Asteroids, volcanoes, and lack of oxygen are just some of the things that almost wiped out life on our planet. Against all odds, humans managed to survive. But how did we go from a handful of molecules to dominating the world? Today we take you on a journey through time to explore the fascinating evolution of humankind. Something absolutely amazing happened here. And I'm not talking about the Big Bang. I am referring to abiogenesis, the emergence of life from inert matter. Imagine a boiling broth of water with ammonia, methane, and hydrogen, which could be cooking in a volcanic chimney at the bottom of the sea. Instead of pepper, Mother Nature perhaps preferred to add sulfur, nickel, and iron to that soup, which continued to heat the molten rock underneath. Millions of years ago, those molecules fused together to form chains of amino acids such as lysine, lactic acid, and urea. Which perhaps formed the foundations of life. Over time, these chains replicated until they began to produce the first cells, the first form of terrestrial life. Well. It may not have been that simple, but that's more or less how the first single-celled organism emerged. Most likely, at some deep-sea hydrothermal vent. I present to you LUCA, the Last Universal Common Ancestor. Humans, animals, fungi, trees, we can all trace our existence back to this creature. For humans to appear on the scene, something more important was required: oxygen. The Earth was immersed in nitrogen, hydrogen, water vapor, and carbon dioxide. And it was thanks to the emergence of cyanobacteria that humans gained access to that vital oxygen. Here, the transformation of the Earth formed the supercontinents. First, Rodinia. And 200 million years later, Pannotia formed around the South Pole. This time coincides with the appearance of the first simple animals from single-celled organisms. At this point, one of the most critical events in our history occurred: the Cambrian Explosion. In short, it was when hundreds of diverse animals invaded the evolutionary scene, sowing the seeds of the origins of all major animals identified so far, including you. Dinosaurs dominated the Earth for years and survived several extinctions. However, 66 million years ago their luck ran out when an asteroid fell to Earth, impacting what we now call the Yucatan Peninsula in Mexico. He scattered tons of debris that blocked the sunlight, which led to the end of the dinosaurs. An event that sounds insignificant today, but this extinction was what allowed mammals to reach the top of the animal kingdom. These mammals include the small, furry ancestors of all primates. And it's curious that those primitive primates looked more like today's small rodents. They lived and reproduced in particular ecological niches, thanks to an omnivorous diet of fruits and insects. And they continued to evolve, increasing both their size and their brains. After millions of years of evolution, the first apes were seen in Africa, Asia, and South America. And its evolution continued, taking another great leap of millions of years, diversifying the orangutan lineage. And soon our turn would come. This is when the common ancestor of chimpanzees and humans, sometimes called "humanzee", split into two branches. For many years, biologists assumed that this separation had been abrupt. But they were wrong. These two lines continued to interbreed, exchanging genes for millions of years. It is most likely that the two species took 4 million years to completely separate. Even today, humans share between 98% and 99% of our genes with chimpanzees. The human race was defined about 5 or 6 million years ago, although our ancestors were still apes. You may have heard of Lucy, our ancestor who lived 3.2 million years ago in Ethiopia. Its brain was small, like that of a chimpanzee. The recipe for a bigger brain was yet to come. And Lucy had long arms, perhaps because she spent her time swinging from branch to branch. But there was one fundamental trait that separated early humans, like Lucy, from apes: they became bipedal. What made them move on two legs? Well, there are two theories that explain upright walking. The first is the savanna hypothesis. According to her, when the forests receded to give way to the savanna, humans turned to gathering. And walking upright allowed them to see further. However, after studying the behavior of chimpanzees in Tanzania, some scientists believe that early humans stood upright on two legs in the trees. Probably to collect fruit from the upper branches or to evade predators. But this theory has gaps: why did humans manage to become bipedal, but not chimpanzees? Primitive humans began to understand the world. Their brains developed and they made the first tools. Which species is responsible? Homo habilis, who carved stones to use as tools. Likewise, it took a million years to invent the hand axe. Anthropologists do not believe that primitive humans were ruthless hunters. Rather, scavengers that fed on the scraps left by other animals. The tools were used to break bones and extract the nutritious marrow. And on a lucky day they might eat a zebra, an elephant or a buffalo, but they were primarily vegetarians. It should not surprise us that these distant ancestors were not very intrepid. They did n't have it easy, according to fossil records from 1 or 2 million years ago that show horrifying deaths. Even if you survived an animal attack, gingivitis could still be the death of you. Scientists have determined, based on collected evidence, that a child could die of sepsis after losing a baby