Purpose: Students will discuss how prevailing theories and personal biases can affect the interpretation of evidence and the development of scientific explanations, make observations of virtual fossils and compare their observations to those of their classmates to identify any biases in their data collection.

Procedural overview:
In preparation for the activity, students will read the online Science News article “Fossils and ancient DNA paint a vibrant picture of human origins” and write an article summary for homework. A version of the article, “ Tracing the origins of human evolution,” appears in the September 25, 2021 issue of Science News.

In class, students will discuss the effects of bias on the interpretation of fossil features and the development of scientific explanations. The class will develop a framework for observing and comparing fossils. Students will then use that framework to make observations about fossils from the Smithsonian National Museum of Natural History’s virtual collection of hominin fossils. Finally, students will compare their observations about the fossils to understand how the same evidence is sometimes interpreted differently and explore how biases influence scientists looking at the same evidence.

Approximate class time:  2 class periods

Supplies:

Paper

Pencils

Student worksheet

Teacher background sheet

Computer with access to the internet

Interactive meeting and screen-sharing application for virtual learning (optional)

Audio or video capture and editing hardware and software (optional)

Want to make it a virtual lesson?

This activity can be performed virtually by using interactive meeting software. Student groups should use screen-sharing and file-sharing applications in which documents can be simultaneously edited in real-time.

Directions for teachers:

The setup

For homework before the first class, have students read the online Science News article “Fossils and ancient DNA paint a vibrant picture of human origins,” and answer the following questions. A version of the article, “Tracing the origins of human evolution,” appears in the September 25, 2021 issue of Science News.

1. What is the main point of the article?

Multiple lines of evidence, including fossils and DNA evidence, indicate that the origin and evolution of humans and other hominins is more complex than a simple lineage from a single point of origin. Much of early human evolution occurred in Africa, but several intermingling species of hominins over a wide geographic area contributed to the suite of behavioral and biological traits that define modern humans.

2. What details does the author provide to support the main point? Identify at least three supporting ideas or details.

Fossil finds that have been dated by using radiometric dating methods suggest that multiple species or groups of hominins lived at the same time and in the same general areas. Mitochondrial DNA evidence indicates that the “mother” lineage of all modern humans came from Africa about 200,000 years ago. However, DNA evidence indicates that 1 to 4 percent of the DNA in modern humans not of African descent came from Neandertals, and Denisovan DNA accounts for 2 to 4 percent of the genome of Melanesian people. These genetic data, along with fossils that contain physical features of multiple hominin species, support the idea that hominin groups were intermingling and mating over time and in widespread areas.

3. What significant fossil finds or scientific breakthroughs affected how scientists looked at the evidence or at the explanations of hominin evolution?

The identification of the Taung Child in South Africa was significant because it showed that organisms with both apelike and humanlike features and that walked upright lived in South Africa millions of years ago. The discovery of Paranthropus boisei and Homo habilis in Tanzania and the nearby hominin footprints also showed that human relatives lived in Africa millions of years ago. The Java Man fossils showed that humanlike creatures with large brains and that walked upright were in Asia over a million years ago. Robert Broom’s discovery of bipedal apes with humanlike teeth helped support Dart’s interpretation of the Taung Child fossil. Radiometric dating of rocks helped constrain the dates of different fossils, which helped scientists develop timelines for evolutionary branches. Breakthroughs in the study of DNA, especially mitochondrial DNA, also helped establish timelines for evolutionary changes.

4. What role did bias play in historical explanations of hominin evolution?

Student answers will vary. Students might say: Several types of bias affected how scientists described features of fossils and their interpretation of what those features meant in terms of human evolution. Some scientists were biased in a way that caused them to seek out evidence to support explanations they already believed. Some scientists were influenced by cultural biases, in which they perceived certain groups of modern humans to be superior or inferior to others. They relied heavily on the first information they received or on information from preferred sources and were less likely to accept new information from less acclaimed sources. They were also biased against other scientists (or amateurs) based on their opinions of those people or their backgrounds.

5. After reading the article, what questions do you have about the fossil evidence for hominin evolution or about explanations of human origins?

