Human Evolution: The Story of Us
How does studying the human genome teach us about the ancient past?
Where does the human species come from? How are we all related? From our earliest days, humans have tried to answer these questions. Nearly every society and religion throughout history has a creation story that tells the tale of how we humans came to live on Earth. Now, the information found in the human genome gives us a new way to explain our human history—a record of our evolution captured in our genome.
Your genome reveals clues about the history of your ancestors, both recent and ancient. Advances in DNA sequencing and genomic tools allow scientists to compare genome sequences among humans—both those alive today and those who lived long ago.
With this information, scientists can trace the evolutionary history of the human species. Plus, comparing genomic sequences of people from different populations can reveal the genomic changes that have occurred in those populations as time has passed.
Genomics can help us fill in the story of the human species. Small genomic changes passed down from generation to generation are the link to our ancient ancestors. The story of our species is written in our DNA. Let’s take a look.
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. You look and behave differently from early human ancestors because of how the entire human species adapted as a whole.
Some traits are favorable, meaning that they give an organism a survival advantage over other organisms. Organisms that survive are more likely to reproduce and give their genes to their offspring. As generations pass, favorable genetic traits become common in a population.
For example, early humans who were able to outrun predators were less likely to be eaten and more likely to survive and pass on those speedy running skills. These genetic changes in a population can affect what a species eats, how it grows, and where it lives.
The Maidu Creation Story by Harry Fonseca, Maidu/Native Hawaiian, 1946–2006
credit: bobistraveling (CC BY 2.0)
For many years, scientists have used evidence such as early human fossils and other archaeological remains to reconstruct the history of humans on Earth. These fossils include bones, tools, footprints, and other evidence of activity left by early people.
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. Changing skull sizes illustrate how the size of the human brain has changed. These clues help paint a portrait of what humans, and life in ancient times, was like.
Although fossils and other archaeological evidence are important in learning the history of human evolution, information from the human genome can shed new light on how early humans migrated around the world. It can help illuminate their connections to other species.
Today, scientists have developed a way to extract small amounts of DNA from ancient fossils, such as bones, fur, or soil. A new field of paleogenetics has emerged to study the past through preserved genetic material found in the remains of ancient organisms.
At first, scientists could read only short segments of ancient genomes because the samples they found were incomplete or damaged and they had no reference genome. As a result, they focused their research on particular genes or narrow regions of DNA, such as the male Y-chromosome or mitochondrial DNA inherited from a mother.
However, these short sequences did not tell the full story of human ancestry. To learn more, scientists needed to be able to sequence an entire genome. Although scientists could now sequence the genome from a living human, tackling an entire ancient genome is a challenge. Ancient DNA is very fragile and can easily fall apart. It can also undergo chemical reactions that change its nucleotide code. In addition, ancient DNA samples are often contaminated with DNA from other organisms.
However, technological advances now allow scientists to read billions of bases from the genomes of ancient humans and other organisms. Here’s how. To extract ancient DNA, scientists clean ancient bones and other surviving tissue in a sterile lab. They crush a part of the bone into powder and dissolve it with chemicals that isolate short DNA strands. The extract becomes a soup of DNA, some from the ancient sample and other material, such as DNA from microbes that lived in the soil surrounding the remains.
After scientists amplify and sequence the ancient DNA, they use certain clues to authenticate it, or make sure it’s a valid sequence. For example, ancient DNA strands are usually shorter than 100 letters. If they find a fragment that is longer, they discard it as a contaminant. Then, scientists use computers to arrange the short strands based on overlapping stretches of code and compare the DNA sequence to other, previously sequenced reference genomes.
Researchers extract Neanderthal DNA from a sample.
credit: Max Planck Institute for Evolutionary Anthropology
Studying ancient human DNA reveals more information than can be learned from fossils or artifacts. It has answered questions—such as whether or not our human ancestors interacted with Neanderthals. According to the DNA, the answer is yes. Ancient genomes show that not only did our ancestors (Homo sapiens) meet a similar species, Neanderthals (Homo neanderthalensis), but they also produced offspring with them between 40,000 and 100,000 years ago. Today, there are still traces of Neanderthal DNA sequences in our genomes.
As scientists find better ways to extract and isolate ancient DNA and improve DNA sequencing technologies, we will learn more about human history. Ancient DNA studies that span 500,000 years and hundreds of remains are already adding to our knowledge of human history and evolution. Because ancient DNA can be used to track the evolution of diseases and how humans respond to them, the data is even useful for medical research.
When an organism dies, its DNA decomposes. How long it takes DNA to decompose depends on a variety of factors, such as temperature, burial conditions, and the number of microbes feasting on the remains. This means the amount of ancient DNA that scientists can study is limited by how well it is preserved.
From human-like species, the oldest DNA fragment recovered is from a Neanderthal species and is estimated to be about 430,000 years old. Scientists discovered the Neanderthal remains in a cave in Spain that stays a cool 50 degrees Fahrenheit (10 degrees Celsius) year-round.
Scientists are starting to have success extracting ancient DNA from fossils found in warmer places. This is especially important because human evolution is believed to have started in Africa, which has a hot climate. Researchers have discovered that DNA in petrous bone is preserved better than DNA in other bones. From this tiny, dense bone on the skull, scientists have been able to recover ancient DNA from fossils found in the Middle East that are estimated to be up to 12,000 years old.
Africa has long been known as the “cradle of humanity.” Our earliest human ancestors originated on the continent. Using genome sequencing, scientists have been able to follow the path of human migration around the world. When early humans first left Africa around 60,000 years ago, they left genetic footprints that can still be seen today.