tooth. By that time, humanity had acquired a fundamental skill. One that would not only alter the course of history, but would have put modern dentists in a difficult position. Humans learned to control fire. Therefore, they stayed warm, scared off predators, and cooked meat, allowing their brains to increase in size. However, this also brought certain disadvantages. Since they no longer had to chew as much, their jaws became smaller. Crooked teeth appeared, simply due to lack of space. Just look at this caveman's teeth. It's perfectly aligned, just because she has a larger jaw. Having secured food and the use of fire, humans were able to improve their shelter. They no longer wanted to live in gloomy, dark caves, and they began to build houses. The oldest one dates back 400,000 years and was found in France. This is also the era in which Homo heidelbergensis appears, a human species that, despite having become extinct, is critical as a precursor to Homo sapiens, Denisovans, and Neanderthals. Between 300,000 and 400,000 years ago, a group of Homo heidelbergensis moved from Africa to Eurasia. A subgroup moved to Western Europe and evolved into Neanderthals. And what were the love affairs between species like? I'll tell you in a moment. Meanwhile in Africa, 300,000 years ago, the gradual evolution of Homo heidelbergensis into Homo sapiens began. And, after another 150,000 years, that archaic version underwent changes to become the human we know today. Fossil remains allow us to observe the transition of their physical features. The size and shape of the forehead, the brow ridge, and the face changed over time. The skull of modern Homo sapiens became larger and more rounded, with a wider forehead. At some point during this transition between the archaic and the modern, a female ancestor was born. Contemporary scientists have given it the name of It turns out that the DNA of each and every one of us can be traced back to it. Imagine her as your great- great-great-grandmother. And well, even though we already had fire, shelter and a great-great-grandmother, life was still difficult. Anything could kill us: a disease, fighting with others over the carcass of a giraffe, attacks from other animals. The average life expectancy did not exceed 30 years and the population did not even total one million on the entire planet. Our fragile existence was threatened at that moment by the eruption of a supervolcano that massively altered the climate. Africa went through increasingly dry conditions. Homo sapiens almost disappeared. Probably between 600 and 10,000 adults of reproductive age managed to survive. Apparently, during these calamitous times, humanity split into smaller, more isolated groups. The drought shrank the rivers, making fishing easier. For a long time, humans ate more fish. Although in the end, they may have become extinct, causing these communities to leave Africa. This is when humans began to migrate out of the African continent. The decrease in water levels cleared new routes, allowing them to initially reach Asia. About 45,000 years ago, they had already settled in Indonesia, Papua New Guinea, and Australia. Do you remember when I mentioned the love affairs with Denisovans? In Asia, Homo sapiens interbred with the Denisovans. As a result, between 3% and 6% Denisovan DNA can be identified in certain modern Asian populations. Homo sapiens arrived in Europe via two routes. To Eastern Europe, via Turkey; but also along the coasts of the Mediterranean Sea. There they interbred with European Neanderthals and acquired genes related to pigmentation and hair from them. Those freckles that adorn the skin of some people come from Neanderthals. About 15,000 years ago, Homo sapiens migrated from Asia to North America. Does that seem unlikely to you? Keep in mind that the topography was very different. Although the continents as we know them had already formed, for 5,000 years there was a natural passage between Asia and America. It was through the Bering Strait, and it connected eastern Siberia with Alaska. And once on American soil, some began to spread towards South America. Furthermore, in this era, coincidentally, all other Homo species become extinct. Homo sapiens was the only one that remained, although we don't know exactly why. Research suggests that it may have been due to their higher survival rate or that, for some reason, the other species simply ended up being subdued. Here we find a major turning point. Humans began to cultivate the land, and to raise goats and cows. The first farmers inhabited the Fertile Crescent, the region we know today as the Middle East. But they were not the only ones: other unrelated communities were exploring agriculture around this time. When the human species became agricultural, its population began to grow much more. By year 1 of the current era, we already numbered around 170 million. Then, in the 14th century, after the devastation caused by the bubonic plague that killed 50 million people, we were 400 million. And it took Europe two centuries to recover that number of lives. It's time for the Industrial Revolution. The world has gone from being rural and agricultural to urban and industrialized. The population has been growing rapidly, reaching 1 billion in 1804. In 2023, we surpassed 8 billion. Our growth has been exponential, using more and more energy as time goes on. After coal, oil, and natural gas, will we depend on solar energy? And will we ever be able to travel to space? Will we become a Type I civilization? Well, that's a story for another episode of What If…?