4 billion years ago something very strange happened on this planet we call it life and I dare any person any of you to tell me that it isn't the most interesting thing that has ever happened thankfully for all of us it's still happening it's happening as much if not more than ever it's squishy slippery and slimy and sticky and spiky and you'll find all the shapes moss-shaped mosquito-shaped Manatee shaped u-shaped and all the sizes too from Tiny to tremendous and everything in between and biology is the study of this thing we call life we study biology for a whole bunch of different reasons from the obvious like making new medicines to the not so obvious like learning how to identify misinformation and we use biology to describe anything that life does quick are you breathing right now that is a biological process life does a lot of stuff but it's not so easy to pin down what life is like okay for sure that ant is alive it responds to its environment like the crummit just found and it must reproduce based on how many of these ants I see coming right now but other things respond to their environment too like if that's our definition is fire alive is a computer virus alive is the robot vacuum I call my personal Butler alive no I'm a bit biased as a biologist and a living thing but life is the most interesting thing to have ever happened on Earth and we are not even sure what it is hi I'm Dr Sammy and this is Crash Course biology wait am I alive but I I am alive right because we just introduced some serious uncertainty here [Music] life feels like something you know when you see it but humans have wrestled with how to define it for centuries like the ancient Greek philosopher Aristotle thought the ability to grow reproduce and react to inner and outer forces set life apart from non-life he was off track with some other observations like he thought that women have fewer teeth than men and that eels are made of mud but on the subject of life he was really on to something modern day biologists tend to agree that life involves a state of chemical balance that reproduces and evolves over Generations in fact that's also the definition that Nasa uses should they ever see signs of such a thing beyond Earth a self-sustaining chemical system capable of evolution but for now here on Earth generations of scientists have developed a list of seven characteristics that sort the stones not alive from the stone fish definitely alive we know you're there stonefish you can't trick us no matter how good that disguise is first living things keep their inner conditions steady as outer conditions change and that's called regulation like on a scorching hot day when I am dripping sweat and my dog is panting our bodies are using two different strategies for the same life-sustaining goal regulating our temperature second living things respond to their environment that includes Dramatics like when a fast and furious cheetah Sprints after a gazelle or house cats imagining that they're cheetahs but they're actually batting at house flies but it's also the slow motion stuff of the plant world like the turn of a flower towards the Sun or a Vine twisting on a branch third living things reproduce passing on genetic information to their offspring and that includes the bouncing baby giraffe who inherited her father's eyelashes and their mother's extra thick tail hair but it also includes a single-celled yeast splitting in two making more of itself in its own image oh yeasty the 52nd carrying on the family name fourth living things also grow and develop based on the instructions in their genes reading those instructions triggers a tadpole to turn into a frog or a teenage boy's voice to change unfortunately jeans can't prevent those instructions from being read the day before his solo of Oh Holy Night terrible timing there when when all in one night your voice goes from Mariah Carey to Barry White oh yeah baby it's a bit of a mess fifth all living things process energy to do their well living things like my body is using nutrients from a Greek salad right now to breathe pump blood and talk to you and all that energy was processed by the living things that supplied my lunch so this episode of Crash Course is brought to you in part by a lettuce plant an olive tree and oddly enough the milk of a sheep sixth life is organized even in living things that seem chaotically arranged to humanize I'm looking at you platypus yeah even those weirdos have a sense of order structuring their bodies from the cells up those groups of cells arranged into tissues organs and organ systems keep whole organisms functioning and last on the list living things have adaptations shaped by a history of evolution these adaptations are traits that help organisms survive and reproduce individuals that survive and reproduce pass on their genes and traits associated with them that's how the Platypus ended up with such a weird cool body that that includes not just webbed feet but also venomous Spurs to battle their Rivals like what are you even doing with those buddy you have almost no native predators don't you point those in me wander around with this checklist of life and you can find evidence of it in lots of interesting forms Beatles ferns algae yourself your neighbor I mean please ask first before looking for that evidence don't don't spy on your neighbor and at the same time some of these traits can be found in non-living things too like take snowflakes definitely organized but they don't process