Transcript 0:00 · You don't descend genetically from all your ancestors. 0:02 · The human genome is made 0:04 · up of more than 3 billion base pairs, the nucleic acids that bind to one another to form the double 0:08 · helix structure of DNA. There are only four types of base pairs: adenine always pairs with thymine 0:14 · and cytosine always pairs with guanine. 0:17 · Now, you might imagine that 3 billion base pairs is a very large number. And you'd be right. But 0:22 · remember that the number of your ancestors doubles every generation. We only need to go back 32 0:27 · generations, to roughly 1200 AD, before each of us has over 4 billion 30th-great-grandparents. 0:34 · If we were to descend equally from all of our ancestors, we would reach a hard limit around 0:39 · this time. With more ancestors than units of DNA, we simply could not descend from everyone. 0:45 · Having said this, one might imagine that it is 0:48 · irrelevant how many theoretical ancestors a person had 32 generations ago. Due to cousin marriage, 0:54 · many of the same people appear in different places in our family tree, so we would have far fewer 1:00 · actual ancestors than base pairs of DNA. In fact, it would not be possible to have 4 billion 1:06 · ancestors living in 1200 AD -- we now estimate that only 450 million people lived on the planet 1:11 · at that time. Even if a person living today managed to be descended from all of them, this is 1:17 · a far smaller number than the limit of 3 billion base pairs. We could say the same thing about 1:22 · earlier human populations: no matter how far we go back in time, there would be fewer people living 1:28 · on the planet than there are base pairs in our genomes. Those individual ancestors would appear 1:33 · over and over again in our ancestry, distributed over many different parts of our family trees. 1:40 · Therefore, we should still be able to get genetic material from all of those ancestors. 1:44 · But this is not how human descent 1:47 · works. We do not descend equally from all of our ancestors, as David Reich explains in his book, 1:53 · Who We Are and How We Got Here. 1:56 · Each of us gets 23 chromosomes from our fathers and 23 from our mothers, arranged in pairs. 2:01 · Two chromosomes determine sex. Men 2:04 · receive an X chromosome from their mothers, but a Y chromosome from their fathers. Because of this, 2:09 · we can trace a man's direct paternal ancestor (his father's father's father, going back millennia), 2:15 · by looking at his Y chromosome. 2:18 · It's impossible to use this method to trace a woman's direct maternal ancestor (her mother's 2:23 · mother's mother, going back millennia), because women receive X chromosomes from both parents. 2:30 · However, we only receive mitochondrial DNA from our mothers. We can therefore trace 2:35 · anyone's direct maternal ancestor by looking at mitochondrial DNA. 2:39 · As a result, 2:40 · each of us is born with 47 distinct pieces of DNA: 23 chromosomes from each parent, and 1 complete 2:47 · set of mitochondrial DNA from our mothers. 2:51 · But we do not transmit those chromosomes fully intact to the next generation. Instead, 2:56 · during meiosis, the pairs of chromosomes are interspliced. When a man forms a sperm, he creates 3:02 · a set of 23 chromosomes that has an average of 26 splices. Any chromosome in a typical sperm 3:08 · will therefore have about half of its DNA from the man's father and half from the man's mother. When 3:14 · a woman forms an egg, she creates an average of 45 splices. So the chromosomes in eggs will be split 3:20 · so that roughly one third comes from one of her parents and two-thirds come from the other parent. 3:26 · Unlike a man's intact Y chromosome, the crossover of genes affects all of a woman's chromosomes, 3:32 · including her X chromosomes, so that the X chromosomes she passes to her children mix the 3:37 · DNA from both of her own parents' X chromosomes. 