WATCH: How Did Life Begin and Change?

How many different species of living organisms do you think exist on our tiny planet? The truth is we don't really know. Estimates range from perhaps five million to perhaps as many as 30 million, and it could be a lot more than that. That's staggering variety. And that's only a tiny fraction of all the species that have existed in the history of life which extends over almost four billion years. Yeah, life really is mysterious, but there's actually quite a lot we know about it. We've seen that... we know some of the main characteristics of life, and we also have a pretty good idea of how and why life changes over time. Now, what we're going to do in this unit is we're going to survey the history of life over almost four billion years. We'll begin with the origins of life. That, remember, is our first... fifth major threshold in this course. And then what we're going to do is we're going to look at a series of smaller turning points. We call them mini-thresholds. These are times when something new appears that seems to be slightly more complex than the things that existed before. And all of this, of course, is pointing to the creation of our own species, Homo sapiens. We begin with the origins of life. Now remember, this is the fifth major threshold of increasing complexity in our course, and let me remind you what we mean by thresholds of increasing complexity. At each of these thresholds, something new seems to appear in the Universe, something with entirely new qualities. Now, each time we've crossed these thresholds, we've asked similar questions. So when we talked about the first threshold, the creation of the Universe, or the creation of stars or the creation of new chemical elements, or the creation of planets, we asked, "What were the Goldilocks conditions that made it possible to cross that threshold?" So let's ask the same question about life. Now, with life, there's a problem because biologists suspect that there's life all through the Universe, but the truth is we don't know. The only place in which we're sure life appeared is on our planet. So that's the only place we can really study it. So let's rephrase the question. What were the Goldilocks conditions for the origins of life on Earth? Now, we can start with the fact that living organisms consist of very, very complex chemicals. They're not just large; they're also organized in very precise structures, whereas non-living things consist of very simple molecules. So to get life, you need environments in which you can do really exotic, really interesting, really elaborate chemistry. So where do you find conditions such that atoms can combine in all sorts of exotic forms? Let's recall that in space, you can do simple chemistry-- atoms can combine to form molecules with perhaps ten, 20, 30, never more than 100 atoms. But rocky planets like our Earth, it turns out, are wonderful environments for good chemistry. There are three reasons for this. The first is that they contain a great diversity of different elements. And above all, they contain those elements that you need for organic life. The crucial ones are carbon... There's lots of hydrogen, of course. There's also oxygen, nitrogen. These are all elements that were formed in dying stars, and also a bit of phosphorous and sulfur. The second Goldilocks condition is energy. But this is subtle. You mustn't have too much. If there's too much, you blast complex molecules apart. But you mustn't have too little. If there's too little, there's not the energy for atoms to combine. The Earth was perfect. It was near a star so it had energy, but not too much. And it also had energy coming up from its hot, molten core. Now, the third Goldilocks condition is slightly subtler. It's the presence of liquids and, above all, of water. Why? Well, think about it. In gases, atoms are moving around incredibly fast and it's very hard for them to link up. But in solids, atoms are stuck. They're stuck in a grid, like bad traffic, like a traffic jam. But in liquids, they can cruise past each other so it's much easier for them to hitch up in all sorts of complex forms. So here are our three Goldilocks conditions. The early Earth was almost perfect for elaborate chemistry. We've seen that all these Goldilocks conditions existed on the early Earth. Probably the ideal place for elaborate chemistry was deep beneath the oceans at cracks in the Earth's crust, mid-oceanic vents. These were ideal because you've got lots of chemicals seeping up from the mantle and you've also got lots of energy. And we know now that under these conditions, it's fairly simple to create the simplest organic molecules that appear in all living organisms. I'm talking about molecules with just ten, 20, 30 atoms in them such as amino acids, which are the basis of all proteins, or nucleotides, which are the basis of DNA. We also know that it's not too hard under these conditions to string those simple molecules together in huge chains to form proteins and nucleotide molecules. What gets tricky is the next stage. How, with lots of these large molecules, to bring them together to form cells with membranes-- they're like skins-- and also with DNA at their center. But most biologists are pretty confident that though there's still some mystery here, they'll work it out.