energy or think about fire it grows but it can't reproduce and then there are some real edge cases the gray areas where our definition of what's living and what's not really gets pushed to the Limit let's head to the thoughtful meet the virus a tiny bundle of genes in a protein jacket even smaller than a cell on their own in their little protein jackets viruses just kind of hang out they can't reproduce they can't grow they cannot process energy they don't even regulate themselves because nothing's really happening they're like really complicated dust but when those jackets Come Off watch out viruses seem wildly alive their genetic information replicates they spread between living things multiplying to infect other cells and just like cockroaches cactuses and chimpanzees they evolve over many generations but there is a catch a virus can't do any of those things without first taking over a host cell they can't take action to infect they have no system for responding to their environment they are totally dependent on bumping into the right cell at the right time behind every life-like virus is a cell that has been commandeered to do all the work in fact viruses like rabies direct their hosts so convincingly that 19th century scientists thought that they were among the simplest forms of life but today most biologists would say viruses aren't alive and I'd like to emphasize most instead viruses sit alongside life in a way they borrow life and yet we also know that viruses have been around a long time almost as long as cells themselves they've evolved to infect everything from single-celled amoeba to 150 ton blue whales so as life has evolved viruses have been along for a lot of the ride thanks thought bubble of course all that said we don't know what life might look like outside of planet Earth in fact there's a whole field of biology called astrobiology devoted to thinking up what extraterrestrial life might look like by examining some of the most extreme forms of life here on Earth biologists might one day decide to add to the list or tweak it depending on what that non-terrestrial life looks like when it's discovered but for now our planet is the only one that we know sustains life ultimately there is still room for debate here our qualifications for life let us draw a circle that keeps life inside and non-life outside but don't be surprised if it's the kind of thing that human thinking continues to change on especially when and if we get better at making things that may or may not be alive debating what does and doesn't count as life might seem kind of up in the clouds but hey if we didn't how else will we know that there are living microbes up in those clouds that affect the weather that's right biology is everywhere it is responsible for the medicine that you take when you get a headache and for the vaccines that protect you from serious illness it's in the cotton of your T-shirt and the Apple in your lunch both brought to you by plants that have captured the energy from the Sun and guess what biology is in you it's in the organs and cells and tissues that make everything about you possible whether that's sneezing or laughing or digesting lunch writing a poem or riding a bike and it doesn't end there no matter what size or shape life takes every living thing is connected to every other living thing every bug bat and bacterium shares a common ancestor a single-celled organism that lived about 4 billion years ago but it's not just Evolution that connects us the very molecules that make up our bodies were born in the heart of a fiery star long before earth was formed and that same Stardust will move on after we're gone to make up new ones in a way you have just as much claim to the title of Heavenly Body as Jupiter or Venus this interconnectedness can be mind-blowing and perspective changing but it's not just something to Marvel at in the abstract it's also a practical puzzle piece that helps us understand ourselves and our future like by knowing that we're connected to mice on the family tree ethically studying them can help us find cures for human diseases and because life is connected by big biological processes we can understand how our actions affect the water the land the climate and in turn us by showing us life's interconnectedness biology can help us and is helping us find solutions to some of our biggest challenges like hunger disease and climate change so yeah biology is everywhere because life is all around us and by highlighting the Wild web of connections between you and every other living thing biology helps us understand ourselves and each other and hopefully create a better future for all kinds of life when biologists and really people of all kinds ask seemingly philosophical questions like what is life and how is everything connected or practical questions like how do cancer cells reproduce and what are the effects of climate change on ecosystems they're participating in something bigger something we call the scientific process but we'll talk more about that next time this series was produced in collaboration with hhmi biointeractive if you're an educator visit biointeractive.org crash course for classroom resources and professional development related to the topics covered in this course thanks for watching this episode of Crash Course biology which was made with the help of all of these nice people if you want to help keep crash course free for everyone forever you can join our community on patreon [Music]