How did we evolve?
Homo sapiens, the newest addition to the Earth's inhabitants, emerged approximately 300,000 years ago. These upright walkers, known as humans, would go on to accomplish remarkable feats such as creating magnificent cave paintings, constructing Machu Picchu, and even inventing credit scores. However, before all of these achievements, humans were simply the ambitious newcomers, joining three other closely related ape species that already roamed the Earth. These early humans were not so different from us; some of them discovered the power of fire and began cooking their food, while others even practiced burial rituals and created art, just like we would eventually do. Although we often perceive ourselves as the dominant figures in the story of humanity, wielding fire and crafting tools, the truth is that for the majority of our existence, we shared this planet with other species. These ancient neighbors challenge our understanding of what truly defines a human. As a scientist, I prefer to reserve the term "human" exclusively for Homo sapiens.
Although the term "human" typically refers to our own species, some scientists use it to describe other species within the hominid family, including our close relatives that share the planet with us. This raises the question: what exactly is a human? Our closest kin have been extinct for over 40,000 years, so today we share more genetic similarities with chimpanzees and bonobos than any other living species. In fact, we share about 99% of our DNA with them. While humans and chimps are both members of the large primate group, which includes distant cousins like lemurs and Tarsiers, as well as not-so-distant ones like gorillas and orangutans, we did not evolve from chimps or any other living ape. Rather, we split from a common primate ancestor around seven million years ago and have been evolving along our own branches ever since, alongside many other branching species, some of which survive to this day and many that do not. Despite our similarities, there is still much that we do not fully understand about the evolution of humans and our place in the natural world.
There are over 20 known homin species that have lived over the past 7 million years, based on fossil and DNA evidence. However, their relationships to each other and to us are still being studied. While they may be related to us, not all of them are our ancestors. One likely ancestor is Australopithecus afarensis, who lived about 4 million years ago in Africa. They were small with long arms and a small brain, but they walked on two legs, which was unusual for apes at the time. Another likely ancestor is Homo erectus, who had a larger head and body proportions similar to modern humans, making them great for long-distance walking. They were the first hominin to leave Africa.
In Europe and Asia, there are places where people can hang out, similar to the original Gap for Backpackers. These two ancient ancestors provide some insight into the way early humans lived. However, our understanding of our relationships with other hominids remains unclear and subject to debate. As we continue to learn more about the hominid family tree, we are realizing that it may not be as straightforward as a tree. Let's explore the fascinating world of human evolution.
In the 1990s, a Chinese paleontologist named Dr. Zen Shi Wu made an intriguing discovery. He came across a 200,000-year-old skull known as the Tali Cranium. This skull had a thick and heavy brow, along with a medium-sized brain. It shared some similarities with Homo sapiens fossils found in Europe, but also had resemblances to fossils of Homo erectus from China. It was a complex mix of characteristics.
Dr. Wu developed a hypothesis to explain this puzzling find. He proposed that this species might not be just another branch on the evolutionary tree. Instead, he suggested that it could be a manifestation of evolution working like a braided stream. In this analogy, populations sometimes become isolated for generations, leading to the development of distinct traits. However, over time, these populations can merge back into the same flow, resulting in a blending of characteristics.
This pattern of isolation and merging can occur when populations do not reproduce with outsiders for extended periods. As a result, they undergo genetic changes and evolve unique traits. However, there are also instances where these isolated populations eventually reconnect with other groups, leading to the mixing of traits once again.
Dr. Wu's hypothesis challenges the traditional view of a linear evolutionary tree and suggests a more intricate and interconnected process. By studying these ancient ancestors, we are gaining a deeper understanding of the complexities of human evolution.
The populations reconnect and exchange genetic material once again, causing the genetic differences to diminish and similarities to reemerge. This pattern of populations alternating between periods of separation and connection, with gene flow occurring during the connected times, is similar to the braided stream analogy. Dr. Woo initially used this analogy to understand human evolution in China, but now many scientists find it useful for explaining human evolution worldwide and the general workings of evolution. With advancements in technology, scientists have been able to extract ancient DNA from fossils, revealing that the braided stream analogy accurately represents the interbreeding between ancient humans and other hominins as they migrated across the planet. Our ancestors interbred with Neanderthals, a hominin species that emerged approximately 400,000 years ago. Like us, Neanderthals practiced burial rituals and created art. As a result of these prehistoric relationships, many individuals today carry a small percentage of Neanderthal DNA, typically ranging from 1 to 4%. However, while humans and Neanderthals share some genetic material, they are not exactly the same species.
