[⪠INTRO] Everything living on Earth is related. Weâre 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 thereâs only one species on a branch,
we call it a monospecific taxon, which is just fancy biologist-speak for âsingle-species
groupâ. 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
arenât always rare or endangered. On the contrary, weâre quite abundant and
widespread, with over 7.5 billion of us worldwide. But in an evolutionary sense, weâre pretty
isolated. We werenât 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 weâre 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 werenât
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 hasnât ended with
just our genus. Weâre 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. Itâs the only living species in its family,
and itâs 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 itâs sometimes called
âtree tumboâ, 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. Thatâs 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 isnât
a life-long juvenile. Itâs just done away with its stem, or, the
top of its stem, to be more precise. Thatâs 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 itâs the botanical
equivalent of an animal species losing its head. Because of its lack of stem, Welwitschia canât
make space for more leaves. And that means, it just keeps extending the
two leaves itâs got continuously, for hundreds to thousands of years! But Welwitschia didnât used to be so unique. For hundreds of millions of years, Welwitschiaâlike
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. Itâs 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 canât. 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 plantâs 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 itâs 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, itâs 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 weâll also get more insight into why
this plant is such an evolutionary loner. This might look like a lizard, but itâs
not. Itâs 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 thatâs not the only ancient trait they
have. On top of their heads sits a third eye called
a parietal eye. Itâs built much like other eyes; it has a transparent outer cornea, a flexible
lens, and a light-sensing retina. But they donât really see with it, because
itâs covered by a scale. Instead, researchers believe itâs 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 tuataraâs 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 tuataraâs 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 theyâre the only mammal
alone in its order. They kind of look like a bunch of other mammals,
though. And thatâs because theyâre 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, theyâre more closely related
to elephants! Aardvarks look more like those other animals
because theyâre 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 donât
really require chewing. Historically speaking, itâs unclear what
happened to the aardvarksâ closest relatives, mostly because, well, we donât 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 theyâre related to. So all we know is that the aardvarkâs 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 itâs in a class by
themselves or a subphylum instead. The lack of certainty stems from the fact
that we havenât known about these critters for
very long, so theyâre 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, theyâve 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 theyâve somehow missed
the males, because theyâre 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 theyâre 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 isnât 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. Weâre certainly big fans of them; so much
so that we made a poster! And itâs 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]
This was fun.
There are many evolutionary loners. The platapus, walrus, beaver. And then theres the most glaring evolutionary loner of all... us. Every species of Homo was the apex predator of its era. Many Homo species coexisted over the past few million years and no other animals posed a threat to them. And yet only we remain today. The apex of the apex.