Student answers will vary. Students might say: How do they figure out what a skull or skeleton looks like from a few small fragments of bone? What features do they use to differentiate between different hominin species? How did hominins travel around Africa, Asia and Europe millions of years ago? How did groups of different hominins interact and interbreed when they looked or acted differently? How did scientists use the rate at which mitochondrial DNA changes to calculate when the “mother” lineage started? Which DNA markers are associated with Neandertals, and how do we know that modern humans have them?

6. Write a three-paragraph summary of the article’s main points.

The study of human evolution and the gathering and interpretation of fossil evidence has been affected by various biases and rivalries. Before the 1920s, most scientists thought that humans evolved in Asia or Europe, but fossil evidence discovered in the early 20th century suggested that Africa was the source of human ancestors. A series of fossil finds throughout the 20th century and the introduction of DNA evidence caused scientists to slowly adjust their theories about the origin of humans.

Multiple lines of evidence, including fossils and DNA evidence, indicate that the origin of hominins is more complex than a simple lineage from a single point of origin. Fossil evidence indicates that at least 23 hominin species lived over the last 7 million years. These species shared some characteristics with the great apes, such as chimpanzees and gorillas, and some characteristics with modern humans, such as upright walking or bipedalism, increased brain size and tooth structures.

Both the fossil record and DNA evidence indicate that human evolution was not a linear, unbroken process of one hominin evolving into another through time. Hominins were diverse, and species overlapped in both time and space. Much of early human evolution occurred in Africa, but several intermingling species of hominins over a wide geographic area contributed to the suite of behavioral and biological traits that define modern humans.

Class period 1

Paired feedback

Pair students and have them read their partner’s article summary. Students will provide feedback to their partners and will discuss how the summaries were similar and different.

1. Read your partner’s article summary. How was your partner’s summary similar to yours?

Student answers will vary. Students might say: Both of our summaries identified the main point of the article as being that the accepted explanations of human evolution have changed over time as new fossil data became available and new scientific techniques were developed. Changes in cultural values also have influenced explanations about human evolution.

2. How was your partner’s summary different from yours?

Student answers will vary. Students might say: My partner focused more on the timeline of discoveries than I did. I focused more on the cultural and scientific developments that affected how scientists thought about fossils of extinct human relatives and modern humans.

3. How could you improve your summary?

Student answers will vary. Students should make suggestions based on how well their summary identified the main points and supporting details of the article and whether their article summary identified the current status of the debate about human origins.

Class discussion

Before beginning the class discussion about “Fossils and ancient DNA paint a vibrant picture of human origins,” make sure students understand that the fossil evidence for hominin evolution is relatively scarce. Few of the fossils of hominin species are complete skeletons; the vast majority of fossils are individual bones and teeth. Before beginning the class discussion, students may also benefit from a review of what bias is and how to identify it.  

Use the following prompts to guide students toward recognizing how bias has affected interpretations of hominin fossils. Students should be encouraged to take notes during the class discussion so that they can refer to these concepts as they evaluate fossils later in the activity.

1. What is bias?

In scientific circles, bias is any systematic deviation between the results of a study and the “truth.” Bias is sometimes described as a tendency to prefer one thing over another, or to favor one person, thing or explanation in a way that prevents objectivity or that influences the understanding of a phenomenon.

2. Describe two potential sources of bias in scientific research. Try to give specific examples.

Students should discuss how researchers can intentionally or unintentionally introduce biases because of their attitudes toward the topic or their general worldview. Bias can also be introduced by methods of measuring, collecting or reporting data. Examples of potential sources of bias include competition in a narrow field of study, the small size of fossil collections, the incomplete nature of the fossils and the tendency to look for patterns in data to confirm existing explanations or opinions the researcher already has.

3. List examples from the article of hominin fossils that were described and interpreted by scientists throughout the 20th century. Identify at least one observation made by scientists about each fossil.

Raymond Dart’s Taung Child: The individual was a juvenile ape “with some distinctly humanlike features, including a relatively flat face and fairly small canine teeth. The foramen magnum, the hole through which the spinal cord exits the head, was positioned directly under the skull, implying the child had an erect posture and walked on two legs.”

Robert Broom’s Australopithecus africanus: Skulls and limb, spine and hip bones of individuals of varying ages (young and old) indicate the organisms were bipedal apes with humanlike teeth.