Certain areas of scientific research have established the concept of species based on their ability to interbreed and produce fertile offspring. According to this definition, both humans and Neanderthals would be considered as the same species. However, when scientists discuss fossils, they typically employ different definitions, such as the morphological and phylogenetic species concepts. These concepts focus more on physical characteristics and ancestry to distinguish between species. Therefore, scientists generally classify Neanderthals and humans as separate species, or at the very least, different subspecies. If you want to delve deeper into the complexities of defining species boundaries, you can explore episode 15. Nevertheless, Neanderthals were not the only ancient relatives that captured our attention. We also interbred with another group called the Denisovans, who were adapted to high altitudes. As a result of this interbreeding, some individuals today possess a genetic variant that enhances their ability to cope with low oxygen levels at high mountain peaks. The extensive hominin family tree is still being unraveled by scientists, who rely on fossils, genetic evidence, and thorough investigation of our ancient relatives' lives, akin to deep snooping on their Instagram profiles at 2 a.m. This approach helps us piece together some of the significant advancements that have occurred throughout our evolutionary history.
As we reflect on our journey towards humanity, we can recall our companion, Granny Afarensis, who walked upright on two legs. Through the study of fossils and biological anthropology, we have learned that early hominids had a spinal cord that passed through the bottom of their skulls, allowing their heads to be stacked above their bodies - a position that facilitated bipedalism. This freed up their front limbs, which eventually led to the development of tools for slicing meat off the bones of large animals. By 1.8 million years ago, our ancestors had a similar body size and shape to us, and were walking long distances on two legs, like Homo erectus who migrated out of Africa for the first time. As they faced new challenges in new environments, they continued to evolve and adapt.
Around 1 million years ago, some of our ancestors had already developed the ability to cope with their bodies and brains. Evidence of their mastery of fire can be seen in piles of ancient ash left behind in caves. With fire came the first attempts at cooking, which makes one wonder why it took so long for us to invent Chicken and Waffles, despite having a million-year head start. During this time, their brains were steadily growing. However, it wasn't until around 800,000 to 200,000 years ago that our ancestors started communicating with symbols, sending messages with shared meanings. This was a significant development, as early as 320,000 years ago, they were fashioning red and black rocks into crayons, possibly using them to decorate themselves or their tools. They were also putting in the extra effort to transport special rocks over long distances. Modern humans arrived on the scene around 300,000 years ago, and by then, our brains had tripled in size since the first hominid ancestors started walking. As evolutionary biologist Steven J Gould said, we stood up first and got smart later. By 80,000 years ago, some groups of our ancestors had made significant advancements.
Throughout history, humans have embarked on numerous migrations across various continents. However, with each new group that ventured out on their own, they eventually encountered the founder effect. This phenomenon resulted in a decrease in their gene pool, leading to a loss of genetic diversity. As a result, the people in sub-Saharan Africa today exhibit greater genetic diversity than any other region on Earth. This is because they are the descendants of those who remained in the area, rather than smaller groups that migrated elsewhere.
As humans continued to move and interact with different populations, they intermingled and procreated, contributing to a significant amount of genetic mixing. Even in present times, there are no definitive genetic boundaries between different groups of people. As our ancestors spread across the globe, various traits evolved based on what aided their survival and reproduction in different environments.
The wide range of human skin colors, for instance, emerged as a result of natural selection adapting to the optimal level of sunlight exposure. In regions with less sunlight, individuals evolved lighter skin tones to facilitate increased absorption of UV rays and produce sufficient vitamin D. Conversely, excessive sunlight can be detrimental to folate, a vital vitamin required for DNA synthesis. Therefore, populations closer to the equator developed darker skin tones to protect against excessive sunlight while still allowing for adequate vitamin D production.
Around 12,000 years ago, humans began domesticating wild plants and animals, selectively breeding them for desirable traits. This gave us an advantage over other hominids, as we were able to control our environment and resources. Dogs were the one exception, as we had already been living with them for thousands of years. While scientists are still unsure why Homo sapiens survived while other hominids did not, studying human evolution can help us answer big questions about what it means to be human. We have learned many important traits from our ancestors, such as sharing resources, caring for each other's children, and receiving care as we age. These traits are not pre-installed, but rather learned through experience and passed down through generations. By looking at our past, we can better understand ourselves and imagine our future as a winding stream, constantly flowing and changing.
In the course of Millennia, and with a touch of good fortune, we shall continue to progress from Millennia onwards. We invite you to rejoin us for our upcoming installment, where we will shift our focus from Evolution and delve into a topic of equal significance, albeit on a smaller scale: carbon and the intricate chemistry that underlies life.