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.