3:41 · In every generation, these splits take place at different spots on the genome, and there 3:45 · are different numbers of splits that form each time. But on average, every generation adds 71 3:51 · more splices than the generation before. Knowing this allows genetic researchers to estimate the 3:57 · familial relatedness of two people by looking at their genomes: if they share identical sections 4:02 · of DNA, the longer those sections are, the closer in time they are related. This is because there 4:08 · have been fewer random splicing events that have disrupted the shared stretches of DNA. 4:13 · This also creates a fascinating genetic lottery: 4:17 · as generations advance, some ancestors will contribute disproportionately to a descendant's 4:22 · DNA, and some will contribute nothing at all. 4:26 · Why? Each generation, the number of DNA stretches only grows by 71 while the number of ancestors 4:33 · doubles. In other words, the units of genetic descent increase arithmetically, but the units of 4:39 · genealogical descent increase exponentially. 4:43 · So, for example, 3 generations back, you have 8 great-grandparents, whose genetic contribution 4:49 · will be divided into 260 pieces. On average, each will contribute about 5 or 6 chromosomes' worth of 4:56 · DNA in 32 large stretches. 4:59 · Nine generations back, say around the year 1800, you have 512 ancestors, whose genetic contribution 5:06 · will be divided into 686 pieces. Although you still have more genetic slots than you 5:11 · have ancestors, some of these ancestors will have passed none of their genes on to you, 5:16 · and some will have multiple stretches of DNA that they gave to you completely 5:20 · intact. At this point, only 72 percent of these 7th-great-grandparents will give you some of your 5:26 · genetics. 5:28 · It only takes a short time for the number of your genealogical ancestors to dwarf the number 5:32 · of your genetic ancestors. Fifteen generations back, say around the year 1650, when the first 5:39 · English colonies began to be founded in North America, you have 32,768 genealogical ancestors, 5:46 · but only 1,112 genetic ancestors. Over less than 400 years, 97 percent of your ancestors contribute 5:55 · nothing to your genetics. 5:58 · The same is true in the reverse direction. If you have living descendants in 400 years, 6:03 · you have only a 3 percent chance of contributing anything at all to their genome, unless you are 6:07 · lucky enough to be s omeone's direct maternal or paternal ancestor. Soon, any contributions you 6:14 · do make will appear in slivers of DNA so tiny that it becomes difficult to distinguish whether you or 6:20 · some other ancestor made the contribution. 6:23 · Within a very short time, we descend from an anonymous stream of humanity, we are 6:28 · individually shaped by a few recent ancestral generations, we strongly shape a few generations 6:33 · of our descendants, and then our contribution passes into an anonymous stream of humanity. 6:39 · With the aid of some speculation, an example 6:43 · from a royal family can help illustrate this. 6:46 · The current King of the United Kingdom is Charles III. At the bottom of the screen, his 6:51 · father's contribution to his DNA is highlighted in purple and his mother's is highlighted in orange. 6:55 · Charles's mother was Elizabeth II, 6:58 · who was Queen for 70 years, from 1952 to 2022. Charles inherited half of his genome from her, 7:05 · including his X chromosome and his mitochondrial DNA. This contribution to Charles's DNA remains 7:11 · highlighted in orange. 7:13 · Her father was George VI, King during World War II. Already at this stage of descent, 7:19 · the randomness of meiosis begins to play a role: grandparents do not contribute equally to their 7:24 · grandchildren. In our hypothetical scenario, we might imagine that George contributed slightly 7:29 · more than half of the genes that Elizabeth passed on to Charles. This contribution remains 7:34 · highlighted at the bottom of the screen. 7:36 · George's father (Charles's great-grandfather) was George V, King during World War I. In our 7:43 · scenario, George V contributed a heavy proportion of the genes that his son eventually passes on to 7:48 · Charles. Note that despite his genetic closeness to Charles, George V's DNA makes no contribution 7:55 · at all to Chromosome 17. 7:57 · George V's father was Edward VII, who was heir apparent for 60 years and King for 9. At this 8:04 · stage, each ancestor should account for 6.25% of their great-great-grandchildren's genes. In our 8:10 · scenario, Edward instead contributed 10.33%. 