The big question for this segment is, what is Threshold 5? Before Threshold 5, there was no life, after Threshold 5 there was life, that's hugely important in this story. [MUSIC] So far our story's been about things that are not alive. They're interesting, they're really interesting, but they're not alive. Threshold 5 brings the emergence of life. So what is life? What's new about life? What makes a living wiggling thing different from a rock? There's no perfect answer to this question, but we know of three things that are extremely important in all life and they are metabolism, reproduction and adaptation. So let me talk about each of these. First, metabolism. Life seems to be much more complex than non life. Even the simplest life form depends on exquisitely precise arrangements of billions of molecules. Everything has to be in exactly the right place. Maintaining these precise structures requires huge flows of energy. So all living things seem to be able to mobilize energy, often in the form of food, from their surroundings. This is metabolism. The way living things seem to seek out energy, makes living things seem purposeful in a way that rocks don't seem purposeful. The second quality of all living things is reproduction. The ability to make copies of themselves, that living things carry information about the way they're constructed. In almost all organisms, that information is carried in the huge helix like molecule with billions of atoms that we call DNA. What's more, that information can be copied so that new copies of each organism can be made. The individuals die, but the templates preserving the information survive, creating multiple copies, and it's those copies that form what we call species. The third crucial quality of all living things is adaptation. This is really a quality not of individuals, but of whole species. As living things make copies of themselves, those copies can change slightly through errors, tiny errors in copying. If some of these variants survive, then you have a slightly different type of organism. This means that living organisms seem to keep changing as their environments change. Now let's talk about evolution or natural selection. How does life adapt, how do species adapt over time? Taken together these three mechanisms I've described explain why life has changed and diversified so much over time, in the process we call evolution. It was Charles Darwin in the 19th century who first realized that over time, only those organisms would survive that were well adapted to their environment. They in turn, would have offspring that would look like them. It was as if the natural environment was selecting which individuals could reproduce. Just as a dog breeder selects which pups will reproduce. But if the environment changes, then it will select slightly different qualities. Darwin realized that such changes over vast periods of time, over millions even billions of years, could explain the appearance of millions of different species. His book, Origin of Species, was published in 1859 and it marks one of the greatest of all scientific breakthroughs. Darwin's work helped us explain how life changes and adapts, but how did the first living organisms appear? That remains a difficult question. Living organisms are really huge structures built from information, energy flows, and very complex chemistry. Now we know that the early Earth was a great place for chemistry. It had a huge diversity of different elements. It had heat from its core and also from the Sun, and above all it had liquid water. Now why is liquid water so important? Well, in solids atoms are stuck, they can't move. In gases they move too fast to hook up. But in liquids such as water, they can cruise past each other and sometimes they can hitch up and form larger molecules, but exactly how the first cells appeared remains a bit of a mystery. The crucial chemicals for life are first amino acids, which make life's basic building blocks, proteins. Secondly, nucleic acids, which make up DNA and the other molecules that take information from DNA into each cell. And thirdly, phospholipids. Which make the semi porous membranes that protect each cell from the outside world, while allowing it to take in food and excrete wastes. We know that these simple molecules could have been made fairly easily in the oceans and seashores of the early Earth. We also know how they could have combined into much larger, chain like molecules. Like the molecules we see today in proteins, or in nucleotides. But biologists still struggle to explain the appearance of life's software DNA, and how DNA, or it's simpler but more versatile relative RNA, began to control life processes. Okay so let's try and summarize what's happened with Threshold 5. Threshold 5 clearly represents an increase in complexity because living things are clearly more complex than non living things. They're structurally much more complex, fantastically intricate structures. But also they have this astonishing capacity to adapt to their environments, to slowly change over time, so as to fit in to their environments. What are the energy flows associated with life? Well, those are associated with metabolism. Living organisms seek out the energy and resources they need to survive, and that's what metabolism does. What are the Goldilocks conditions for the creation of life? Well. The Goldilocks conditions seem to have existed on our early Earth. We can list several of them. First, you've got a great chemical diversity. The early Earth we've seen, contained a vast diversity of chemical elements. And secondly, you've got gentle energy flows, heat from the center of the Earth and heat also from the Sun. These energy flows are not so violent that they prevent complex chemistry. Third, you've got water, a fabulous environment for chemical processes to occur, for molecules to combine in more and more complex combinations. There's one more factor. There was no free oxygen in the atmosphere of the early Earth, and that may be critical, because oxygen is incredibly disruptive. It's the chemical that makes fire for example, so it's likely that if there'd been free oxygen, it would have disrupted early organisms, but there wasn't free oxygen on the early Earth. So those are the conditions, Goldilocks conditions, for early life. And finally, what emerged at Threshold 5? Well, what emerged was life. The possibility frankly, of you and me. [MUSIC]
