10 Pivotal Moments for Life on Earth

Life is everywhere on Earth, from the smallest microbes at the bottom of the ocean to the birds in the sky and everything in between. However, it wasn’t always like that. For most of Earth’s history, life only existed in the form of simple, single-celled organisms, until a few pivotal moments changed everything.

These were the turning points that allowed species to grow more complex, diversify, and eventually dominate the planet. Some of these changes were slow and gradual, taking thousands—even millions—of years to unfold, while others were sudden and dramatic. All of them allowed new species to emerge and thrive, resulting in the incredible diversity of life we see all around us today.

Related: 10 Weird Critters That Lived Alongside the Dinosaurs

10 Self-Replicating Molecules

There were no signs of life on Earth until around four billion years ago. Then, something curious happened. Some organic chemicals began linking together into longer chains, and at some point, a few of them gained the ability to copy themselves. It was the birth of self-replicating molecules that could store genetic information, make copies of themselves to reproduce, and even perform basic chemical reactions. Over time, they started evolving through natural selection—the better a molecule was at copying itself, the more it multiplied, leading to evolutionary improvements.

This was the moment when random chemicals turned into the building blocks of life itself, which would later evolve into DNA and proteins. These self-replicating molecules allowed information to be passed through generations, setting the blueprint for all life on Earth today.^[1]

9 The Great Oxygenation Event

Around 2.5 billion years ago, Earth’s atmosphere had almost no free oxygen. At the time, life mostly consisted of simple bacteria that relied on iron and methane for survival. That changed with the emergence of the first microbes capable of photosynthesis, cyanobacteria, as they started converting sunlight to energy and releasing oxygen as a waste byproduct. The environment was soon flooded with oxygen, triggering the first mass extinction event in Earth’s history.

Oxygen was actually toxic to most of these life forms, which is why this period is also sometimes called the “Great Oxygen Catastrophe.” It was also a climate disaster, as the oxygen started reacting with methane—a potent greenhouse gas—to form carbon dioxide. Global temperatures plummeted rapidly, resulting in an ice age that nearly wiped out life altogether.

Thankfully for all of us, there were exceptions. A handful of life forms adapted to the new, oxygen-rich world. As oxygen was far more efficient for energy production, that adaptation eventually led to the evolution of complex life forms.^[2]

8 The Cambrian Explosion

Imagine a world where life existed, but in slow motion—simple, soft-bodied creatures drifting through the ocean with no predators, no competition, and no urgency to evolve. That was what life on Earth was like before around 540 million years ago. Then, in just a matter of 10 to 20 million years—a mere blink in evolutionary time—most of the major animal body plans we see today appeared.

This was the Cambrian Explosion, one of the most significant creative bursts in evolutionary history. Suddenly, the oceans were full of all kinds of life forms, each one locked in its own evolutionary arms race to survive the longest. Creatures developed hard shells and complex eyes, and terrifying predators—like the giant, nightmarish Anomalocaris—were suddenly patrolling the waters. Among all these strange creatures was a humble, worm-like organism called Pikaia gracilens, with a flexible rod along its back that would become the basis for all vertebrate spines, including ours.

While most of these creatures eventually went extinct, they established the fundamental body plans that all complex life, including humans, would later build upon.^[3]

7 The Ordovician Radiation

While the Cambrian period established the basic evolutionary features of modern-day animals, the Ordovician period—around 485 to 443 million years ago—was when those early designs developed into Earth’s first complex ecosystems.

Before this period, marine ecosystems were still relatively simple, dominated by leftover species from the Cambrian era. As the continents shifted and sea levels rose, a variety of new habitats opened up in the shallow seas, creating opportunities for life to expand in ways never before possible.

What made this period important wasn’t just the proliferation of marine species, but also how they began to organize themselves into more intricate and interconnected communities. For the first time, predators became specialized, food chains grew more elaborate, and organisms started occupying distinct layers of the ocean, much like the different levels of a rainforest. Coral reefs formed vast underwater cities, while burrowing creatures reshaped the seafloor. This diversification established complex food webs that hadn’t existed before.

The ecosystems that emerged during the Ordovician became the foundation for marine life throughout the Paleozoic Era, setting the stage for creatures like modern fish, land vertebrates, and humans.^[4]

6 The K-T Extinction

Dinosaurs were the dominant branch of life on Earth for about 160 million years. Everything changed around 66 million years ago, when a massive asteroid crashed into Earth and wiped them out, along with three-quarters of all existing species at the time. Now known as the K–Pg (formerly KT) extinction event, it was an apocalypse for life on Earth, with wildfires, tsunamis, and years of darkness killing off thriving ecosystems around the world.