Piltdown Man: Skull bones indicated the organism “had a human-sized brain, but his primitive jaw had a large, apelike canine tooth.” The fossil was surrounded by fossils of extinct creatures, which indicated that it was very old. (This specimen was later shown to be a hoax composed of a human skull and an orangutan jaw with its teeth filed down.)

Mary Leakey’s Paranthropus boisei (Zinjanthropusboisei): This fossil was a “skull with small canine teeth like Australopithecus” and “giant molar teeth, flaring cheekbones and bony crest running along the top of the skull where massive chewing muscles would have attached.”

The Leakeys’ Homo habilis: This organism “lived at roughly the same time as Zinjanthropus but had smaller, more humanlike teeth and a brain notably bigger than that of both Zinjanthropus and Australopithecus. Because of the [larger] brain size and details of the hand, the Leakeys argued that this hominin was the one who made the tools at Olduvai Gorge.”

4. For each example, describe how the scientist’s background or biases might have affected which features of the fossil they focused on.

Raymond Dart was trained as an anatomist, so he was familiar with human body structures, including the structures of the brain and the skull. He was able to apply his understanding of the position of the foramen magnum to infer that the specimen walked upright on two feet.

Robert Broom was an expert on reptile evolution. Because the features of juvenile organisms commonly differ from those of adults, he recognized that they needed to find fossils of Australopithecus africanus adults to confirm Dart’s interpretation of the Taung Child’s features. Broom excavated caves to gather many fossils that represented individuals of all ages of the species to verify that the humanlike characteristics Dart had identified existed in adults of the species.

British scientists at the time thought that a big brain was the first trait to distinguish human ancestors from other apes. The Piltdown Man’s
skull bones indicated the organism had a human-sized brain and a primitive jaw. Other evidence suggested that the fossil was very old.

The Leakeys had found many stone tools in Olduvai Gorge, so they were expecting to find fossils of hominins that had more humanlike features, which they associated with the intelligence and motor skills required to make and use stone tools.


5. What types or sources of bias influenced the scientists’ interpretations of fossil evidence and explanations of human origins?

The article describes different types of bias displayed by the scientists. Many scientists were skeptical about Dart’s interpretation of the Taung Child fossils. At the time, the prevailing theory was that the human family tree was rooted in Asia because up to that point almost all fossil hominins had been found in Asia and Europe, and the Taung Child was only the second African fossil to be linked to the human lineage. The characteristics of the Taung Child did not conform to the existing theory and were the opposite of the favored “Piltdown Man” fossil, which was later shown to be a hoax. Many people, including scientists, considered African people to be “more primitive” or “less evolved” than Asian or European people, which added colonialist, nationalist and racial biases to the debate. In addition, Dart’s reputation among other scientists and his position beyond the scientific mainstream biased many scientists against his claims.

6. As a class, create a framework that can be used to analyze and compare hominin fossils. This framework could include specific language for describing fossils or a structured template for recording observations. When considering how to describe fossil features, look at the comparison of three fossil skulls in the article under the subhead “Homo sapiens arrives, somehow.”

Consider the following questions as you develop your framework.

What parts of the body and which features will you look at?

Why are those structures or features important for distinguishing different species?

How will you describe the features you observe?

What will you use to collect your data?

What language or measurements will you use to make comparisons?

How can you structure your framework to account for bias in your observations?

Answers to above questions will vary. Sample answer: We will look at features of the skull and jaw, including the size and shape of the forehead, the size and shape of the cranium, the size of the ridge on the top of the skull, the size and shape of the eye sockets and the nasal cavity, whether or not the eye sockets appear to be enclosed, the size and shape of the brow ridge and any other ridges on the skull, the shape and prominence of the cheekbones, the relative sizes of the different types of teeth (incisors, canines and molars), the shapes of the teeth and the flatness of the face. Those features can be used to indicate general characteristics of the face in terms of how humanlike it is. We will use words and phrases to describe the features, such as “the forehead is high, and the skull is short and round” or “the forehead is low, and the skull is flat and long.” If we all use the same words or phrases and describe the same features, we will avoid the bias caused by describing a fossil as “apelike” or “humanlike.”

7. As a class, agree on a framework everyone will use to observe and compare hominin fossils. Record the framework and any instructions or rules for conducting observations.

