Chapter 3
The History of Life on Earth

What makes life on Earth so special? And what is it, really? We still don't know exactly how life began, but we know what it does. It metabolizes to stay alive, self-regulates to stay stable, reproduces to stick around, and adapts to whatever the Universe throws at it. Life is endlessly inventive and annoyingly fragile. Just ask the dinosaurs.

Chapter at a Glance

40 Minutes

1 Threshold

10 Videos

2 Galleries

How Did Life Evolve on Earth?

Four billion years ago, our planet had just the right ingredients and just the right conditions for life to form. We are just the right distance from the Sun. Earth was a rocky planet with liquid water. Molecules could mingle, split, and recombine. Earth's atmosphere stabilized, and a magnetic field shielded the surface from radiation. When the planet finally cooled enough for oceans to form, chemistry crossed a line in the deeps.

Threshold 5: Life on Earth

How life evolves, adapts, and thrives.

Mini-Thresholds of Life

As Earth took shape, it became a laboratory for new kinds of complexity, and as far as we know, the Universe hadn't tried this before. Deep beneath the oceans, volcanic vents blasted out heat and chemical soups rich in energy. Somewhere in that chaos, the first microbes sparked to life. These tiny, self-replicating chemistry experiments would soon take over the planet.

The Evolution of Life

The brothers — © National Oceanic and Atmospheric Administration (NOAA)

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Spewing life

Scientists believe that some of the earliest life forms developed around deep sea vents gushing chemically rich material from the Earth.

The first named cell

While examining a thin slice of a dead cork plant, English natural philosopher Robert Hooke described what he saw as a "cell." He said it reminded him of the walled compartments monks live in.

Two billion-year-old colonies

Cyanobacteria have been on Earth for at least 2.8 billion years, often forming large "colonies" producing oxygen that enables further biodiversity. Their blue-green blooms can be seen from far above the Earth's surface.

Making oxygen

Chloroplasts capture sunlight and carbon dioxide to conduct photosynthesis in green plants. They transform these materials into energy-rich molecules like ATP and NADPH and, eventually, glucose.

A single-celled predator

The eukaryotic amoeba captures a paramecium. The amoeba will change its shape to surround and absorb the tasty prey.

Moving to dry land

The Tiktaalik roseae is considered a missing link between fish and the first land animals. Its front fins evolved into arm-like legs, similar to those on reptiles like crocodiles.

From mouse to man?

Some of the earliest mammals about 50 million years ago were thought to be small, mouse-like creatures. They prospered when an asteroid wiped out the dinosaurs.

Mini-moments of life

Life is so rich it can be broken into six mini-thresholds. These are the times when life seems to have become distinctly more complex.

What Is Life?

A mountain and a whale are both made of molecules. Both follow the same chemistry. Both change over time. What makes one a landscape and the other a living thing?

The answer is: Purpose. Or at least something that looks a lot like it. Living things don't just exist. They do. They repair themselves, feed, and reproduce, relentlessly copying their genes into the next generation. Mountains, magnificent as they are, don't try to make baby mountains. They just sit there, eroding politely.

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How Does DNA Work?

The young Earth was a brutal place. Volcanoes raged, meteorites blazed across the sky. But deep in Earth's oceans, the perfect chemical ingredients for life were quietly brewing. But what does it mean to be alive? Well, we know that life has four essential qualities:

Infographic detailing four characteristics of life with accompanying images: Metabolism showing green plant cells, Self-regulation showing an infrared rodent, Reproduction showing a sperm cell on the surface of an egg, and Adaptation showing four variations of finch heads. — Credit: 1. Chloroplasts in leaf cells, by Kristian Peters, CC BY-SA 3.0. 2. Thermal image of a rat, by Lakmus3041, CC BY 4.0. 3. A sperm cell fertilizing an egg cell, public domain. 4. Darwin's finches, public domain.

Tying all four powers together is DNA, life's master blueprint. Inside this elegant spiral, DNA stores the instructions for building cells, repairing damage, and passing traits forward. With the development of this single molecule, Earth's chemistry crossed a line. It stopped just reacting and started remembering. DNA passed information down to offspring, over and over, changing a little at a time until it created beings that could walk and think and doomscroll.

What Is Evolution?

DNA is very good at copying itself, but occasionally, it screws up. These genetic fails are called mutations. Tiny variations appear in every generation, and the ones that help an organism survive tend to stick around, passed down to its descendants. Over time, these small successes can add up to big changes—new species, new ecosystems, and platypuses.

We call this process evolution. Evolution works through something called natural selection, a term coined by English scientist Charles Darwin in 1859.

The idea of evolution wasn't entirely new when Charles Darwin published On the Origin of Species, but he provided the instruction manual. Darwin showed that life constantly changed and adapted to suit its environment.

The notion that humans were just another branch on the tree of life offended a lot of people in the nineteenth century. Today, 150 years later, plenty of people still find the idea uncomfortable.

Evolution doesn't care. It keeps rewriting life, one mutation at a time.

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Charles Darwin and Natural Selection

On the Origin of Species — Bettmann/CORBIS

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An evolutionary idea

Before the 19th century, scholars generally assumed that organisms remained as they were created, never changing. Charles Darwin shattered this idea in his masterwork, On the Origin of Species, which presented evidence that species change over time in a process he called "natural selection."

Similar insights, different islands — Reproduced With Permission From John Van Wyhe, Ed., The Complete Work Of Charles Darwin Online

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"Natural" evidence growing

A five-year voyage surveying the coast of South America and the Galápagos Islands exposed Darwin to a significant number of specimens supporting the growing "natural selection" theory.

