Sex: A Two-Billion-Year Story

How Sexual Reproduction Transformed Life on Earth

My husband has many hobbies, but one that people usually relate to him is his love for marine life… which translates to a saltwater aquarium that takes a good part of our living room and storage space. Oh, I love that everywhere I look at the house, I find yet another aquarium-related item.

However, I have to agree that it provides us with an amazing tool for kids’ education and guest entertainment. They spend hours looking at the neon lights and the colorful organisms. We are the house with the blue lights!

The other night, we had a very curious guest who inundated us with questions we enjoyed answering. However, I wasn’t ready for this one.

You see, the coral in the tank keeps growing by multiplying themselves, something we call “asexual reproduction.” This is why we buy them as little fragments and eventually establish lavish colonies that can take over the whole tank if you don’t watch out.

Our friend had seen one of these David Attenborough documentaries where all corals sync to release eggs and sperm and start new colonies far away from their parents. What we call coral spawning. Our friend was curious to know whether we had ever seen anything like that in our tank.

And while this can indeed happen in corals in captivity, we haven’t had any event like that ourselves. Not yet, at least.

Of course, this conversation led us to talk about the meaning of sexual and asexual reproduction and reminded me that I should take the time to write about it here. After all, this is a Two-Billion-Year Story!

From birds and bees to the plants in your garden, sex is a constant in nature. But it wasn’t always this way. In fact, the origin of sexual reproduction was a game-changer for life on Earth. Some may say it was one of the most important evolutionary adaptations, one that made adaptation possible, or at least faster.

But why is that? Let’s explore how it all began and why it matters so much, shall we?

A long time ago, Earth was a pretty simple place. We’re talking about a period roughly 2 to 2.5 billion years ago when single-celled organisms like bacteria and archaea dominated the planet.

But these early life forms multiplied straightforwardly: they cloned themselves. That’s it.

Each cell made a copy of its single chromosome, split in two, and voilà, two identical offspring. Actually, this approach worked fine for billions of years. Why mess with success?

Well, cloning has its limits. When offspring are genetic copies of their parents, there’s no variation in traits and adaptations. If the environment changes, these identical clones may not have the flexibility to adapt because none of the organisms in the population may have the right set of traits for the new environment.

Sure, bacteria can mix things up a bit by swapping small pieces of DNA with one another or picking up stray bits from their surroundings, but that’s not quite the same as the genetic shuffling we see in sexual reproduction.

If you want to learn more about bacteria and their modes of reproduction from an expert, watch the video below!

But back to the evolutionary history behind this story.

Somewhere between 2 billion and 1.5 billion years ago, a new kind of life appeared. We call it the Last Eukaryotic Common Ancestor, or LECA for short. LECA wasn’t the first eukaryote (organisms with a nucleus in their cells), but it’s thought to be the ancestor of all eukaryotic life forms today — everything from fungi to animals to plants to us humans!

While we don’t have fossils of LECA itself, we know it must have existed because all modern eukaryotes share certain traits, including sexual reproduction.

However, sexual reproduction wasn’t an instant innovation. It took time and likely evolved as a response to changing conditions. The essence of sex is that it allows offspring to inherit DNA from two different parents. But I’m sure you knew this much from hearing conversations surrounding newborns with daddy’s nose and mommy’s eyes.

Cells divide their genetic material, creating gametes that have half the usual number of chromosomes. These gametes then seek out others from another parent. When they combine, the result is a new mix of genes, producing genetically unique offspring. Not exactly like mom, not exactly like dad. However, seeing my son, I wonder whether my genes contributed at all.

But back to the science, this genetic mixing is the magic of sex (biological sex, don’t get too excited). It creates diversity, giving populations a better shot at surviving and thriving when the environment shifts.

When ecosystems become more complex, as they did millions of years ago, having different genetic combinations helps species adapt. Those with favorable traits survive and reproduce, while others fade away — a heartbreaking basic but crucial part of evolution.

Fast forward a bit, and the first signs of sexual reproduction in the fossil record start to appear in the form of animals called Funisia. These creatures lived around 565 million years ago during the Ediacaran Period. They formed colonies that looked like tiny underwater forests. Initially, their fossils were misinterpreted, but scientists later realized they were animals.

What’s fascinating about Funisia is that their fossil colonies show groups of individuals that are all about the same size, much like modern oysters, which release gametes into the water to reproduce. These gametes meet up, develop, and eventually settle in clusters. The similarity in pattern led paleontologists to conclude that Funisia might have reproduced sexually.

Indeed, it’s quite common for paleontologists to make interpretations based on what we know about the present world.

Interestingly, Funisia wasn’t stuck with just one reproductive strategy. The fossils also show evidence of asexual reproduction through budding — a process where a new organism grows directly from the body of the parent, becoming a clone.

However, this dual approach isn’t unique to Funisia. Even today, some animals, like Hydras, reproduce this way. And if you are curious about what these ancient creatures looked like, watch the short video below. They blow my mind every time!

So why the need for two methods? Asexual reproduction makes perfect sense when conditions are stable. If an organism is well-adapted to its environment, producing exact copies ensures its successful traits are passed on fully.

There’s no risk of genetic variation introducing something less beneficial. It’s a strategy of consistency — if it isn’t broken, don’t fix it.

But when conditions shift, that’s when sexual reproduction shines. By mixing genes, organisms can develop new traits to help them survive in a changing world. This flexibility offers a critical advantage when the environment is unpredictable or when new challenges arise. It keeps organisms ready for whatever happens.

The ability to switch between asexual and sexual reproduction gives species like Funisia the best of both worlds. When life is good, and everything’s predictable, they can stick with cloning. But when things get rough, they can switch to sex, rolling the genetic dice for a chance to adapt.

The story of sex isn’t just about a single event or a single species. It’s a process that took millions of years to develop, shaped by the ever-changing environment on Earth. Today, every eukaryote — every plant, animal, fungus, and even you — owes its existence to that ancient evolutionary leap: Sex.

Thanks to the mixing of genes, life became more resilient, capable of surviving and evolving through the countless changes Earth has seen.

From the humble beginnings of Funisia colonies to the complex ecosystems we know today, the story of sex is one of adaptation and survival. It’s proof that sometimes, the best way to face the future is to mix things up a little.