- Ask students if they have ever heard of Charles Darwin, and what they know about him. Tell students that Charles Darwin was a British scientist who traveled around the world from 1831 to 1836 on a ship called the HMS Beagle. Darwin was originally asked to join the ship’s crew simply as a dining companion for the captain, but quickly fell into the role of ship’s naturalist, and took the position very seriously. During his time on the Beagle Darwin observed plants, animals, and geological features in and around South America and Australia, and recorded his observations in a diary, which was later published as a book titled Voyage of the Beagle.
- Tell students that you are going to have them read an excerpt from Charles Darwin’s diary. You may ask students to visit “Excerpts from Charles Darwin’s Voyage of the Beagle”, or you may wish to distribute hard copies of the excerpts to each student. Ask half of the class to read the excerpt titled Buenos Aires, Argentina: 34ºS, 59ºW August 24, 1833 and half to read the excerpt titled Galapagos Islands, Ecuador: 0ºS, 90ºW September 15, 1835. Encourage students to ask about or look up any unfamiliar words in a dictionary. (if the language or vocabulary in the excerpts is too complicated or advanced for your students, you may wish to read the excerpts together as a class.) Ask students to underline or highlight any text that refers to the number or quantity of different species, or the physical structure or appearance of individuals within species. Give students 10 – 15 minutes to read their excerpts.
- Ask each group to present a brief summary of its assigned excerpt, and ask students to share what they underlined/highlighted. Make sure the following concepts are addressed:
- Many animals, both in the same location and in different locations, shared similar characteristics.
- Animals that seemed to be different shared some characteristics.
- Any given location seemed to be home to many species.
- Animals on the mainland were similar but not identical to animals on the surrounding islands.
- Several species were perfectly suited to their unique environments. Ask students if, at this point, they have any of their own theories that might link these observations together.
- For Darwin, these observations led to his curiosity about the relationships between species and the origins of new species. He developed the idea that many species could emerge from one original species. He thought that species could change—or adapt–over time, and that all species were related to a common ancestor. These ideas were the foundation of Darwin’s famous theory of evolution.
- Eventually Darwin came to the idea that the key to evolution was variation, or different characteristics within a species. He observed that within a species individuals had varying characteristics, such as eye color in people. Individuals with traits useful in their environments, such as finches whose beaks can crack a specific nut, have a better chance of surviving to reproduce and pass along those traits. Conversely, individuals with harmful traits, like an inability to digest a nut that is the only food source, would not survive, and would not pass those traits to future generations. Darwin called this process “natural selection.” It is also sometimes know as “survival of the fittest,” but Darwin himself did not use that term.
- Darwin started writing about his new theory almost immediately after returning to England after the Beagle voyage, but did not publish his famous work On the Origin of Species by Means of Natural Selection until 1859. By the time of his death in 1882, his theory of evolution was widely, but not universally, accepted. In later works Darwin discussed how natural selection applied to the process of human evolution. He was faced with skepticism and controversy when he suggested that humans shared common ancestors with primates like apes or monkeys, but today it is a commonly accepted theory. Ask students how they think natural selection has factored into human evolution.
THE HUMAN SPARK
- Ask students to think about their family trees. How far back can they trace their ancestors – 3, 5, maybe even 10 generations? According to Darwin’s theory that all life on earth evolved from a common ancestor, a person’s family tree actually goes back thousands, maybe millions of generations! Tell students that the human “family tree” does not look at individual branches (like uncles or cousins) but presents a broader view of the early species that are directly and indirectly related to modem human beings. Modern humans – known scientifically as Homo sapiens –evolved over millions of years through the process of natural selection, like all other species on earth.
