Why You Wouldn't Exist Without Viruses

this episode of real science is brought to you by skillshare home to thousands of classes that can teach you a new life skill in a year when a single virus has killed millions it's hard to think of viruses as anything other than bad our understanding of them is largely based around the death and destruction they cause hiv has caused the deaths of over 33 million people since the start of that epidemic in the 80s the ebola virus kills up to 90 percent of people who get infected with it in a gruesome and awful way polio still paralyzes people zika dengue yellow fever still ravage the tropics so from where we're standing it very much seems that viruses are not our friend there are an estimated quadrillion quadrillion individual viruses 10 to the 31st power that exist on this earth with us more than all the stars in the universe but they aren't all bad only a small few have the ability to cause us harm and the vast majority don't affect us at all but over time some have become beneficial to us in fact a part of us viruses have been infecting us and our ancestors for as long as we've been around and around a hundred million years ago our mammalian ancestor was infected with a virus that would shape the tree of evolution from that point on the traces of this infection and many others exist as viral fossils within our genome around eight percent of our dna is not human at all but viral and in recent years scientists have started to see that these viral gene sequences are not simply leftover genetic baggage strange and purposeless but are sequences that code for essential proteins in human development how can it be that our evolution depended on a virus getting into our genome how did these viral segments of dna become a part of us and what do they do for us that's so important no one knows for sure where viruses came from whether they existed before any cells did assembling themselves from molecules in the environment or whether they are derived from cellular life leaking out of or being reduced from a cellular existence but regardless of this chicken and egg scenario viruses are perfect parasites they are tiny particles of genetic material enclosed in a protein coating and their existence depends on invading cells to hijack the cellular machinery to replicate themselves to invade a cell a virus recognizes and binds to the cell via a receptor molecule on the cell surface and then breaks in once inside the viral genes are expressed viral proteins are created and new virus particles are assembled and are ready to be released in some cases the exiting viruses leave the host cell intact so it can continue cranking out more virus particles but sometimes the viruses cause the host cell to burst killing it the viruses then float within the extracellular space poised to infect more cells this is likely the story of viral infection that you've heard it's the primary mechanism of spread for most viruses but some viruses like hiv measles and the herpes simplex virus can also spread directly between two cells that are in contact with each other in a process called cell to cell transmission this is particularly insidious as it enables viruses to evade the body's immune response viruses that can spread in this way have a few mechanisms to achieve it perhaps most notably genes that code for proteins that force host cells to fuse together allowing the viruses to jump from cell to cell and these genes that seem absolutely harmful to human kind by allowing some viruses to wreak havoc on our bodies have also enabled us to exist at all when these particular genes got inserted into our ancestors genomes a hundred million years ago the course of human evolution was changed forever the human genome contains around a hundred thousand fragments of viral dna and the story of how it got there begins with retroviruses retroviruses are viruses that have an rna genome and work backwards from how other viruses work instead of using dna to make rna which is then used to make viral proteins retroviruses insert rna to make dna using reverse transcriptase this dna then makes its way to the nucleus and inserts itself into the host cell's dna thus changing the genome of the host cell hiv for example is a retrovirus that incorporates itself into the genome of a cell where it lives as a template for creating more hiv viruses the hiv virus infects immune cells but some viruses infect the cells that will be passed through the generations and this is where our history with viral genes begins if retroviruses infect reproductive cells the cells that produce eggs or sperm the virus inserts its dna into the heritable genome of the host once a retrovirus has embedded itself in this way it's known as an endogenous retrovirus at first endogenous retroviruses force cells to make more retroviruses that can infect other cells but over the generations the viral dna mutates and endogenous retroviruses eventually lose the ability to infect new cells much of the viral dna within our genomes is therefore thought to be dormant evidence of our ancestors past infections but nothing that affects us now but for some endogenous retroviruses the story is more complicated scientists have realized that even after being disabled and no longer being able to fully replicate itself some retrovirus sequences can still make some of their proteins and over time two of these viral proteins became essential in human development evolution co-opting viral genes for human needs [Music] in human pregnancy there are many biological marvels and the placenta is at the front of that list it's an organ that develops in the uterus during pregnancy and provides oxygen and nutrients to the growing baby and removes waste products from the baby's blood the placenta attaches to the wall of the uterus and is where the baby's umbilical cord arises from and it may have never developed without the help of viral proteins since sit in one and since sit in two around five days after fertilization the embryo has differentiated into two distinct cell types the inner cell mass which will develop into the fetus and eventually become the baby and trophoblasts which will develop into the placenta and external membranes around the seventh day the blastocyst starts to implant onto the uterine wall where it will remain attached until birth at this point the syncytiotrophoblast starts to form around the developing placenta it's a structure that determines which substances cross the placenta such as nutrients and oxygen and which substances do not such as certain maternal hormones and toxins it's both life-giving and protective and is the structure that results from the once viral synthetic proteins in viruses the syncident proteins allow them to fuse host cells together so they can spread from one cell to another in the cell to cell viral transmission i mentioned before now the proteins allow placental cells to fuse together to form this vital layer next to the uterus without syncidin the placenta does not form correctly and if it had never been introduced into the human gene pool the placenta would not have evolved as it is today without it internal pregnancy would have developed very differently or perhaps not at all viral sequences that live in our genome act as raw material for new adaptations lines of code that should be useless to us turned into something valuable in what feels like a miraculous turn of events it reminds us that evolution has no plan there is no single track on which it operates it's a machine of randomness and variability and in all likelihood our history of past retroviral infections and even current ones will continue to shape our evolution as a species behind the scenes at real science and real engineering we are constantly trying to improve our creativity and workflow to bring you all better and better videos i am personally bad at multitasking and juggling several different projects at once to help deal with this challenge i turn to our friend and fellow youtuber thomas frank's productivity class on skillshare called productivity masterclass create a custom system that works this class helps you create a task management system that you'll actually stick to it taught me how to manage our calendars better and set up a task management system that i now use religiously and as our team grows we've also been expanding our animation capabilities with more and more 3d animations made in blender and cinema 4d with still plenty of 2d and character animations too skillshare has fantastic courses in multiple animation platforms like blender and after effects when i started making videos i didn't know how to animate like at all thinking the software was too intimidating but once you dive in you realize it's not as hard as you thought especially with the classes skillshare has right now i'm working on honing in my skills with this class called animating with ease in after effects which is teaching me how to use keyframes to their maximum potential skillshare is an online learning community with thousands of inspiring classes for creative and curious people it's curated specifically for learning meaning there are no distracting ads and it's less than ten dollars a month with an annual subscription but the first thousand people to sign up with the link in the description will get a free trial of skillshare premium so there's no risk in trying it out and looking for a class that piques your interest as always thanks for watching and if you'd like to see more from me the links to my twitter instagram and patreon are below you

Channel: Real Science

Views: 161,281

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Id: 7zmKaschEew

Length: 11min 4sec (664 seconds)

Published: Sat Feb 20 2021