The shape of survival

At the Galápagos, Darwin studied the accumulated effect of natural selection by an island's particular environment and the available food sources. For instance, the shape of a beak that best fed a finch gives it a competitive advantage. That finch survived and reproduced, passing along the selected trait to its offspring.

The evidence builds

In a letter to Darwin, naturalist Alfred Russel Wallace shared his similar thoughts about evolution prior to On the Origin of Species after exploring present-day Indonesia and Malaysia

Artificial selection

Darwin knew that breeders often mated particular animals so their offspring will carry desired traits forward. Likewise, people have cultivated vegetables to make more suitable food sources. Modern understanding of DNA and new "genetic engineering" techniques have produced new possibilities for artificial selection with new questions about life.

Satellite image of the GalÃ¡pagos Islands in the Pacific Ocean. — Credit: NASA image by Jeff Schmaltz, LANCE/EOSDIS, public domain.

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Galápagos Islands

You don't need a time machine to see evolution. You just need weather and patience. In the 1980s and '90s, Peter and Rosemary Grant studied Darwin's finches on the Galápagos. Their research showed that finch beak sizes can shift measurably within a few generations when droughts change which seeds are available, thus proving that we can watch evolution play out on the scale of a human lifetime.

The Tree of Life

Four Billion Years and Still Growing

For centuries, humans believed that we were the pinnacle of evolution. We strutted around on two legs, patting ourselves on the back with our opposable thumbs, assuming we were evolution's mic-drop moment. Adorable. The more we learn, the more we understand that the tree of life never stops branching, and we are only one leaf among many. At the center of this biological foliage sits the last universal common ancestor (LUCA)—a single-celled overachiever whose survival and reproduction started everything about 4 billion years ago. So no, humans aren't the apex. We're just the latest patch in life's endlessly buggy operating system.

The tree of life — © The Big History Project

How many times did life evolve on Earth? (Podcast)

From the microscopic spark of LUCA, life split into three domains:

Infographic comparing the three domains of life: Archaea with images of microorganisms, Eukaryota with images of plants, fungi, and animals, and Bacteria with microscopic views of bacteria. — Credit: Archaea: *Methanohalophilus mahii*, by Spring, S. et al., CC BY 4.0; *Pyrococcus furiosus*, by Michelle Kropfl, CC BY-SA 4.0; *Haloquadratum walsbyi*. by Rotational, public domain; *Promethearchaeum*, by Hiroyuki Imachi eta al., CC BY-SA 4.0. Eukaryota: *Boletus edulis*, by Hans Hillewaert, CC BY-SA 4.0; *Cetonia aurata*, by I, Chrumps, CC BY-SA 3.0; *Ranunculus asiaticus*, by Leif Stridvall, CC BY-SA 2.5. Bacteria: *Streptomyces*, by CDC, public domain; *E. coli*, by NIAID, public domain; *Streptococcus pneumoniae*, by CDC, public domain; *Campylobacter jejuni*, by USDA, ARS, EMU, public domain.

The History of Extinction

Evolution is a picky editor. It selects for whatever traits get the job done. Traits like camouflage, sharp hearing, fancy feathers, the ability to hold your breath, or store water in a desert. But the infinite complexity of life also makes our biosphere a fragile place. Sometimes the rules change fast. Every so often, Earth hits a hard reset with a catastrophe so massive that most species don't make it. When that happens, the survivors start the story again.

Extinction Events

Since the 1980s, scientists have agreed on five major mass extinctions. Many biologists now warn we're in the middle of a sixth extinction today. This one's different: It's caused by us.

No one knows exactly how many species share the planet. Our best guess is around 8.7 million, not counting microorganisms. For many of these species, the future is grim. One-third of plant and animal species could vanish within 50 years. If current trends hold, we could lose half of all known species by the end of this century.

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What Causes Mass Extinctions?

So, what triggers mass extinction? Scientists are still debating the exact recipe, but the usual suspects include:

Massive volcanic eruptions that choke the air and darken the skies.
Rapid climate shifts so extreme, ecosystems can't adapt.
Asteroid impacts that rearrange the biosphere in an afternoon.
Anoxic events that starve the oceans of oxygen.
The slow but relentless drift of continents, which change ocean currents and the climate over millions of years.

Earth, in short, has a habit of clearing the board. The difference today? The extinction-level threat has thumbs.

Activity

Mass Extinctions

Journey through the five major extinction periods on Earth

Ordovician Extinction

50 Estimated %
of species lost

All known life at the time existed in the seas and oceans, and marine invertebrates suffered the heaviest losses during this extinction.
Late Denovian Extinction

30 Estimated %
of species lost

Most marine groups were impacted, with reef ecosystems ceasing to exist completely.
End-Permian Extinction

60 Estimated %
of species lost

Nicknamed the Great Dying, this is believed to be the most intense extinction event, with marine and land animals greatly impacted.
End-Triassic Extinction

35 Estimated %
of species lost

Thriving mammal-like reptiles and large amphibians disappeared, as well as many dinosaur groups.
K-T Extinction

75 Estimated %
of species lost

Named for occurring at the boundary between the Cretaceous (K) and Tertiary (T) time periods, this event signaled the end of the dinosaurs.

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How Do We Know What Killed the Dinosaurs?

The history of Earth is written in rock layers. These layers have helped geologists solve the biggest whodunit in history: Why did the dinosaurs suddenly vanish?

The smoking gun was a massive crater buried underwater near Mexico's Yucatán Peninsula. The clues fell into place. A colossal asteroid struck Earth about 66 million years ago, triggering wildfires, darkness, and a mass extinction that wiped out three-quarters of all species.

It was the worst day in dinosaur history. But it was a lucky break for the small, adaptable mammals waiting in the wings—your ancestors.

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Chapter 3
The History of Life on Earth