Chapter 3
Life

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.

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.

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.

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)

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

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.

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