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 which characteristics or abilities they noticedif they noticed any specific characteristics or abilities, like skull and brain size, or new tools and inventions. Follow up by askingAsk students how 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?