The big question of this segment is where did life come from? [MUSIC] Different people have different ideas about where life first arose. Some people think it occurred in a tidal pool, other people think that it occurred at deep sea volcanic events. No one's really sure. So, what was the early Earth like? Well, as far as we know, the early Earth was bombarded with meteors up until about 3.9 billion years ago. That made it really difficult for life to develop because the Earth was continually being smashed up by asteroids from outer space. But by about 3.8 billion years ago, we have the first chemical signatures of life on the planet. What were conditions like in that 100 million years? Well we know there was lots of volcanic activity, and volcanoes spew out gases, and that makes the atmosphere. So the early atmosphere was composed, probably, of carbon dioxide, nitrogen, ammonia, methane, and water vapor. Importantly, there was no oxygen in this atmosphere. Oxygen's a highly reactive gas, and it tends to degrade complex compounds. Think about when you cut an apple or a potato, and it turns brown. That's oxygen at work. Where there's no oxygen, complex organic molecules can accumulate and they're stable. So we can end up with large concentrations of them in the early ocean. One of the key pieces of evidence for how life arose on Earth was an experiment performed by Stanley Miller in 1953. He assembled an apparatus like this one and filled it with gases that formed with the early atmosphere. He then boiled water in the apparatus to make steam, and circulated that through an electric spark which simulated lightning. He cycled that for several weeks, and after that time, a brown sludge appeared in the bottom of the glassware. When he analyzed that sludge, it contained sugars, the precursors for RNA and DNA, and it contained amino acid. We know that this experiment reflects natural processes and the same kinds of compounds can accumulate even in outer space. Sometimes, meteorites fall to Earth and when we look inside them, we find thousands of complex organic compounds just like the ones that were made in the Miller experiment. What this means is that early oceans became a concentrated soup of literally billions of different organic compounds. Over 100 million years, these building blocks joined up to form the complex molecules that we know in living things today. So amino acids joined up to make proteins. Sugars joined up to make polysaccharides. And most importantly, nucleic acids formed. That's DNA and RNA. One of the important properties of nucleic acids is that they're self complementary. What that means, is that a single strand of nucleic acid, in other words, one single strand, can direct the synthesis of it's partner strand. Kind of like a zipper. That complementary strand, which the other half of the zipper, can now separate from the original. Now you've got two single strands, each of those can make a complementary strand as well. That's replication and that's the basis of life. Life is based on cells, so where did cells come from? I told you before that amino acids join up to make proteins. It turns out that early proteins in the prebiotic soup could associate with other complex molecules, and if you heat them and cool them, they spontaneously form up into what are called microspheres. These are little spherical bodies that have cell-like properties. They can accumulate material from the primitive ocean. When they do that, they grow. When they get too big, they can divide. These are just like cells. Now what I want you to imagine is that one of these microspheres forms around an RNA molecule. What you've then got is a self-replicating molecule inside a cell that can grow by accumulation. When the cell body now divides, a new RNA molecule can go into each cell. One of the things about RNA is that it can actually perform enzyme-like activity. That means take material from the prebiotic soup and stitch it together into other molecules or convert them. So if you have an RNA molecule inside a microsphere that can accumulate material from the prebiotic ocean, that's the first metabolism. And we're well on our way to the first living, dividing cell. Eventually, over the 100 million years between 3.9 and 3.8 billion years ago, the replication and information storage functions of RNA were replaced by the molecule that all living things now have called DNA. That individual cell is what we call the last universal common ancestor. That first kind of cell probably gained energy and materials by harvesting the organic molecules that were present in the early ocean. Errors in replicating that DNA generate diversity. That leads to altered functions in the cells, resulting from that replication. That diversity provides the food for natural selection. Natural selection can act to favor cells that grow faster or can use more diverse compounds, and that is evolution. Once evolution kicks into gear, you can get more and more complex cells including the cells that harvest the sun's energy. In photosynthesis, that generates oxygen that gradually changes the atmosphere to the one that we know today. The processes that I've described of generating these organic compounds are the simple result of heat and lightning working on volcanic gases. These properties are the same all over the universe. That means that life is likely to arise on planets that have similar conditions to the early Earth. One day, we might observe life actually forming on a planet, and that's going to confirm all the ideas that I've been outlining in this segment. [MUSIC]
