WATCH: Unit 5 Overview - Life

Confession time. I don't fully understand DNA. But you can't hold that against me, there's a lot of it. Each human cell has about six feet of DNA.   If you took the DNA from a single person and stretched it out it would be billions of miles long. So, it's a complicated topic, all right. Still, I do know that we share 90% of our  DNA with a cat and 85% with a mouse. Ugh! But don't judge Mr. Tibblesworth too harshly. We do the same thing! Like eating a hamburger. Yeah, that cow shared 80% of your DNA. And.... What was that other thing? [cell phone rings] Yellow? Ah, that's right! Thanks. Sixty percent of banana DNA is the same as ours. The point is, everything that's alive has a lot in common. [instrumental music] Hi, I'm Rachel Hansen. And this is Unit  5, life. and don't worry eating a banana   And don't worry, eating a banana doesn't make you a cannibal.   Those DNA percentages are a little misleading. Of course, we share DNA with all these other forms of life.   We all evolved from a common multicellular eukaryote ancestor. But, why do we care about DNA in a history course? Well, our understanding of DNA has had some big consequences for the organization of human communities.   Throughout our history, many people tried to make others believe that some groups of  people were inferior or fundamentally different   based on things like skin color or place of birth.   But, pick any two humans, say you and me, the genetic similarity between us is 99.9%.   That remaining 0.1% percent, that's the tiny sliver of our genes that changes stuff like eye color, height,  and whether or not you think cilantro is gross. We're all part of the same species. The overwhelming majority of scientists agree that there is no genetic evidence to support the  idea of separate races. Race is a category created and given significance by human societies, not by our DNA.  How do we know this about ourselves  and about our plant and animal relatives? Collective learning, of course.   From ancient taxonomies to Charles Darwin, we've continued  to improve our understanding of life on earth. Beginning in the 1990s, scientists from  dozens of different countries collaborated   to build the Human Genome Project a  13-year study that mapped human DNA.   Mapping the human genome has helped scientists  understand why some people are more at risk of certain diseases. It's helped us understand how our species evolved. And it's confirmed that humans might be individually very different,  but on a genetic level we share much more in common. Understanding the grand story of life  on earth from its single-celled origins to you   helps us understand our human community  and our connections to all life on earth. Armed with this information, perhaps we can  begin building better more fair societies. [instrumental music] In Unit 4 we investigated how we went from a  supernova explosion somewhere in the milky way   galaxy to the creation of our solar system and earth. Dying stars like this supernova generated new chemical elements. Those new elements began  spinning under the force of gravity to eventually come together in a series of violent collisions  to form our solar system. Next, we traveled to the  Haitian Eon, when an early earth was a lava-covered  asteroid-pelted noxious gas-ridden rock.   We also learned why plate tectonics make earth so special,  giving us soaring mountains and dramatic volcanoes. The movement of plates also created the initial  ingredients for the evolution and diversity of   life that we see today.  Finally, we examined how  collective learning helped us figure all this out. Scholars like the ancient  natural philosopher Eratosthenes; geologists like Charles Lyle, Harry Hess, and  Walter Alvarez; meteorologists like Alfred Wegener; and many others contributed to our knowledge of  the origins of our solar system and our planet. The only place in the universe  that we know can support life. [instrumental music] And that brings us to a new  threshold of increasing complexity: life! What is life? Other than a terrible board game  from the 1960s. Well, that's a complicated question without a super clear scientific agreement.   But, for this course, we define life as something that has four characteristics: reproduction, the ability to  replicate; homeostasis, the ability to self-regulate-- that basically means continue living-- adaptation, the ability to respond to the environment; and metabolism, the ability to process energy.   Oh, and we can't forget about DNA and RNA all living organisms, including you and your lettuce, contain this blueprint for life. To emerge on earth life needed new complex chemical compounds  like DNA and RNA, just the right amount of energy and water. This we know. What we don't  know exactly is how life sprang from, well, not life. The most accepted theory is that these  complex chemical compounds made their way to deep   oceanic vents, where lava spewed from the earth's  mantle, into the oceans, thanks to plate tectonics. Here the lava escaped from  fissures in earth's crust, mixed with the complex chemicals in water  and formed single-celled organisms called prokaryotes. So, how do we get from these basic  single-celled organisms to us?  Well, that required a few billion years of trial and error called  evolution, which did not always go smoothly. For example, those ocean-dwelling prokaryotes  gradually moved from the deep ocean to the surface. As surface dwellers, prokaryotes evolved to  take energy from the sun through photosynthesis, which also meant they changed earth's atmosphere by releasing oxygen. The atmosphere became so oxygen rich   it triggered an event scientists call  the oxygen holocaust as tons of bacteria went extinct. This is just one example of how life and  earth have interacted over the last four billion years. There are plenty of others. Volcanoes have erupted and asteroids have impacted the earth setting off other mass extinctions like  the one that killed the dinosaurs.   Today, humans burning fossil fuels have already launched a new  mass extinction event as climate change worsens   How do we know about the origins of life, how  it evolved, and how life and earth affect each other? Well, as we like to do here at the  big history project we'll ask the experts, starting with biologist Charles Darwin  and his writings on natural selection. Then, we'll investigate how chemists like Rosalind Franklin built upon Darwin's theory and provided   even more evidence for how life evolves including  the structure of DNA.   Pay attention in this unit. Well, pay attention in all of them, but this one  sets us up for a really important threshold, the one involving you and me.   And without the emergence of our ancient single-celled ancestors there'd be no you, no me, no cat, or mouse,  no banana.   That's an important fact to keep in mind as we think about our place in the grand scheme of  life on our planet and in a universe of complexity. Great, the battery's dead.