Our closest relatives in the animal kingdom, the great apes, share many similarities with us in both physical traits and behaviors. Orangutans use tools and learn from each other, chimps laugh and form friendships, gorillas use sign language, and bonobos show empathy and share food. Despite these connections, humans have evolved significantly since our last common ancestor with these primates about 6 million years ago. Our brains have grown larger, enabling us to dominate the planet, write literature, build cities, and explore space.
Over the past few million years, members of the genus Homo have undergone significant evolutionary changes, setting the stage for modern humans. These adaptations have allowed us to thrive in diverse environments and shape our surroundings. But what exactly set us apart from other primates? Was it just our large brains, or were there other factors involved? Understanding the key steps in our evolutionary history helps us comprehend how we became the complex beings we are today.
For a long time, the idea of ancient human species was unimaginable. Although animal fossils were found, early human fossils remained elusive. The age of the Earth was debated, and many believed humans were created in their current form a few thousand years ago. Even after Darwin’s “On the Origin of Species” in 1859, the application of natural selection to humans was doubted.
However, a groundbreaking discovery in 1856 in Germany’s Neander Valley changed everything. Workers unearthed a partial skeleton with unique features, later identified as Homo neanderthalensis, a species distinct from modern humans. This discovery, along with others in Belgium and Gibraltar, marked the beginning of our understanding of human evolution.
In 1891, another significant find occurred in Indonesia. Eugene Dubois discovered a fossil known as Java Man, later classified as Homo erectus. This species had an upright posture and a relatively large brain, bridging the gap between apes and humans. By the early 20th century, scientists believed that large brain size was a precursor to walking upright, but this theory was soon challenged.
In 1924, the discovery of the Taung Child in South Africa provided new insights. This fossil, belonging to Australopithecus africanus, had a small brain but showed evidence of bipedalism. The position of the foramen magnum indicated an upright posture, suggesting that bipedalism preceded large brain development.
In 1976, Mary Leakey and her team discovered ancient footprints in Tanzania, preserved in volcanic ash. These footprints, dating back 3.75 to 3.59 million years, showed clear evidence of bipedal walking. The alignment of the big toe and the presence of an arch were crucial adaptations for walking on two legs.
Dating these footprints was possible through potassium-argon dating, which measures the decay of radioactive potassium in volcanic materials. This method provided a precise age for the footprints, confirming that bipedalism existed long before large brains evolved.
Several theories attempt to explain why our ancestors began walking upright. The Savannah hypothesis suggests that climate changes in Eastern Africa led to more open landscapes, encouraging bipedalism for efficient travel and resource access. Other theories propose that bipedalism evolved for foraging, energy efficiency, thermoregulation, social status, or freeing the hands for tool use.
Bipedalism was a pivotal moment in human evolution, paving the way for further developments like tool use. The oldest stone tools date back to 3.3 million years ago, coinciding with the existence of australopithecines. While the exact reasons for bipedalism remain debated, its impact on our evolutionary path is undeniable.
The story of human evolution is unique, marked by a series of adaptations that led to the intelligent, emotional, and world-shaping species we are today. This article is part of a series exploring the major steps in human evolution, from tool-making to language and consciousness.
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Examine images and descriptions of key hominin fossils, such as Homo neanderthalensis, Homo erectus, and Australopithecus africanus. Discuss with your peers how these fossils contribute to our understanding of human evolution. Consider the physical traits that distinguish each species and what these traits reveal about their lifestyles and environments.
Participate in a debate on the various theories explaining the evolution of bipedalism. Divide into groups, with each group defending a different hypothesis, such as the Savannah hypothesis or the tool-use theory. Present arguments and counterarguments, and engage in a critical discussion to evaluate the strengths and weaknesses of each theory.
Work in small groups to create a detailed timeline of significant events in human evolution. Include key fossil discoveries, the development of bipedalism, and the emergence of tool use. Use visual aids and digital tools to enhance your timeline, and present your findings to the class.
Research how climate changes in Eastern Africa may have influenced human evolution. Focus on the environmental shifts that occurred during the time of early hominins and how these changes might have driven adaptations such as bipedalism. Present your research in a multimedia format, incorporating maps, graphs, and images.
Write a reflective essay on what sets humans apart from other primates, considering both physical and cognitive traits. Discuss how these differences have enabled humans to create complex societies, develop technology, and explore the universe. Share your essay with classmates and engage in a discussion about the implications of these unique traits for the future of humanity.
Here’s a sanitized version of the transcript, removing any unnecessary filler words and ensuring clarity:
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Relatives, we are reminded of our particular place on the evolutionary tree. The chimps, gorillas, bonobos, and orangutans all resemble us in many ways, both in body and behavior. Just like us, orangutans can learn from each other and are often seen using tools. Chimps laugh when tickled, cultivate friendships, and even go to war with neighboring groups. Gorillas have been taught to use sign language and can even use it to lie, while bonobos are quick to share food and display empathy.
