Chapter 3

What makes life on Earth so special? How do you explain its diversity? And what exactly is it? How life emerged remains a mystery, but we know that it possesses four qualities: It can metabolize, self-regulate, reproduce, and adapt. We also know that life is fragile in the face of gradual and sudden changes to the environment. Just ask the dinosaurs.

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Chapter at a Glance
71 Minutes
1 Threshold
6 Videos
3 Galleries

Life Begins

From Chemicals to Consciousness

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As the Earth formed, it posed opportunities for new complexities not yet seen in the Universe. The prevailing hypothesis is that volcanic vents deep under the sea were spewing very hot, chemically rich compounds into the ocean, which led to the first microbial organisms that spread throughout the oceans — setting the stage for life to inhabit the Earth.

The great oxidation event

A huge number of prokaryotes rose from the ocean and flooded the atmosphere with oxygen, a by-product of photosynthesis. While poisonous for many species, new life forms thrived in the oxygen-rich atmosphere, which also protected life forms from the Sun's harmful ultraviolet rays.

The Earliest Life Forms

What Makes Life, Life

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As new complexities start to populate the air, sea, and land, it might be a good time to ask, "What's the difference between non-life and life?" What is the difference between a mountain and a whale? Both are made of molecules. Both engage in chemistry. And both change through time.

True life is about purpose. Living things are compelled to self-generate and self-maintain. They strive to pass genome copies on to offspring. Mountains are splendid, to be sure, but in the end they have no purpose. They just are.

For about 3.5 billion years life has been doing its thing on Earth. Hank and John Green take a Crash Course look at where the first single-celled organisms came from and how DNA keeps life going.

The Tremendous Diversity of Living Things

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While the young Earth was a dangerous and unfriendly place, germinating deep in the oceans were the perfect ingredients and conditions for the emergence of life. But what is life? Well, we know that life has four qualities:

  • Metabolism: the ability to take in energy from surroundings to keep going
  • Self-regulation: also known as "homeostasis," an organism's ability to regulate itself to maintain stability
  • Reproduction: the ability to create copies, allowing life to preserve itself and go on
  • Adaptation: the ability to change from generation to generation and become better suited to environments

Linking all these qualities together is DNA. It contains the instructions an organism needs to regenerate cells and pass traits to its offspring — providing the Earth with its diverse forms of life.

Life Adapts

The Origin of Species

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Poof and you're gone! Extinction events are no joke. The Greens and Emily Graslie talk about why they are such a big deal, and what they have to do with evolution.


A wide range of traits can be naturally selected for, depending on the environmental niche: for example, camouflage, burrowing skills, bright plumage, acute hearing, or the ability to retain water in desert climes. Yet sometimes events on Earth are so catastrophic that a large portion of its species may die off — making room for newly formed life.

Since the 1980s geologists and paleontologists have agreed upon five major extinction events. And today, many biologists agree that a sixth major extinction is currently underway. This one is unique — the result of humans degrading and destroying the habitats of other life forms. This extinction apparently began about 50,000 years ago when humans moved into Australia and then the Americas, causing the disappearance of many species.

No one knows how many species currently exist on Earth. The best estimate is about 8.7 million, not counting microorganisms. A 2003 study by the World Conservation Union suggested that over the next several decades, extinction threatens one in four mammals. If the present trend continues, biologists fear that we could lose 50 percent of all known living species by the end of this century.

What causes mass extinctions?

The discussion about what causes mass extinctions continues. While scientists do not yet fully understand the reasons, some of the possible explanations are:

  • Sudden massive volcanic activity, as evidenced by vast lava plains whose dates coincide with extinction events
  • Rapidly changing climate
  • Impact or multiple-impact events
  • Anoxic events (the middle or lower layers of ocean becoming deficient or lacking in oxygen)
  • Ever-changing position of oceans and continents (plate tectonics)


Mass Extinctions

Journey through the five major extinction periods on Earth

  • 600
  • 500
  • 400
  • 300
  • 200
  • 100
millions of years

Layers of History

Before dinosaurs perished 50 million years ago, they left us clues about their existence in rock records.

The history of our planet — along with clues about its future — is written in the layers of rock. Rock detectives — or as they prefer to be called, geologists — study these clues about the past and can also observe Earth's current changes firsthand.

The layers of rock laid down over the eons resemble the chapters of a book. These chapters accumulate from the sediment laid down on ocean floors, from the lava flowing from volcanoes, and from the uplift of mountains.

Geologists are carefully picking away at mountain cliffs, chipping rock in quarries, taking samples, analyzing, and speculating. They're reading layers, trying to piece together what happened here 100,000 years ago, 100 million years ago, even two billion years ago.

