[SciShow intro plays]   Michael: Where did life come from, and how did it come to this? The path from bacteria to baseball wasn’t simple. In this miniseries, we’re going to explore the evolution of life on Earth.   It’s been a wild journey, with plenty of twists and turns, and there were lots of times we almost didn’t make it through. This first episode is about two of the earliest geological eons: the Archean and the Proterozoic.   Eons are the second-largest way to divide up Earth’s history, after supereons, and there was one geological eon before the Archean: the Hadean, which was 4. 5 to 4 billion years ago. But because Earth’s rocks are constantly being destroyed and reshaped, literally all the rocks that old are gone. It’s hard to have geology without rocks, so we’re going to start with the Archean.   The Archean began about 4 billion years ago, and ended around 2. 5 billion years ago when the chemistry of rocks began to change, and plate tectonics -- the way Earth’s crust moves around -- started to become more of a thing. That kicked off the Proterozoic Eon, which continued until 542 million years ago. These two eons are where life began and developed in complexity, though it took its sweet time about it. It’s also the time when life transformed the atmosphere, making it more suitable for some living things--but more dangerous for others.   Earth and the creatures that live on it have a complex relationship: we’ve influenced each other. The geology of the planet has guided the evolution of life, and life has shaped Earth just by existing. So Earth’s geology during the Archean shaped what were probably the first forms of life, but you wouldn’t recognize the planet.   The atmosphere contained lots of methane, ammonia, hydrogen, and carbon dioxide -- a type of chemical mixture that’s called a reducing environment, which means it can make electrons available for chemical reactions. That was probably good news for the molecules that would eventually turn into biological molecules -- they were able to react with each other and start becoming more complex.   Carbon dioxide and methane are greenhouse gases, so the Earth was much hotter then, even though the sun was younger and fainter. In fact, there’s no evidence for polar ice caps or glaciers in the Archean. It was too hot for ice.   Meanwhile, the continents were solidifying. Plate tectonics have shuffled them around a whole bunch since then, but the innermost rocky centers of most of today’s continents date to the Archean. Volcanoes belched carbon compounds and water into the atmosphere.   The oceans condensed pretty quickly from that water. And because there was almost no oxygen in the atmosphere, there was no ozone layer. UV radiation might have been pretty intense. If you hopped in the TARDIS and time-traveled to Archean Earth, you might think you were on Venus. That’s very different from the temperate climate and nitrogen-oxygen atmosphere we have now. So what changed?   Well, life happened. At some point in the warm oceans and carbon-rich atmosphere, biological molecules that contained the necessary information to copy themselves, and had the chemical ability to do so, formed from organic compounds. These became encapsulated in an oily membrane that kept them safe from the outside world -- the first things to resemble a living cell. And eventually, this early life transformed the atmosphere and climate.   Some scientists believe the earliest biological molecules were RNA, the molecule our DNA now uses to send messages outside a cell’s nucleus. It’s called the RNA world hypothesis. RNA is very similar to DNA, but it’s easier to form from simple components. It also stores genetic information the same way DNA does, but can tangle itself into shapes that make it easier for chemical reactions to happen, the way proteins do in our cells now. The RNA world hypothesis would explain why RNA is the go-between for DNA and protein in our cells. But some scientists argue that it would have been too complicated for life to switch over from RNA to DNA and protein, and it’s more likely that all three evolved together.   Whether life started as an RNA world or not, by the time the common ancestor of everything alive today came along, the system was based on DNA. The earliest fossils show that life, in the form of bacteria, existed 3. 5 billion years ago, but a recent finding shows life could be even older than that. A team of researchers from California found evidence of life that’s 4. 1 billion years old.   Now, I know I just said that there aren’t any rocks older than 4 billion years. And there aren’t, which means there aren’t any fossils either. But the team found resilient little crystals called zircons, which can be preserved when the rock surrounding them is destroyed.   Then they get incorporated into new rocks. The zircons in question, found in Australia, contain traces of 4.1 billion year old carbon. There are lighter and heavier forms of carbon, called isotopes, and living things tend to have more of the lighter ones compared to the heavier ones. And so do these zircons.   For many scientists, it’s hard to believe life could possibly be that old. For one thing, Earth was pelted by asteroids 3.8 billion years ago in what was called the Late Heavy Bombardment. Next time you look up at the moon, check out the craters, a lot of them come from the Late Heavy Bombardment.   