While many characteristics separate humans from other animals on Earth, we all have one thing in common—evolution! Evolution is not something that just happens to other animals, plants, and bacteria. It has happened and is happening to humans, too.
Human evolution is the process of change during a long period of time by which modern humans descended from our ancestors. Evolution does not change any one individual. Instead, it changes the inherited traits in a population.
ESSENTIAL QUESTION
What can we learn about the present and future of the human race by studying ancient peoples?
paleoanthropology: the study of human evolution.
archaeological: having to do with archaeology, the study of ancient people through the objects they left behind.
Genetic traits that increase survival and the ability to reproduce are more likely to be passed to offspring. Parents pass genetic changes to their offspring and eventually these changes may become common in a population. Genetic change in a population can change what a species eats, how it grows, and where it lives.
Humans evolved as new genetic variations in early populations favored abilities to adapt to the changing environment.
Paleoanthropology is the study of human evolution. When paleoanthropologists search for the origins of human’s physical traits and behaviors, they attempt to discover how evolution has shaped humans and our defining traits.
EVIDENCE OF HUMAN EVOLUTION
Scientists rely on several types of evidence to reconstruct the history of humans on Earth. Early human fossils and other archaeological remains are some of the most important clues about ancient humans. These fossils include bones, tools, footprints, and other evidence of activity left by early peoples. Most of the time, these human remains were buried and preserved. Later, they are found on the surface after being exposed by rain, rivers, and wind erosion. They can also be excavated by digging in the ground.
DID YOU KNOW?
Some people are uncomfortable with the idea of human evolution because it does not always easily fit with religious and other traditional beliefs about how humans and the world were created.
artifact: an object made by a human being.
dexterity: skill in performing tasks, especially with the hands.
phylogenetic reconstruction: the process of comparing characteristics of modern humans with those of ancient humans.
primate: any member of the group of animals that includes human beings, apes, and monkeys.
Scientists have found early human fossils of more than 6,000 individuals. Fossilized bones give scientists information about what early humans looked like and how their appearance changed with time. The size of the bones, their shapes, and markings on the bones from muscles tell scientists how early humans moved and how they held tools. Skulls shows scientists how the size of the human brain has changed. They can use the fossils to understand how well adapted an early human species was for walking upright or living in different climates. Fossils can tell scientists how quickly or slowly children of early human species grew up.
Scientists also study archaeological evidence, such as markings or objects that early people made, and the places where they are found. Stone tools are one type of artifact that provide evidence about how early humans lived, made things, and evolved.
Some of the earliest stone tools date back at least 2.6 million years.
Because stone is less likely to be destroyed than bone, many archeological sites have stone artifacts. These tools provide evidence about where and when early humans lived, how they migrated, and how they survived in different habitats. They also give clues about the dexterity and mental skills of the early humans.
Many people think of dinosaurs when they think about fossils, but there are lots of human fossils to find! You can look at the collection of human fossils at the Smithsonian Museum of Natural History here. Can you find fossils that look similar to the bones found in our bodies today?
Learn more about early human species here.
human origins fossils 
human origins species
Although fossils and other archaeological evidence are very important in learning the history of human evolution, scientists also use other types of evidence to recreate human history. Phylogenetic reconstructions help scientists understand how humans fit into the greater tree of life. Human DNA studies give information about how early humans migrated around the world and their connection to other species. For example, comparing human DNA with the DNA of Neanderthal, one of our closest relatives, gives scientists a better understanding of the relationship between the two species.
DID YOU KNOW?
The DNA of all modern humans is 99.9 percent identical, even as there is so much variation in how we look.
OUR PRIMATE COUSINS
Humans are primates. So are monkeys, apes, and lemurs. Even in the eighteenth century, Carolus Linnaeus classified humans with monkeys, apes, and other primates in his classification system. Although we have a lot of differences, there are several key physical and behavioral traits that link humans to these other primates.
gestation period: the period of time an offspring is carried by its mother before birth.
Primates have well-developed hands and feet that have fingers and toes. They have opposable thumbs that allow them to grab objects. Their eyes face forward in the skull, giving them the ability to perceive depth and allowing them to judge distance. Their brains are large and highly advanced, which helps them control and manipulate their environments. Primates can also use complex language and communication skills. A highly developed visual center allows them to see different colors.
Primates can also run upright, using only their hind legs. They are born fully formed after a long gestation period. Mothers have a strong bond with their babies and care for their young for long periods of time. Primates are also very social and form strong bonds with family and friends.
