed learning about these weird, wonderful species.

We're certainly big fans of them; so much so that we made a poster! And it's really the kind of artwork that pulls a room together, you know? If you want to check it out for yourself, you can head on over to dftba.com/scishow or scroll to that little merch shelf.

And as always, thank you for watching SciShow! [♪ OUTRO].