It’s hard to gaze into the eyes of any of the great apes and not see a reflection of ourselves. However, as much as we may connect with our primate cousins, it’s been about 6 million years since the death of the last common ancestor we shared with them. Since then, humans have become quite distinct from the rest. Our brains have more than tripled in size during this time, and with our large brains, we’ve become the most dominant species on the planet, capable of writing novels, building skyscrapers, and exploring the cosmos. We are simply not the same as the other great apes.
In the past few million years, individuals belonging to the genus Homo experienced profound evolutionary developments that now serve as the foundation of the human race. These increasingly complex adaptations have allowed humans to fully dominate the globe, living in almost any habitat and controlling their environments. But how exactly did we diverge so drastically from other primates? Is it just our big brains that make us different, or is there something more? What were the defining steps in our evolutionary history that made us the walking, talking, thinking, feeling, society-building beings that we are?
For centuries, the idea that there could have been an ancient species of human that existed before our own was inconceivable. Even though many animal fossils had been found over the years, no early human fossils had been discovered. The age of the Earth was still disputed, with most believing that humans were created in their present form a few thousand years ago. Even after Darwin published “On the Origin of Species” in 1859, few believed that natural selection could apply to humanity. Darwin himself was surprisingly silent on the matter.
However, a pivotal discovery in 1856 during quarrying operations in the Neander Valley in Germany would shake the scientific world, pushing it closer to an understanding of the origin of humans. Mine workers found a partial skeleton of a male individual, which had never been seen before. It was a specimen with an oval-shaped skull, a low receding forehead, distinct brow ridges, and thick strong bones. The fossil was from an individual that had a big brain like us, but its other features were quite distinct. Initially, even great minds like Darwin did not believe the fossil could be an early human ancestor, but rather just an odd Homo sapiens specimen. Soon, scientists realized that what they found was not a human like us, but a different species altogether. By 1864, the specimen became the first early human species ever named Homo neanderthalensis, estimated to be 40,000 years old.
Several years after Neanderthal 1 was discovered, scientists realized that prior fossil discoveries in 1829 in Belgium and 1848 in Gibraltar were also Neanderthals, even though they weren’t recognized at the time. These discoveries were just the start of our understanding of the immense evolutionary journey of humanity. Soon after, in 1891, another specimen was found in Indonesia, this time a much older fossil. Eugene Dubois made it his life’s mission to find the missing link between humanity and apes, wanting concrete proof that Darwin’s theory of evolution and natural selection applied to mankind. His discovery, later called Java Man, would help cement our evolutionary story. The Java Man specimen had a pronounced brow ridge like the Neanderthals, an upright posture, and a relatively big brain, although smaller than the Neanderthals and Homo sapiens. This hominid would be named Homo erectus and lived around 1.5 million years ago.
At the turn of the 20th century, scientists began to draw the reasonable conclusion that the unifying feature between us, the Neanderthals, and Homo erectus was our big brains. The assumption was that large brain size was a precursor to bipedalism, that we started walking upright to accommodate our big brains. However, this assumption would soon be challenged with the discovery of a skull of a small child in 1924. This skull, so unprecedented, would be called the most important anthropological fossil found in the 20th century. The Tong child, as it was named, was found during quarry operations in South Africa. Its brain was one-third the size of modern humans, so small that scientists initially thought it was just the remains of an ancient ape. But this was no ape; one important feature of the skull made this clear. The foramen magnum, the void on all vertebrate skulls where the spinal cord attaches to the brain, is located at the back of the skull in four-legged vertebrates. In human skulls, this hole is distinctly in the middle, allowing the body to be oriented vertically for walking upright. The same is true for the Neanderthals and Homo erectus, and the foramen magnum on the Tong child was also located centrally, more similar to Homo sapiens than to chimps or other apes. The Tong child would later be classified as Australopithecus africanus, a type of hominid that predates the entire genus Homo. The Tong child is estimated to have lived 2.8 million years ago. The fossil record indicates that Australopithecus is ancestral to the genus Homo, including modern humans.
Specimens discovered in the following decades would help confirm the placement of the foramen magnum in this species. Among the most spectacular of these discoveries was a fossil nicknamed Lucy, unearthed in 1974 in Ethiopia. Paleoanthropologists Donald Johanson and Tom Gray found several hundred bone fragments, which represented 40% of a single hominin. The skeleton included parts of the pelvis, legs, and ankle, proving that Lucy had a body capable of walking upright. Lucy was classified as Australopithecus afarensis, and the rock in which she was found dated back to 3.18 million years. Lucy helped solidify the conclusion that human evolution began with the adoption of two-legged walking while brains were still essentially ape-like. Bipedalism defines the hominid lineage and was the first evolutionary breakthrough that began our journey to humanity. The first step to true humanity was indeed the first actual upright steps taken.