And as you can probably guess from what happened to the dinosaurs, asteroid impacts aren’t great for the survival of living things. But some analyses suggest that the Late Heavy Bombardment would have only wiped out most life, not all of it, so if there were living things that old, some of them could have survived. Either way, we’re more confident about those fossils that are 3. 5 billion years old. They’re called stromatolites, and they’re made up of layers laid down by films of bacteria.   Stromatolites are pretty uncommon today, although you can find them living in Australia’s Shark Bay. But pretty much up until the time grazing animals evolved half a billion years ago and started eating all the bacteria lying around, they were an extremely widespread form of life. That means they basically ruled Earth for a solid three billion years.   They may look like slimy lumps of rock, but stromatolites are actually kind of incredible. Even if life only goes back 3. 5 billion years, that’s still pretty amazing, because it means that Earth was about a billion years old when life first emerged. That’s not a whole lot of time, especially since the continents and oceans and stuff were still forming. So it wasn’t that hard for life to get started. The hard part was staying alive once there was oxygen everywhere.   A few kinds of microorganisms make stromatolites. One of them is cyanobacteria. And cyanobacteria are pretty special. Instead of acquiring energy from their environments, like the other first organisms, cyanobacteria could capture energy from the sun. They could photosynthesize. Photosynthesis seems to have evolved in the Archean, but didn’t really kick into high gear until early in the Proterozoic. That’s because the water cycle needed time to work on the brand new continents.   Water weathers continents. It carries sediment into the sea, and that creates a shallow region around the shore: the continental shelf. The continental shelf is great for photosynthesis because it’s shallow and gets plenty of sun. But photosynthesis has a nasty, chemically voracious, toxic byproduct: Oxygen, one of the most greedy, electron-stripping elements on the periodic table.   It reacts with practically anything. It’s where we get the word oxidizing, for chemically stealing electrons. Once continental shelves formed, the cyanobacteria started pumping out oxygen like there was no tomorrow. And for many of the anaerobic, or oxygen-intolerant, life forms on the planet, there was no tomorrow. They weren’t used to oxygen and couldn’t handle it. It was poison to them -- it reacted with and destroyed them. All that oxygen even changed the rocks, it reacted with iron that had been dissolved in the oceans, laying down bands of iron ore that we still find today. The Earth basically rusted.   But there was another problem. The cyanobacteria also pulled those carbon-containing greenhouse gases out of the atmosphere. Earth’s temperature plunged. In the early Proterozoic -- and again near the end, maybe twice -- the planet became a snowball.   Life nearly poisoned and then froze itself to death. Cyanobacteria changed the atmosphere so much they destabilized the climate and changed the composition of the Earth itself. Eventually, the volcanoes coughed up more greenhouse gases and the snowball thawed out. Cold-tolerant forms of life managed to survive the crisis. And some organisms learned to not only put up with oxygen, but to use it to make energy. They became aerobic life forms, but many anaerobic organisms had to either find a place to hide or snuff it.   Life is easy, but surviving is hard. And so is becoming any more complex than a single, simple cell. Life stayed single-celled for 2 billion years or so, and it only changed because something really weird happened. That something weird was endosymbiosis.   Sometime in the Proterozoic, before 2.1 billion years ago, an anaerobic cell ate an aerobic bacterium, one that used oxygen to produce energy. And the bigger cell never digested the smaller one. The smaller one kept on living, using oxygen and producing energy, and there was so much surplus energy that the bigger cell benefited from having the little guy in there.   Eukaryotic cells -- the ones with nuclei -- have the descendants of these aerobic bacteria inside them. They’re our mitochondria. That convenient energy arrangement paved the way for life to become more complex, and all because of a weird fluke. Except... maybe it wasn’t that weird, because it happened again. The ancestor of plants engulfed a cyanobacterium, which kept right on doing its photosynthesis thing, and now plants have chloroplasts.   The earliest evidence for multicellular life is 2.1 billion years old. But it could have just been a colony of single-celled organisms. The earliest multicellular eukaryotes we can be fairly confident about are 1.5 billion years old. Animals didn’t show up until 600-800 million years ago. Late in the Proterozoic and into the next geologic time period, animal life established the groundwork for every kind of animal body plan that still exists today.   And that’s probably a good place to pick up next time: the start of the Phanerozoic Eon, which includes the animals’ rise to power, and the biggest extinction of all time.   Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon. If you want to help support this show, just go to Patreon.com/SciShow. 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