So sex is everywhere. I’m not the only one to notice
that, right? Like, I don't have to tell you this? I mean you got your birds, you got your bees and you got your...internet. But sex is also plants, and it's fungi, and it's basically
every kind of complex organism you’ve ever seen. And sexual reproduction has been around so
long that the common ancestor of all complex life probably had sex. Of some kind. Now we don’t know which living thing was the
very first to arrive at the totally revolutionary process that is sexual reproduction. But we can follow the history of how, and
why, sex became a thing -- and show you some of the earliest animals that we know … umm
.. did it. Ok...first, let’s talk about the time before
there ever was sex on Earth. Between 2 billion and 2.5 billion years ago,
the planet was home to simple, single-celled organisms, like bacteria and archaea. And their main mode of reproduction was, and
still is, to basically clone themselves. They duplicate their single chromosome, and
then split it in two, with each offspring getting a copy of the original. That means the offspring are always genetically
identical to their parents. And for a long time, probably billions of
years, that worked out just fine. Now, bacteria do have ways of swapping small
amounts of DNA among themselves, and of picking up DNA from their environment, which changes
their genetic makeup. But sexual reproduction in the strictest sense
-- where offspring get an equal share of two parents’ DNA -- that didn’t show up until
well into the Proterozoic Eon. And it’s in there, somewhere between 2 billion
and 1.5 billion years ago, that a brand new kind of thing evolved. Today it’s known as the Last Eukaryotic
Common Ancestor, or LECA. You can think of it as the great-great great-great
great-grandparent of all the eukaryotes that are alive today. That includes everything that has a nucleus
in its cells. So, all the plants, all the animals, all the
fungi, all the protists -- basically every living thing that you’ve ever seen with
your naked eye. Now, LECA isn’t really a specific thing
that we’ve found in the fossil record. We don’t know exactly what it was. But we can infer its existence, because all
eukaryotes that are alive today are thought to have descended from a single eukaryotic
species. So LECA isn’t the very first eukaryote -- it’s
the most distant common ancestor of all modern eukaryotes -- the ones we know today. And we can infer what characteristics LECA
had, based on what all eukaryotes today have in common. And one thing that we all have in common is
sex. Now … you know what I mean by sex in this
context, right? Should I get you a pamphlet? When I talk about sexual reproduction, I’m
talking about cells that divide their chromosomes into matching sets, duplicate those sets,
and then separate them out into many new cells. Those new cells, with just one set of chromosomes,
are gametes. And they’re looking for compatible gametes
from another parent that have their own set of chromosomes. Since each parent has a slightly different
combination of genes in its chromosomes, when their gametes combine, the offspring that
they produce will be different every time. This whole process probably took a lot of
evolutionary time to master, and it may or may not have been perfected by LECA’s time. But like I said, we don’t really know what
LECA was. So what’s the earliest evidence for sex? Well, we do have some pretty old fossils of
eukaryotes, including red algae that are about 1.6 billion years old. But red algae reproduce by using tiny spores,
and we haven’t detected any direct evidence of those in fossils of that age. We animals, however, are a little more obvious
about our mating habits. And the oldest fossil we have that’s interpreted
as direct evidence of sex is of an animal. Actually, many animals. They’re known as Funisia, and their fossils
don’t look really … scandalous or anything. In fact, until scientists figured out that
they were a new type of animal, their fossils were mistaken for … poop. They date to the Ediacaran Period at the end
of the Proterozoic eon, about 565 million years ago. Funisia lived in huge colonies that looked
kinda like a tiny underwater bamboo forest. But it was neither plant nor algae. It seems to be most closely related to either
sea sponges or soft corals, but we’re not sure. So, if we can barely tell this thing is an
animal, why do we think that it had sex? Well, it comes down to the size, age, and
arrangement of its fossils. Fossils of Funisia are often found in groups
of individuals that are all about the same size. And this kind of distribution is very similar
to how some modern animals are arranged. Specifically, oysters. Oysters reproduce by broadcasting their gametes
into the ocean. The gametes join up and start to develop while
floating around in the water. But as they start to change into their adult
forms, they settle to the seafloor. And that results in big groups of oysters
that are all about the same age and size, a lot like Funisia. But, here’s an interesting little wrinkle! We think that Funisia might have been able
to reproduce asexually too! That’s because, some of them fossilized
when they were in the process of budding, sprouting little buds off the main stem that
would become clones of the original. This is a technique that’s still used today,
by freshwater animals called Hydras. So, because we’ve found both budded offspring
and synchronized groups, we think Funisia could reproduce BOTH sexually and asexually. Which, actually, is what Hydras do too. But, why was the development of sex such a
big deal in the first place? And if sexual reproduction is so great, then
why could Funisia, hydras, and other animals still reproduce asexually? Well, sexual reproduction creates new combinations
of genes, which allows for new variations in the offspring. And some variations are more successful than
others. That’s the basic idea of natural selection. And since the world is always changing, it
can help a lot if you can change, too, and quickly. But if your environment isn’t changing much,
then there isn’t much selective pressure. In which case, mixing up your genes might
not be such a great idea. After all, if you’ve already found a formula
that works, why mess with it? Asexual reproduction lets you pass on ALL
of your genes, not just half of them. So if you’re really well adapted to your
environment, it guarantees that your offspring will be too. So, organisms like Funisia and Hydra seem
to use asexual reproduction when their environments are stable, but they can switch to using sex
when they’re under pressure to adapt and survive. They’re just keeping their options open,
and that’s cool. So for sex to have become a thing when it
did actually makes sense, when you think about it. For Funisia, sex would have come in very handy
in the Ediacaran, when ecosystems were suddenly becoming more complex than they ever had been
before. So all of us eukaryotes living today owe LECA,
Funisia, and their descendents a debt of thanks, for the evolutionary breakthrough that they
discovered and perfected, in the form of sex. Thanks for listening to me talk about sex
for seven minutes! And as always, I want to know what you want
to learn about! So leave me a note in the comments below! And be sure to go to youtube.com/eons and
subscribe. Now it might help to have sex if you
have a brain and if you want to learn about psychology and neuroscience and why humans
do the things that they do, I implore you to check out Brain Craft, a show by our friend
and colleague Vanessa Hill. Your brain--if you have one--will thank you.
Why the fuck is this NSFW?
Good video but it didn't tell me anything at all about HOW sex came to evolve.
You might also be interested in this: Why did two sexes evolve?