But how far back did bipedalism happen in our lineage? Some answers can be found not in fossils but in footprints. Mary Leakey and a team of paleoanthropologists discovered a remarkable number of footprints in 1976, preserved for millions of years by volcanic ash. There were tracks from ancient giraffes, baboons, rhinoceroses, hippos, and about 70 footprints that looked surprisingly similar to those of modern humans. Not only did the heel touch the ground first in these bipedal walkers, but the feet clearly had an arch and a big toe aligned with the other toes. These adaptations are essential for bipedal walking. In chimps and other great apes, the big toe is more like an opposable thumb, but for bipedalism, it shifts position to provide more stability and balance.
Dating footprints might seem challenging, but the Laetoli prints were preserved in volcanic ash. Freshly erupted lava and ash contain a form of radioactive potassium that decays into argon, and this half-life is used by scientists to pinpoint the age of such fossils with an accuracy of about 20,000 years. Using the potassium-argon dating method, scientists dated the footprints to somewhere between 3.75 and 3.59 million years ago. Given the clear evidence that our hominin ancestors were bipedal before they evolved to have larger brains, the next question becomes what behavioral and environmental pressures led to bipedalism. There are many theories but no conclusive answers at this point.
Early on, scientists proposed the Savannah hypothesis, suggesting that the climate in Eastern Africa was becoming hotter and drier around 4 million years ago. This new climate changed the landscape from woodlands to a patchwork of trees and open grasslands, possibly pushing our pre-bipedal ancestors out of the trees and onto the ground, where they could more easily travel and reach other food sources. More recent discoveries have identified an even older hominin, Ardipithecus ramidus, whose skeleton shows some evidence of bipedalism and dates back to 4.4 million years ago in Ethiopia, when the habitat was still very forested.
Other researchers have suggested that bipedalism might be tied to particular foraging strategies. From the ground, chimps stand on their hind legs to reach fruit in trees, and this may have been such a successful strategy for ancient hominins that it slowly evolved into outright bipedalism. There is also a bioenergetic and thermoregulation hypothesis: walking upright expends 75% less energy than chimps walking quadrupedally, making it easier for our ancestors to cover long distances. At the same time, walking upright reduces the amount of sunlight the body is directly exposed to, which was important in a hot environment without much shade. The upright posture could have also had advantages for social status; standing on two feet makes you look taller and possibly more capable of attracting a mate. One researcher suggests that the bipedal posture frees up the hands for striking, meaning bipedalism allowed for more fistfighting, which might have been an element of males battling over mates. Most importantly, bipedalism freed up hands to carry things, from children to food to tools.
Tool use was the next big step in the story of humanity. Although there are no tools directly associated with Australopithecus afarensis, the oldest stone tools from Kenya date back to 3.3 million years ago, when the australopithecines lived. While we may not understand the precise factors that led to bipedalism, it’s clear that this evolutionary adaptation was a watershed moment in human history, opening the gates for millions of years of profound and unique evolutionary changes.
The evolutionary story of humans is unlike anything else, which is why we made an entire series about it—about how one adaptation cleared the path for the next, allowing us to become the upright, hairless, emotional, intelligent, and world-dominating beings that we are today. The video you’ve just watched is episode 1 of our series “Becoming Human,” and the rest is available exclusively on Nebula. The next four episodes of this series discuss the major steps in human evolution, from tool-making to hunting, long-distance running, language, and the birth of consciousness as we know it.
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This version maintains the core information while removing extraneous elements for clarity.
Evolution – The process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth. – Charles Darwin’s theory of evolution by natural selection explains how species adapt over time to their environments.
Bipedalism – The condition of using two legs for standing and walking, which is a key characteristic of humans and their ancestors. – The development of bipedalism in early hominins is considered a significant evolutionary step that allowed for more efficient locomotion.
Ancestors – Organisms from which others have descended, often referring to earlier forms in the evolutionary lineage. – Fossil evidence suggests that our ancestors began using tools millions of years ago, marking a pivotal point in human evolution.
Adaptations – Inherited characteristics that enhance an organism’s ability to survive and reproduce in a particular environment. – The thick fur of polar bears is an adaptation that allows them to survive in the Arctic’s freezing temperatures.
Fossils – Preserved remains or traces of organisms from the remote past, often found in sedimentary rock. – Fossils provide crucial evidence for understanding the evolutionary history of life on Earth.
Species – A group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. – The concept of species is fundamental in evolutionary biology, as it helps scientists classify and understand the diversity of life.
Tools – Objects used by organisms to perform tasks, often considered a sign of advanced cognitive abilities in evolutionary studies. – The discovery of stone tools alongside early human fossils suggests that tool use was a significant factor in human evolution.
Climate – The long-term pattern of weather conditions in a particular area, which can influence evolutionary processes. – Changes in climate have historically driven evolutionary adaptations, such as the development of thicker fur in mammals during ice ages.
Primates – An order of mammals that includes humans, apes, monkeys, and others, characterized by large brains and complex behaviors. – Studying primates provides insights into the social structures and behaviors that may have been present in early human ancestors.
Brain – The organ in vertebrates that is the center of the nervous system, responsible for processing sensory information and directing behavior. – The increase in brain size among hominins is a key focus in understanding the cognitive evolution of humans.