A Neanderthal skull discovered in France in 1908
credit: Luna04 (CC BY 2.0)
How human ancestors spread across the globe from Africa. The numbers show approximately how long ago the migration happened.
Biointeractive skin color
By mapping the appearance and frequency of genetic markers in modern humans, scientists are building a picture of where and when our ancient ancestors moved around the globe. Both fossil and DNA evidence support the idea that our species, Homo sapiens, originated in Africa around 200,000 years ago. According to both the genetic and paleontological evidence, our human ancestors began to leave Africa between 60,000 and 70,000 years ago. Scientists believe that major cooling in the earth’s climate at the time may have driven our ancestors to find new places to live.
Remember, the human genome has about 3 billion bases. While 99.9 percent of our genome is identical, the tiny differences that are passed down from generation to generation can be used to track the movement of populations around the world. By sequencing the DNA of groups of people who have lived in the same area for generations, scientists can analyze and identify the base differences that are common for people in that area.
Based on fossil records and the new information provided by genome sequencing, our human ancestors divided into several different groups and moved around the African continent and the world in multiple waves. One of the first waves leaving Africa traveled along the coastline of Asia toward Indonesia. Some traveled thousands of miles until they reached Australia. Evidence to support this idea was found in people living in isolated Indian villages along the route—people who share a rare genetic marker with those who migrated to Australia. Those who still have the genetic marker are probably descendants of the original coastal migrants.
A little later, another group traveled north into the Middle East and southern Central Asia. From these areas, smaller groups broke off and traveled to Europe, northern Asia, and beyond. Later migrations moved our ancestors into Western Europe and Siberia. About 20,000 years ago, ancient humans crossed an ice bridge that connected Asia and present-day Alaska. These migrants and their descendants formed populations in North and South America.
As humans migrated out of Africa, the genomes of the different groups also began to evolve as they faced new climates, diets, and diseases. Those who carried or developed a genetic mutation that gave them an advantage in their new environment were more likely to survive and produce offspring.
Check out this interactive map of human migration! Where do your more immediate ancestors come from? Does this make you think about how all humans are related?
Nat Geo human journey
As time passed, the favorable mutations became common in people living in a specific region. Also, those who lived in the same region were more likely to meet and produce offspring. As a result, two people living near each other were more likely to share the same genetic patterns than two people who lived far apart. This made people living in India more genetically similar to each other than they were to people living in Ireland.
Researchers with the PopHumanScan collaborative catalog project have found genetic evidence of adaptations in more than 2,800 regions of the human genome. Remember, an adaptation is a trait that has evolved through natural selection because it is favorable and gives an organism an advantage. These genetic variations show us how our ancestors were shaped by the different environments and conditions they faced around the world.
One example of a genetic adaptation is the ability to digest lactose, a sugar found in milk. In the human genome, a gene codes for an enzyme that the body uses to digest lactose. In ancient humans, the gene was turned on to produce the enzyme in young children so they could digest their mother’s milk. After the child was old enough to be weaned, the gene usually turned off and the lactose-digesting enzyme was no longer produced.
However, around 5,000 years ago, a genetic adaptation occurred in the cattle-herding people of northern Europe. This population relied on cattle and cattle milk for many nutrients.
People who had a gene mutation that allowed them to digest lactose as adults had a survival advantage because it was easier for them to consume more nutrients. The genetic adaptation that kept the lactose gene turned on into adulthood spread throughout the population.
Genomic studies have found that this genetic adaptation for lactose tolerance was not limited to the northern Europeans. Researchers have tested multiple ethnic groups in East Africa and discovered three separate mutations, all of which were different from the one found in the Europeans, all of which kept the lactose gene turned on in adults.
Lactose tolerance had evolved independently at least four times in the human genome—an example of convergent evolution. Natural selection allowed people with the different mutations to pass on that mutation to future generations. In Africa, people with the lactose gene mutation produced 10 times more offspring, which gave them a strong advantage in spreading the mutation across the population.
Researchers studying other genes have also found adaptation evidence for genes that control skin color, salt retention, resistance to malaria, and more. And the change has not stopped. The story of human evolution is still being written. As scientists learn more about the human genome, they hope to unlock more chapters in the human story. Who knows what the future might bring? We’ll study at least a few answers to that question next!
VOCAB LAB
KEY QUESTIONS
•Why might it be important to know what region of the world someone’s ancestors lived when considering their health and well-being?
Ideas for Supplies
•three types of beans, each in a different color, 50 beans per type
•two different colored backgrounds, such as colored paper
Natural selection is the process by which some organisms with certain traits that help them better adapt to their environment tend to survive and produce more offspring. In this activity, you will demonstrate how natural selection works.
•To start, spread all of the beans onto one colored background. Close your eyes for about 30 seconds. Open them and pick up the first bean your eye is drawn to. Close your eyes again for 10 seconds and repeat. Repeat 20 times.
•Count the remaining beans on the background. Count the beans removed.
•Create a chart that shows how many of each bean you removed.
•Repeat all steps using the second colored background. Create a data chart for your results.
•Based on your results, think about the following questions.
•On background #1, which bean survived the best? Which bean was the worst survivor? Why do you think this happened? Predict what will happen to this population of beans over time. Explain your prediction.
•On background #2, which bean survived the best? Which bean survived the worst? Was this a different result from background #1? Why?
•Why do different beans survive better on different backgrounds?
•How does this activity simulate natural selection? How does natural selection help explain the story of human evolution?
To investigate more, is there a background/environment in which none of the beans would have an advantage? Why? What might happen to this population?