8:15 · His mother (Charles's 3rd-great-grandmother) was Queen Victoria, who ruled England for most of the 8:20 · 19th century and whose 9 children intermarried with the various royal houses of Europe and thus 8:25 · fought on both sides of World War I. 8:28 · Her father was Prince Edward of Kent and Strathearn, who had an unimpressive 8:32 · military career that ended prematurely after he was stripped of command of the outpost of 8:36 · Gibraltar following a mutiny. He theoretically remained Governor of Gibraltar but was forbidden 8:42 · to travel there again. Because Edward was Charles's 4th-great-grandfather, 8:47 · one of 64 ancestors contributing to 473 genetic slots, it is almost certain that he 8:53 · passed on genes to Charles. In our scenario, he contributed an outsized 5.3% of Charles's genome. 9:00 · Edward's mother was Charlotte, a 9:03 · German who married George III and became the Queen of Great Britain during the American Revolution. 9:08 · In reality, she very likely contributed to King Charles's genetics, although on average, 9:13 · ancestors of her generation contributed less than a percent each. In our scenario, she contributed 9:18 · 2.6% of Charles's genome. 9:20 · Charlotte's father, Charles Louis Frederick, was three months old when his own father died. He 9:26 · never inherited the duchy of Mecklenburg-Strelitz and lived in a small castle, where he enjoyed 9:31 · playing the transverse flute. There is a 91% chance that he contributed to King Charles's 9:36 · genetics. 9:37 · Adolphus Frederick II was himself born posthumously and ruled over the small 9:42 · province of Mecklenburg-Strelitz. There is a 72% chance that he contributed to King Charles's 9:47 · genetics. By this point, 9 generations deep into a family tree, the average ancestor contributes a 9:54 · single snippet of DNA to their descendant. In our scenario, Duke Adolphus contributed a hefty 0.66% 10:03 · of Charles's genome. 10:04 · The Duke's father, Adolphus Frederick I, reigned as Duke of Mecklenburg-Schwerin throughout the 10:09 · first half of the 1600s, except for a brief time when Catholics under von Wallenstein seized the 10:15 · province during the Thirty Years' War. There is a 50% chance that he contributed something to King 10:20 · Charles's genetics. 10:23 · His mother Sophia had married the Duke of Mecklenburg, who later split the duchy in half 10:27 · for his two sons. There is a 32% chance that she contributed anything to King Charles's genetics. 10:33 · Sophia's mother Christine was one 10:35 · of ten children of the Landgrave of Hesse and was almost married to the King of Sweden, but instead 10:41 · married the Duke of Holstein-Gottorp. There is a 19% chance that she contributed anything to 10:46 · King Charles's genetics. In our scenario, she contributed exactly the same stretches of DNA 10:53 · to Charles that her daughter did, making their genetic contributions to him indistinguishable. 10:57 · Christine's mother, also 11:00 · named Christine, was married to Philip of Hesse, who felt disgusted by her and openly practiced 11:05 · a bigamous marriage with another woman. There is an 11% chance that she contributed anything 11:10 · to King Charles's genetics. In our scenario, we have arrived at a single tiny snippet of DNA 11:16 · shared between Christine and her descendant Charles, amounting to 0.05% of his genome. 11:21 · Christine's mother Barbara was married 11:24 · to the Duke of Saxony in front of more than 6,000 Polish and German nobles. There is a 6% chance 11:30 · that she contributed anything to King Charles's genetics. In our scenario, her contribution to 11:35 · Charles mirrored that of her daughter Christine. 11:38 · Barbara's father Casimir IV Jagiellon ruled Lithuania and Poland from the 1440s until 1492, 11:45 · when that territory stretched from the Baltic Sea to the Black Sea. There is only a 3% chance that 11:49 · he contributed anything to King Charles's genetics. In our hypothetical scenario, 11:54 · he is the first genealogical ancestor in this line of descent to contribute nothing 11:59 · to King Charles]s genome. All of Casimir's myriad ancestors would also contribute nothing at all to 12:05 · Charles's genetics. 12:07 · As Charles's case illustrates, had our genealogical ancestors not existed, 12:12 · we would not exist, but their genetic contributions to us disappear with time, 12:17 · much like our memories of them. 12:19 · The same will become true for us and our own genetic contributions to our future descendants. 12:25 · In a few short centuries, those descendants will resemble us only insofar as we are all made up 12:30 · of the DNA of our ancient forebears, long since shattered into a cacophony of tiny fragments.