If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

WATCH: Why Human Ancestry Matters

This week, Emily Graslie is teaching you about human ancestry and geneaology, how we got to be the species we are, and why that matters in our zoomed out look at Big History. For more information, visit the Big History Project: http://www.bighistoryproject.com Crash Course is on Patreon! You can support us directly by signing up at http://www.patreon.com/crashcourse Thanks to the following Patrons for their generous monthly contributions that help keep Crash Course free for everyone forever: Mark, Les Aker, Bob Kunz, mark austin, William McGraw, Jeffrey Thompson, Ruth Perez, Jason A Saslow, Shawn Arnold, Eric Prestemon, Malcolm Callis, Steve Marshall, Advait Shinde, Rachel Bright, Khaled El Shalakany, Sam Hickman, Ian Dundore, Asif Ahmed, Tim Curwick, Ken Penttinen, Dominic Dos Santos, Caleb Weeks, Frantic Gonzalez, Kathrin Janßen, Nathan Taylor, Yana Leonor, Andrei Krishkevich, Brian Thomas Gossett, Chris Peters, Kathy & Tim Philip, Mayumi Maeda, Eric Kitchen, SR Foxley, Tom Trval, Andrea Bareis, Moritz Schmidt, Jessica Wode, Daniel Baulig, Jirat -- Want to find Crash Course elsewhere on the internet? Facebook - http://www.facebook.com/YouTubeCrashCourse Twitter - http://www.twitter.com/TheCrashCourse Tumblr - http://thecrashcourse.tumblr.com Support Crash Course on Patreon: http://patreon.com/crashcourse CC Kids: http://www.youtube.com/crashcoursekids.

Want to join the conversation?