COMPARING DNA
While scientists suspected that humans and other primates were closely related because of their physical and behavioral traits, DNA analysis adds further evidence. The number of differences and similarities in DNA between two species shows how closely they are related. DNA analysis has shown that the difference between human DNA and chimpanzee DNA is only about 1.2 percent. The DNA of the bonobo species, a close relative of the chimpanzee, also has only 1.2 percent difference with human DNA. This shows that humans, chimpanzees, and bonobos are closely related.
In fact, humans, chimps, and bonobos are more closely related to each other than any of them are to other apes. The difference in DNA between humans and gorillas is about 1.6 percent. Chimpanzees and bonobos show the same amount of difference from gorillas. Human DNA differs from the orangutan by about 3.1 percent and from rhesus monkeys by about 7 percent of their DNA.
All of this DNA analysis provides evidence for one fact—humans, chimpanzees, and bonobos are more closely related to each other than any of them are to gorillas or other primates. This DNA evidence supports the idea that the human evolutionary tree is part of the great apes’ tree.
In 1871, Charles Darwin looked at the similarities between humans and African apes. He predicted that Africa was where the human lineage branched off from the apes’ lineage. Africa, he believed, is where the common ancestor of chimpanzees, gorillas, and humans lived. Darwin concluded that human evolution began in Africa. More than a century later, DNA evidence supports Darwin’s conclusion.
The Evolution of Bigger Brains
Millions of years ago, the brains of early humans were smaller and less complex. However, as early humans faced new environmental challenges, larger brains were an advantage. Larger, complex brains can process and store a lot of information. As early humans migrated around the world, they encountered new environments. Larger, more complex brains helped early humans interact with each other and their new environments. For example, during periods of dramatic climate change, a large brain able to process new information was a big survival advantage. This selective pressure led to an increase in brain size. Through evolution, today’s human brain has tripled in size from that of early humans. Our brain is the largest and most complex brain of any living primate in the world.
bipedalism: the ability to walk on two legs.
DIVERGING FROM A COMMON ANCESTOR
DNA shows that our closest living biological relatives are chimpanzees and bonobos. But we did not directly evolve from chimpanzees or bonobos, or any other living ape for that matter. Instead, the human species and chimpanzees diverged from a common ancestor that lived about 6 to 8 million years ago.
DID YOU KNOW?
Every person living on Earth today is part of the same species: Homo sapiens.
We can trace our origins to Africa, where the earliest humans evolved. They lived in a variety of climates and environments. They evolved into new species or went extinct. About 2 million years ago, some populations began to spread beyond Africa into Asia. They faced new environments and climates. New species continued to evolve and eventually go extinct.
One of the earliest-known members of the human family were the australopithecines. One of the most famous Australopithecus fossils is called Lucy. In 1974, scientists discovered bone fossils that made up about 40 percent of the skeleton of a female in the species Australopithecus afarensis. They named the fossil Lucy. Scientists believe that Lucy lived approximately 3.2 million years ago. She was about 3½ feet tall and weighed about 60 pounds. She was also bipedal—she walked on two legs. Scientists believe that bipedalism evolved more than 4 million years ago.
Finding the missing link between humans and apes is an impossible task—there isn’t one! Chimpanzees or any other ape did not evolve into humans. Instead, both lineages have a common ancestor. The two lineages descended from this ancestor and then branched off, going their separate ways. So, instead of looking for the missing link, scientists are trying to answer the real question: Who was the most recent common ancestor? So far, the question remains unanswered.
About 200,000 years ago, a new species evolved in Africa called Homo sapiens. This species was the first modern humans, the ancestors of everyone alive today. We can all trace our DNA back to these early Homo sapiens. As time passed, populations of Homo sapiens spread beyond Africa. They moved into western Asia and Europe. There, they faced new climates and environments.
Homo sapiens are an adaptable species. As they moved around the world, they developed different skin and hair colors to adapt to different environments. Our ability to use our traits to meet new challenges enabled Homo sapiens to survive while other early human species became extinct. Modern humans spread around the world. About 17,000 years ago, Homo sapiens could be found worldwide. They were also the only surviving human species.
HI? HI!
Have you ever heard a gorilla roar or monkeys chatter? Many primates use a variety of sounds to communicate with each other. Gorillas and chimpanzees, some of human’s closest relatives, can even learn sign language to communicate. What makes humans different is our use of rich and complex language.
BCE: put after a date, BCE stands for Before Common Era and counts years down to zero. CE stands for Common Era and counts years up from zero. This book was published in 2017 CE.
melanin: a brown pigment in skin.
Language allows us to record information, imagine different scenarios, and express complex ideas. This is a human trait that makes us different from other primates. Scientists are not sure when humans started talking, but they believe the use of language is a fairly recent event in the history of human evolution.
Spoken language does not create fossils, so there are very few clues about when early humans began to speak. However, some of the objects that humans made, as far back as 350,000 years ago, were complex enough that they probably required spoken language.