- Explain to students that the path of human evolution is not a straight line. On the human family tree, some branches kept growing from the beginning, some split into two or three different branches, and some stopped growing altogether. Each of these branches played a role in human evolution, but only one grew directly into Homo sapiens. Tell students that you are going to show them a video clip that explores some of these branches of our family tree. Give students a focus for watching the clip by asking them to observe and note specific characteristics and abilities possessed by our ancestors. Play “Links in the Evolutionary Chain” clip. When clip has finished, ask students if they noticed any specific characteristics or abilities, like skull and brain size, or new tools and inventions. Ask students why they think those characteristics and abilities changed over time? Why did some things change while others remained the same? Why did different groups have different characteristics and abilities?
- In pairs or small groups, have students log on to Who’s Who in Human Evolution. Assign each group or pairs one of the four subgroups presented in the interactive: early hominins, Australopithecus, paranthropus, Homo. (Depending on how many students are in your class, some or all of the subgroups may be assigned to more than one pair or group of students.) Have students click on the fossils in their group, read the text, and write in their notebooks which characteristics they think are similar to humans today and which are different. Give students 5 – 10 minutes to complete the activity. When finished, project the interactive on a screen for the class. Ask students to share their noted observations, and click on the fossils being described for all students to see. Ask the class which characteristics made it all the way from early hominins to Homo sapiens? Which were lost? Which were picked up along the way? Why do they think that happened?
- Point out to students that the human family tree featured in the interactive stopped 7.6 million years ago. Ask students what, if anything, they think came before that in the history of human evolution? Explain that the human family tree does go back even further, as Darwin speculated, to primates including gorillas, monkeys, and our closest living relatives, chimpanzees.
- Project or distribute copies of “A timeline of life on earth” (if projecting, cover the top half so only “From primitive primates to people” shows). Explain that this timeline represents when different species branched off from their ancestors and became a new distinct species. Project/distribute “How Ida fits into the primate family tree” for a more detailed representation of the different branches. Make sure students understand that as part of the evolution process new species branched off to form new ones but the original species still existed in some form for some period of time. This image depicts primate ancestors that still exist today, but potentially several primate ancestors species (like humans) branched off and then became extinct.
- As students may have observed, all branches of the hominin family tree have died off except for Homo sapiens. However, many of our earlier ancestors still populate the earth. Explain to students that this access to our early ancestors can give scientists unique insight into the origins of human behavior. Tell students that you are going to show them a video clip relating to our primate ancestry. Ask students to observe the evolutionary distance between modern humans and the primates mentioned in the clip. Play clip. Follow up by reviewing the focus question. Point out that we can see that the shared traits between humans and primates go beyond physical characteristics – they apply to behavior patterns as well. What behavioral characteristics did students observe that are similar to modern humans behavior patterns? What is different? What makes us distinctly human?
- Explain that some scientists feel that the evolutionary gap between humans and apes is still too wide and they are looking for what they call “the missing link” – an ancestor that clearly ties the two branches of the family tree. “Ida,” shown on the two timelines, was a recent fossil discovery that some scientists believed to be a crucial link in the chain of human evolution. Others are not so sure. What traits would you expect this creature to have? Would it be classified as an early hominin or a primate? What would its body look like? How would it act around others? What characteristics would it need to possess in order to be a direct ancestor of Homo sapiens?
- Remind students that according to Darwin’s theory, all species evolved from a common ancestor, even further back than the earliest primate 55 million years ago. Reveal the top half of “A timeline of life on earth” to show that before primates, our family tree included early mammals, reptiles, even fish and worms!
- How can we tell? Just as we can observe similar traits among humans and primate ancestors, we can perceive shared traits with other species. In pairs or small groups have students log on to The Zoo of You. Have students click through the interactive and ask them to make a note of any “Oops” characteristics. Give students 10 – 15 minutes to complete the interactive. Ask students if they found anything particularly surprising or interesting.
- Return to Darwin’s theory of natural selection, where beneficial variants allow individuals to survive and reproduce. Ask students how they think the “oops” characteristics fit into this theory? Why have our modern human bodies evolved to include them?
Proceed to Video Segments.