Video transcript

Hi there, I’m Emily Graslie and welcome to Crash Course Big History. Today, we’ll be discussing human ancestry and genealogy – important for understanding where we come from and how we relate to each other today. Looking at the modern science on the topic, the evidence points toward conclusions that unify humanity into a closely knit family, reminding us of our common identity. INTRO Our species, Homo sapiens, came into existence approximately 200,000 to 250,000 years ago, foraging for generations in East Africa. These early humans were a very small population with fairly limited genetic diversity. In fact, studies of human mitochondrial DNA, the small amount of DNA that exists outside of the nucleus of a cell, have concluded that the ancestry of every human alive today goes back to a single common ancestor from around this time. Over the next 100,000 years we spread across Africa, down to the southern Cape, and into West Africa. The low population numbers of human foraging groups and their relatively disconnected communications with each other, allowed for the genetics of African foragers to begin diversifying. with population growth over the next several tens of thousands of years, more connections were made between foraging groups, and human genetics became more uniform again all over Africa. Let’s go to the Thought Bubble. Around 100,000 years ago, humans moved out of Africa into the Middle East, and maybe making it as far as India. According to genetic studies, the migration may have been due to a population boom, as humans got better and better at hunting and gathering. We can already see collective learning at work. Then disaster struck. Around 74,000 years ago, the gene pool contracted significantly and shrank to only about 3,000 to 10,000 people in the entire world. The best explanation we have for this, among a number of contenders, is the super-eruption that happened around that time at Mount Toba on the island of Sumatra, in present day Indonesia. It may seem odd that an explosion halfway around the world afflicted the inhabitants of Africa and the Near East. But Mount Toba exploded with the estimated force of 1.5 million Hiroshima-sized nuclear bombs. A layer of volcanic ash, an average of 15 cm thick, settled over everything in South and East Asia, but also in India, Arabia, and as far as East Africa. Much more ash was flung up into the atmosphere, darkening the skies, and obstructing sunlight, in the middle of what was already an Ice Age. A decade of perpetual winter for the entire globe followed, with long-lasting effects for centuries. The population appeared to recover quickly after the weather improved. Over the next 10,000 years there are indications that humans went through a population boom. In fact, the growth was so great that it is one of the primary hypotheses for why 64,000 years ago humans made another journey outside of Africa into our largest mass migration yet. Thanks, Thought Bubble. For a while, migrant humans stuck to warmer climates, spreading across South and East Asia. In a very short space of time, maybe a few centuries and at most a few thousand years, humans had colonised everywhere from Pakistan to Korea, avoiding the chillier continental climates further from the coasts. Approximately 60,000 years ago, humans spread down the land bridge that existed between Indonesia and the Asian mainland. Through the use of these bridges and perhaps even seafaring rafts, humans did what no hominine group had ever done before: they landed in Australia. Eventually humans managed to adapt to colder climates, spreading into Central Asia and Europe approximately 40,000 years ago. A few thousand years later, we were even living in Ice Age Siberia. but, we weren’t the first hominine group to have done all this. In fact, there’s concrete evidence that humans ran into the now extinct Neanderthals and even interbred with them. Anyone outside of Africa of Eurasian descent, about 1.5 to 2% of their DNA is of Neanderthal origin. Another thing that no previous hominine had done was migrate into the Americas. The dominant theory is that humans simply walked from Asia to the Americas along the Bering Strait, though recent discoveries have thrown the exact dates into question. but, genetic tests show that humans emigrated out of Siberia, perhaps hunting animals, and moved into the land-bridge between Russia and Alaska as early as 30,000 years ago. At this point we would have been prevented from traveling past the glaciers that blocked us from entering the rest of the Americas. But either by the thaw of those glaciers or a coastal route, or both, humans spread into the Americas around 13 to 16,000 years ago, rapidly heading south. But let’s jump backwards a bit. In evolutionary terms, 74,000 years ago is not long ago at all. And a few thousand people is a tiny gene pool of relatives. Humans are very closely related for a mammalian species. When humans left Africa 64,000 years ago, they left in small foraging bands of a few dozen at a time. It was a decidedly smaller population that eventually ballooned from all the new resources the rest of the world had to offer. What differences there are between ethnic groups are pretty small. Certain diseases are more common in some groups than others, but you see the same pattern occur within individual family histories. Largely superficial and cosmetic traits differ from region to region. One of the most prominent traits by which we have historically distinguished human from human is skin color, which is caused by the melanin levels in our skin. Melanin is useful because it dissipates UV radiation and helps prevent the occurrence of skin cancer -- particularly useful in areas with lots of sunshine. 100,000 years ago, every human on the face of the Earth was African. Melanin levels were roughly the same. And it was only when we spread out into other regions of the world 64,000 years ago, into different climates, with differing degrees of UV radiation, did that slowly begin to change. According to genome research, the humans who arrived in India, East Asia, and Europe between 60,000 and 40,000 years ago had dark skin tones. And until very recently they retained that appearance. Specific genes that lead to depigmentation only became active when humans entered regions where they weren’t getting enough sun and thus not as much vitamin D. But even this change took thousands upon thousands of years. Researchers have determined, for instance, that until 8,000 to 10,000 years ago, there were no white people. These ethnic distinctions are blurred even further by the extreme mobility of different peoples across the ages and especially since 500 years ago, when all the world zones began uniting into a single global system. A great many people have mixed ancestries as a result. In fact, if you got your genome tested, there are reasonable odds that your ancestry is pretty diverse too . It’s worth asking why we’ve had a long tortured history, involving war, murder, slavery, and genocide over our differences. Plenty of primates, from New and Old World Monkeys to great apes like Gorillas and Chimpanzees, show hostility toward their own species in inter-group encounters, arising out of competition for food and mates. For instance, chimpanzees, are known to range around their territories to find strangers to ambush, mutilate, and sometimes kill. Hostility, fear, wariness, and caution, are visceral traits that go back millions of years in our evolutionary history. Humans, who aren’t much better except when we apply logic and reason, distinguished between groups by tribal markers. Humans are good at understanding symbolism, and here we put it to poor use. Different body paints, dialects, and fashions in groups that were otherwise ethnically indistinguishable led to great rivalries and generations of conflict. Once humans started diversifying in appearance, as languages grew more distinct, as religions became more distant, and as battles for resources continued, these tribal markers also intensified. From a gut instinct to be wary of strangers, came the learned behaviour of judging a person by their outward traits. Then came Charles Darwin and the theory of evolution, a great leap forward. Soon after came the abuse of evolution, which was a great leap backwards. In the 19th and early 20th centuries, all the old prejudices found a new explanation, displayed with a thin veneer of scientific credibility. Darwin’s On the Origin of Species (1859) made almost no mention of humans. Deliberately. While evolution had the makings for hard science when applied to different species, when applied hastily to Homo sapiens, it had all the makings of pseudo-science. On the one hand, until the 1950s and the discovery of DNA and the synthesis of various parts of Darwinian theory, humans had a very poor understanding of genetics and how closely related humans really were. A lot the discoveries about our close ancestry were only uncovered in the second half of the 20th century by more archaeological findings and the sequencing of the human genome. On the other hand, Natural selection, which proved revolutionary to biology, was sometimes inappropriately applied to human groups. Differences in behaviour that we ascribe to cultural learning today were given a biological explanation. For instance, even Darwin in the Descent of Man (1871) hypothesized that more emotional cultures and more reserved cultures were the result of inherited traits, rather than just how people are raised to behave in those societies. Some of this was more sinister. Karl Vogt, a German pseudo-scientist, set out in the 1860s to prove that Africans were a different species from Europeans. Ernst Haeckel in the 1880s put forth a theory that humans evolved in Asia, not Africa, and that different major ethnic groups had actually separate primate ancestry, meaning we were divided not by a few thousand years but by millions and millions of years of evolution. He then talked about superior and inferior groups. With European imperialism on the rise, pseudo-scientists grasped toward unsupported theories, cranial measurements, and a whole range of justifications for their biases. In the 1890s, as competition between Western empires heated up, figures like Georges Vacher de Lapouge and William Ripley, subdivided Europeans into different superior and inferior groups as well, this contributed to the intensification of scientific racism in 20th century Europe, and fed the rise of fascism. This kind of thinking spread all over the world in the 20th century and some element of scientific racism can be found in the Sino-Japanese wars, and conflicts in Rwanda, Sudan, and others on a tragically long list. Ignoring our common ancestry continues to have modern consequences. We humans have been warring for millennia and it pays to remember the story of our origins and how closely related we actually are, especially as we move forward as a global society into the trials and tribulations of the 21st century. Just like the Big Bang reminds us of a single point of origin for all matter and energy in the Universe, linking us together, the story of humanity reminds us of our common identity. What diversity there is, is cultural. Which I think is something to celebrate. We should specially celebrate the diversity that is most crucial to human genius and collective learning -- the diversity of thought. See you next time.