Sumerian writing from about 3100 and 2900 BCE
Different Skin Colors
Why do people from different parts of the world have differently colored skin? Different skin colors are an adaptation to different environments. Skin that was darker provided better protection against the sun’s damaging rays. It also prevented the sunlight from stripping away folic acid, a nutrient that is needed for the healthy development of unborn babies. Melanin, the skin’s brown pigment, is a natural sunscreen. People with more melanin in the skin who lived in tropical environments were better able to survive the harmful effects of the sun and preserve needed folic acid. But some sunlight is beneficial to humans. In colder environments with less sunlight, people with lighter skin had an advantage because they were better able to absorb what they needed. In these places, natural selection favored lighter skin.
As early humans evolved to speak, changes occurred in their anatomy. The voice box dropped lower in the throat. The area above the vocal cords grew longer, allowing them to make a wide variety of sounds.
There is a little more evidence for the development of writing and when it happened. Around 8,000 years ago, humans used symbols to represent words and ideas. Later, traditional forms of writing emerged.
DID YOU KNOW?
Scientists have found fossils in Africa that show the first 4 million years of human evolution took place in Africa.
OUR FAMILY TREE
For a long time, many people believed that there was a single line of human evolution. One species evolved into the next species, until reaching today’s modern Homo sapiens. Today, we know this is not true. Like other mammals, humans are part of a large family tree. Fossils show that the human family tree has many branches and long roots. Most scientists recognize about 15 to 20 different species of early humans. More branches are being discovered all the time.
In fact, since the discovery of Lucy in 1974, the human evolutionary tree has nearly doubled its number of branches and its length of time. During some time periods, three or four early human species lived at the same time. Today, Homo sapiens are the only surviving human species.
While most scientists agree that there is a human evolutionary tree, they disagree on the tree’s size and shape. How many branches are there? How are those branches connected?
Scientists also debate how to identify and classify different early human species and the factors that influenced the evolution and eventual extinction of each.
hominin: all humans, including all bipedal ancestors.
agriculture: the practice of farming plants and animals.
Most early human species, except Homo sapiens, have no living descendants. Instead, scientists must use fossils and other evidence to help them learn about these species and their evolutionary relationships. New fossil discoveries can help fill in the missing information.
DISCOVERY OF HOMO NALEDI
In 2013, two recreational cavers stumbled across some ancient bones in the Rising Star Cave, about 30 miles northwest of Johannesburg, South Africa. When scientists returned to the site to excavate the bones, they found nearly 1,500 fossils in total. These were bones from at least 15 distinct individuals.
In 2015, scientists announced that these bones were from a previously unknown species in the Homo genus called Homo naledi. Homo naledi’s fossils showed it had a mix of ancient and modern human features. It had traits similar to the genus Australopithecus, along with traits of the genus Homo and traits not shown in other hominin species.
Homo naledi finger fossils (Lee Roger Berger research team)
Homo naledi was found in a cave that can be reached only through extremely narrow tunnels. If the explorers who discovered this cave had been just a little thicker around, they wouldn’t have been able to squeak through the rocks to find what might be the most important archaeological discovery in a century! You can read more about the details of Homo naledi’s discovery at this website.
new species of human ancestor
Its brain was small, about the size of an orange. Its hands were human-like, but the finger bones were locked into a curve, which suggests that the species climbed and used tools. Homo naledi stood about 5 feet tall, had long legs, and feet almost identical to our feet today. This suggests that they could walk long distances. Many scientists believe that Homo naledi could be one of the earliest members of the Homo genus.
DID YOU KNOW?
Many advanced traits in humans, including complex language, art, and agriculture, emerged mainly in the past 100,000 years.
There are many questions still surrounding Homo naledi. How old are the bones? How did they get into the cave? Scientists continue working to answer these questions and learn more about how Homo naledi fits into the history of human evolution.
The story of human evolution is not finished. Humans continue to evolve. Today, modern evolution is being driven by our culture, biology, and even the technology we have invented. Other animals are evolving, too, especially in response to changes in the environment made by humans. We’ll learn more about these changes in the next chapter.
ESSENTIAL QUESTION
Now it’s time to consider and discuss the Essential Question: What can we learn about the present and future of the human race by studying ancient peoples?
Mapping Early Human Migration
Scientists have found fossils of early humans around the world. These fossils have helped scientists study the history of human evolution and trace a migration path around the world. In this activity, you will follow fossil discoveries of several early human species and map their migration.
Explore the Smithsonian National Museum of Natural History’s online collection of early human fossils. You can find the collection online here.
human origins fossils
Select five species of early humans to follow. Use the website’s sorting feature to find all of the fossils related to a species you have chosen. Study several fossils and create a chart to organize what you have learned about each fossil, including type of fossil, approximate age, and location found. Repeat for each species you have selected.