KT Boundary

Solving the Dinosaur Mystery

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It just may be the biggest murder mystery in history. How did the dinosaurs, some of the biggest and certainly one of the most ferocious species this planet has ever seen, become extinct?

The detective who solved the case is Walter Alvarez. While doing research on the K-T boundary in an unusual quarry in Italy's Apennine Mountains, he noticed an abundance of fossilized remains of tiny sea creatures in the limestone of the Cretaceous period, but not the Tertiary period. The scientist wondered: Could this be the result of a mass extinction event?

His suspicions grew when he saw that the dividing line of clay was dated around the same time that the dinosaurs went extinct. Also interesting were the off-the-charts levels of iridium, an element that's rare on Earth, but common in meteors.

Decades later, a break in the case.

Finally, in 1991, geologists came upon a site that showed what appeared to be the sediment from a massive tsunami 65 million years ago. They calculated that it came from a gigantic crater in the sea discovered near Mexico's Yucatán Peninsula. All the puzzle pieces fell into place. Alvarez's hypothesis about a cataclysmic event turned out to be right.

That K-T Boundary, revealing the impact of a colossal meteor, was bad news for the dinosaurs — and an estimated 75 percent of all species on Earth. But this cataclysmic event also created great conditions for mammals who contained the proper DNA qualities to adapt and thrive.

The crater of doom

The crater of doom

In 1950, oil geologists had noted the unusual features of a crater in the sea off of Mexico's Yucatán Peninsula. They thought it was a buried volcano. When the K-T boundary geologists learned about it, they were able to prove it was their meteor impact site.

DNA: Life's Little Instructions

Surviving through change

After a major extinction, the weakest DNA dies off, while strong DNA is retained. But how it happens still puzzled biochemists by the middle of the 20th century. They accepted that creatures evolved like Darwin suggested, but still needed to figure out how parent organisms pass traits to their offspring. Most acknowledged that DNA, short for deoxyribonucleic acid, delivered the instructions. Yet they were baffled about its structure and how it worked.

In 1953, three English biochemists were racing to determine the double helix structure of the DNA molecule. They collected knowledge from Linus Pauling, an American biochemist, who was using X-ray technology and had already successfully shown that the general shape of DNA must be a helix, or spiral.

At the University of London, Rosalind Franklin recorded very precise X-ray diffraction images showing the basic form of DNA. Two lab mates at Cambridge University, James Watson and Francis Crick, caught sight of Franklin's images and began building models of sheet metal and wire. They were able to pair up DNA's chemical bases and construct a chemically stable structure.

Crick, Watson, and Franklin

Crick, Watson, and Franklin

Franklin produced clear and accurate diffraction images of DNA crystals, which Watson and Crick used to pair two bases into a spiral staircase running in opposite directions — solving the complexity of DNA's structure.

Baffling DNA

The alphabet of life

The twist on life

The discovery of the double helix structure of DNA was a major step in understanding the complexities of life.

DNA consists of four main "bases" connecting two nucleotide strands in a double helix structure. The bases pair up: adenine (A) with thymine (T) and guanine (G) with cytosine (C) to connect the two strands, forming a spiral ladder. Different sequences of these four bases spell out the different genetic instructions for all life forms.

The alphabet of life

Chromosomes in a cell

In human cells, long sequences of DNA are contained in chromosomes. These chromosomes are packed inside the nucleus of the cell, protecting this important genetic information. DNA is so tightly wound within a chromosome that the 51 million to 245 million base pairs in one human chromosome are estimated to be up to two meters long unraveled.


Shared DNA

See the percentage of DNA these animals share with humans

The Tree of Life

3.5 Billion Years Old and Growing

The tree of life

It was first believed that humans represented the pinnacle of evolution. But as biologists continue to study life on Earth, and as new species are discovered and more evidence comes in, the Tree of Life will continue to grow.

At its center is LUCA, or the Last Universal Common Ancestor. It's believed it was a single-celled organism born more than 3.5 billion years ago. From there, the Tree of Life separates into more than 190 different species and three domains of life.

  • Eukaryota, all the plants, animals, fungi, and some single-celled organisms
  • Bacteria, single-celled organisms functioning without a membrane-enclosed cell nucleus
  • Archaea, single-celled organisms often living in extreme environmental conditions

Quiz: Threshold 5

Life On Earth

  • Ingredients
  • Goldilocks Conditions
  • New Complexity
  • Which of the following are critical to life emerging?

  • Which of the following is NOT a Goldilocks Condition needed for life on Earth to emerge?

  • Which of the following is NOT a new ability of newly complex organisms?


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