It was also a time of opportunity for mammals and other small species. While mammals existed before the extinction event, they were small, nocturnal creatures living in spaces that the dinosaurs couldn’t reach. While most mammal species also died off, the ones that survived suddenly found themselves in a world stripped of its dominant predators. The survivors weren’t the most specialized or impressive mammals—just the toughest, most adaptable ones. Over time, these generalists evolved into new species, from tiny fruit-eaters to the first tree-climbing primates.^[5]

5 The First Land Plants

About 500 million years ago, Earth’s landmass was harsh and lifeless, with only the hardiest bacteria and fungi barely surviving on its barren, rocky surface. Then, a small group of freshwater algae species adapted to survive outside water, giving birth to the planet’s first land plants. This transition required some tough changes, like the development of rigid cell walls to support upright growth, protective coatings to shield against deadly UV radiation, and internal systems to survive droughts and extreme temperature swings.

Over the next few millions of years, those early plants evolved increasingly sophisticated ways to sense and respond to their environment. By absorbing carbon dioxide and releasing oxygen, they helped stabilize the atmosphere, while their roots broke down rock into soil, creating the foundation for diverse ecosystems. Land plants didn’t just survive; they transformed the planet, allowing complex life to evolve and thrive on land.^[6]

4 Eukaryotes, or the First Complex-Celled Organisms

For billions of years, life on Earth existed in the form of simple, single-celled organisms called prokaryotes. Despite their adaptability, these creatures remained constrained by their rudimentary structure.

Between 1.6 and 2 billion years ago, cells merged with other cells and gave birth to early eukaryotes—organisms with complex internal structures like nuclei and mitochondria. Unlike their prokaryotic ancestors, eukaryotic cells could generate energy more efficiently, grow in size, manage more genetic information, and form multicellular organisms.

This was a game-changing moment in evolution. The newfound cellular complexity allowed organisms to evolve into the diverse life forms we know today. Without this leap, life would have likely remained microscopic and simple, and complex creatures—including humans—would have never existed.^[7]

3 Sexual Reproduction

Sexual reproduction first appeared around 1.3 billion years ago as an alternative to asexual replication. While it was slower and more energy-intensive, it allowed species to have more genetic diversity with each generation. Mixing genes from two individuals increased the overall survivability of species, as they could now evolve better defenses, find new food sources, and better adapt to changing climate conditions. It also helped them develop genetic defenses to diseases, enabling further complexity and resilience over time.

Today, the most abundant forms of life on Earth—animals, plants, and fungi—reproduce sexually. While asexual reproduction still exists in some bacteria and other organisms, sexual reproduction’s ability to create genetic variety made it a far more successful long-term strategy.^[8]

2 The First Flowering Plants

Flowering plants—or angiosperms—showed up over 100 million years ago, though they didn’t have any real impact on life on Earth until 66 million years ago, when the dinosaurs went extinct. It was the Angiosperm Terrestrial Revolution, when they rapidly diversified into countless new species and transformed Earth’s ecosystems.

Unlike older branches, flowering plants were far more efficient at harnessing sunlight and producing energy, which trickled down through food chains and sustained diverse habitats like rainforests. They also played a foundational role in the rise of human civilization, as nearly all our food crops, from grains to fruits, are flowering plants. Their ability to form dense, layered forests allowed them to outcompete older plant groups like conifers and ferns.^[9]

1 The First Multicellular Creatures

Multicellularity shows up in the fossil record many times, and it’s unclear exactly when single-celled creatures decided to “team up” and form complex, multi-celled life forms. The earliest evidence points to multicellular microbes existing as far back as 3 billion years ago, with more definitive evidence of algae appearing around two billion years ago and primitive animals like sponges appearing roughly 750 million years ago.

It transformed life on Earth. Cells could now specialize and focus on specific functions—like movement, digestion, and reproduction—opening new doors for survival and adaptation. That allowed the evolution of bigger, more complex organisms with abilities far beyond what single cells could achieve. It was the foundation of every major evolutionary leap since, from the first fish crawling onto land to the rise of mammals. It was also a critical turning point for humans—without multicellularity, the complexity of tissues, organs, and ultimately brains would have never developed.^[10]

fact checked by
Darci Heikkinen