Plot the location of each fossil on a small world map, using a separate color for each species. The colored points show the path of migration for that species.
Looking at the map and your chart, which species overlap in time and location?
What biological adaptations would you expect to find in each species based on their migration path? Explain.
CONSIDER THIS: Did the time period in which the species lived affect its migration path? Explain.
The Leakeys
While most scientists today believe that early humans originated in Africa, this was not always the case. In the 1930s, Louis Leakey began to search for human fossils in East Africa. Many of his colleagues doubted he would find anything. For several decades, Louis and his wife, Mary, explored sites in Tanzania and Kenya. In 1959, the paleoanthropologists uncovered the first of many early human fossils at Olduvai Gorge, located in Tanzania. This first fossil, classified as Australopithecus boisei, was called “Dear Boy” by the Leakeys.
Create a Timeline of Hominin Evolution
Different early hominin species have lived during the past 6 to 7 million years. Hominins include all humans and their extinct bipedal ancestors and cousins. Sometimes, two or more species overlapped and lived at the same time. In this activity, you will create a timeline of hominin evolution.
Using the Smithsonian National Museum of Natural History website, create a chart that lists each known early human species and the date ranges the species lived on Earth.
human origins species
Using the information from your chart, create a timeline of hominin evolution.
Using your timeline and the Internet, investigate the following questions.
When was the first hominin fossil found?
When did bipedalism evolve? Which species was the first early bipedal hominin? When did the first evidence of tool use appear? In what species?
Which was the first hominin to leave evidence of culture?
What clues did hominins leave behind that reflect their cognitive abilities?
What cultural adaptations allowed Homo erectus to migrate beyond tropical and subtropical environments into cooler climates?
Which hominins lived at the same time? How do you think this was possible?
DID YOU KNOW?
In 1961, Mary and Louis Leakey’s son, Jonathan, found a second type of hominin. The Leakeys named this individual Homo habilis or “man with skill.” They believed this species had made the stone tools found at Olduvai Gorge.
CONSIDER THIS: What natural selection pressures do you think influenced the evolution of each hominin species? Did the different species experience the same pressures? If so, how did each respond?
WORDS TO KNOW
cognitive: activities related to thinking, understanding, learning, and remembering.
subtropical: an area close to the tropics where the weather is warm.
In 2015, scientists discovered the fossils of a new species of early human, which they named Homo naledi. To learn more about Homo naledi and figure out where it fits into the human evolutionary history, scientists compare the bones and skull to those of other early human fossils. Scientists compare and analyze the similarities and differences between the species to understand their relationships. In this activity, you will study and compare the skulls of several hominin species.
There are several photos of seven skulls available here.
Indiana University skulls2
There are four pictures from different angles of each skull. The skulls are identified by their label “skull.xy” in which the “x” represent the species, while the “y” represents the picture view (f = front; r = right side; t = top; u = under). The skulls are in this order.
b – female chimpanzee
c – Cro-Magnon man
d – modern human
e – Neanderthal man
f – Homo erectus, female
g – Australopithecus boisei
h – Australopithecus africanus
Compare each of the skulls. Arrange them in order from oldest to youngest. What are the similarities and differences you notice in the size, shape, bone structure, nasal bones and openings, and jaw bones? What similarities and differences do you notice in the teeth—size, shape, number, and position?
What features do the skulls have in common?
What features are most useful for telling the difference between species?
What changes have occurred through time? Explain why you think these changes occurred.
TRY THIS: How do you think human skulls will change in the future? Draw a picture of what you think a skull will look like in 5,000 years.
You are bipedal, which means you walk using two legs. The earliest humans climbed trees and walked on the ground. This enabled them to move in diverse habitats and changing climates. Around 6 million years ago, one of the earliest humans, Sahelanthropus, left evidence that it walked on two legs. Did bipedalism help this species survive in the habitats near where it lived? In this activity, you will compare movements with two and four legs and analyze the evolutionary advantage it gave early humans.
How much faster can you run on two legs? Time yourself running a short distance on two legs. Then repeat and time yourself using your arms and legs to simulate running on four legs. Repeat the experiment over different types of terrain, including pavement, grass, flat surfaces, and hills.
Chart your results and compare.
How much faster did you move on two legs?
What evolutionary advantages did bipedalism give early humans? Write a paragraph to explain your answer.
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Two Legs |
Four Legs |
Pavement |
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Grass |
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Hills |
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Climbing |
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CONSIDER THIS: Why do you think other animals did not evolve to become bipedal? Explain your answer.
