This is Harrison Harrison's favorite hobbies include pooping peeing not sleeping and costing me a fair amount of money not exactly the number-one qualities you go looking for in a friend yet I knew that he was the world's most perfect human the instant that I laid eyes on him I knew that I'd give up everything for him even my life without hesitation and all that even though just a few months ago we've never even met the reason why evolution of course staying alive long enough to have healthy offspring and getting our traits from one generation to the next is how we win the big picture game of evolution it's kind of the whole point some traits whether it's being tall having stripes or six fingers on each hand helped us win more than others if they increase in organisms chances of surviving and reproducing then they'll become more common in the future this is one of Chuck T's basic principles in his theory of natural selection the measure of how successful a trait is at making it to the next generation is called Fitness a trait that's more likely to be passed on has higher Fitness compared to one that's less likely to be passed on what we know today and what Darwin didn't know back when he was figuring all this out is that all these traits from how our bodies are built to how we think are the product of thousands of genes all interacting with each other and the environment knowing that makes you realize we are survival machines we exist to get our genes into the next generation Congrats now you know the meaning of life Harrison represents my genes in the next generation well half of them anyway he's my son did I not mention that before did you think I just stole someone's baby now just for fun let's say a pack of velociraptors tries to snatch him away but I step in and sacrifice my life to save his which I would totally do because he's awesome this self-sacrifice is an extreme example of altruism which basically means your pain for someone else's gain thing is this kind of risk or ultimate sacrifice doesn't immediately make sense when you think of evolution if I become velociraptor food my future chances of reproducing are now zero because I'm dead so why would I give up a hundred percent of my genes the ones in my body to save just half of them the ones he carries it might seem like a really bad exchange on my part but it actually makes sense and some pretty basic math can help explain why whether an act of altruism is worth it depends on the relationship between the two parties involved the biologist W D Hamilton actually came up with an equation for this it factors in the cost to you see the benefit to the other individual be and how closely related the two of you are are let's unpack this equation of it if the cost to you is less than the benefit gained by the other individual multiplied by that individuals relatedness to you then acting altruistically is worth it in the eyes of evolution when two individuals aren't related at all our will be zero let's say a bus full of velociraptors is careening towards a stranger completely unrelated to me if I push them out of the way and die myself I completely lose while the stranger completely wins they might print something heroic about me in the newspaper but technically this is not the kind of behavior that evolution rewards but if the relatedness is greater than zero things work differently let's say the Raptor bus is careening towards my first cousin and I die saving them because my cousin and I have 1/8 of our genes in common our R is 0.125 in this case I don't completely lose I only 7/8 lose if I push my sister out of the way of the Raptor Express well my sister and I have one-half the same genes so I only half use understanding this biologist JBS Haldane reportedly said that he was prepared to lay down his life for 8 cousins or two brothers but let's say I'm given an even more difficult choice two buses of velociraptors one heading towards my brother and the other my child my brother and Harrison both share 50% of my genes I'd have lose either way how do I decide what to do I really really hope I never actually have to make this choice because I love my brother too but I suspect most parents would save their kid first and evolution has a good reason for that because Harrison is a tiny baby with his whole life ahead of him and my brother and I are adults Harrison has the greatest reproductive potential among us that's a fancy way of saying Harrison is likely to have more children from this point on than either my brother or I will this is where we see the influence of genes through generations each one at Harrison's future offspring will share about 25 percent of my genes if he has just two kids I break even on my sacrifice and if he has more than that or if any of his kids have kids of their own I'd actually be ahead all these potential grandchildren and great-grandchildren mean sacrificing myself for my child is a better decision for the future of my genes than by letting my child die which I would I would never let happen okay buddy daddy promises now we've been focusing on the most extreme examples of altruism giving up your life but the fact is altruistic acts are fairly common among social animals meerkat guards keep watch over the colony and raise an alarm if they spot a predator which puts them at risk and vampire bats regurgitate blood meals to neighbours that don't get enough food and scrubbed a chicks hand you're on the nest after they grow up and hope their parents raise their siblings if the individuals in these groups were completely unrelated it'd be difficult to explain why they do this but if the group members are related to each other even only distantly genes that underlie altruistic behaviors but caring for children protecting members of their groups sharing food might spread through a population not by upping the odds of an individual's own survival but by helping an individual's genes survive in their relatives it's a mechanism known as kin selection kin selection can help us understand how complex human behaviors might have evolved of course none of us sit there and run equations in our head to decide if it's worth helping the people we care about and we can't trace emotions as complex as love or kindness directly to a gene or two but the way humans behave has been influenced by evolution the same as any other helping each other helps us survive and not just in this generation it is here some part of us will make it in for the next and the next and the next so why do we care for our family why would I or any other parent sacrifice everything for our children it's evolution I mean at least in part and you can't spell evolution without love right stay curious