Student answers will vary. Students should provide an example of the framework they will use for the fossil interpretation portion of the activity. This framework can be a structured table or chart, a spreadsheet or simply a set of rules. Students should list rules related to how observations will be recorded, such as what features will be described, what information must be included for the description to be “complete,” what terms or phrases will be used to describe certain features and whether features will be described using numerical comparisons or ratings.

Sample Framework:

Fossil (exhibit name or number, species name, site/location and estimated age)Skull size and shapeFace shapeEye socket and brow ridge size and shapeTeeth size and shapeOther notable features
Specimen 1     
Specimen 2     

Rules:

Only skulls and jaws will be observed and compared.

All descriptions will include observations of the following features: size and shape

of the skull, shape of the face (nose, cheekbones, mouth), size and shape of the eye

sockets, size and shape of brow ridges and size and shape of teeth (incisors, canines

and molars).

All observers will use the same language to describe each feature. The terms used to describe each feature will be drawn from an approved list.

Descriptions of the skull will use the following language:

Forehead: high low, sloping, vertical

Skull: long, flat, low, short, high, round

Face shape: flat, sloped, protruding (“Flatness” of the face will be rated on a scale of 1 to 5, with 1 being the most sloped or protruding and 5 being the flattest or least protruding, or the most like modern humans.)

Nose: flat, wide, narrow, protruding

Cheekbones: wide, narrow, sharp, flat, prominent, delicate

Ridges (brow or skull): heavy, thick, delicate, thin, light, pronounced

Eye sockets: enclosed, open at temple, round, oval

Teeth (incisors, canines, molars): small, large, flat, pointy

Class period 2

Fossil interpretation

Working individually, students will use this online fossil collection from the Smithsonian National Museum of Natural History to compare hominin fossils. Students will look at the photographs and 3-D scans of two hominin fossils and record observations using the framework developed during the class discussion. Remind students to be aware of which specimens are fossil fragments and which are reconstructions or composites. If all students have internet access at home, this first part of the fossil interpretation could be assigned for homework.

To ensure that students have partners to work with in the second half of the activity, you may want to assign fossil pairs to students rather than allow students to choose their own.

Each student should answer the following questions while examining the specimens. Students’ answers will vary, so example answers are provided.

1. Choose two fossils from the Smithsonian National Museum of Natural History online fossil collection. Record the exhibit name or number for the fossils you choose and identify the locations or sites where the fossils were found, the fossils’ estimated ages and the species names.

I chose KNM-ER 1813 (Homo habilis; Koobi Fora, Kenya; 1.9 million years old) and STS 5 (Australopithecus africanus; Sterkfontein, South Africa; 2.5–2.1 million years old).

2. Following the class framework, list observations about each fossil.

KNM-ER 1813: The skull is relatively complete. It has a low forehead, and the skull is flat and long. The eye sockets are large and round, and the brow ridges are thick across the tip with a slight hollow at the temple and a pronounced ridge runs from the top of eye socket up and back along the side of the skull. The face is flat, the nose is flat, and the cheekbones are not prominent, but the teeth project forward from the bottom of the nose. Some of the teeth are missing, but the canines are not pointy, and the molars are wide and thick. The cranial capacity is 510 cubic centimeters.

STS 5: The skull is relatively complete, but all the teeth are missing. It has a low, sloping forehead, and the skull is flat and long. The eye sockets are large and are wider than they are tall. The brow ridges are thin across the top, but the skull is hollow behind the ridge at the temple. The face is not flat but slopes forward from the prominent cheekbones down to the teeth. There are no teeth in the skull.

3. Use the framework to describe how the two fossils are similar to and different from each other.

The KNM-ER 1813 specimen has a larger brain, rounder skull and flatter face than the STS 5 specimen has. Overall, the KNM-ER 1813 specimen looks more like a modern human skull than the STS 5 specimen does. However, the KNM-ER 1813 specimen also shares some apelike features, such as the protruding mouth, the relatively small brain (as compared to modern humans) and the hollow at the temple.

4. What questions, if any, do you still have about the fossils, about the framework or about how fossils are interpreted or compared?

How can my observations be made more quantitative and less qualitative? How do scientists know how to reconstruct skulls or fossils that have gaps or broken pieces? How do scientists know whether the specimen walked upright on two legs?

Peer review and discussion

Once each student has completed their observations, they should pair up with a classmate who looked at the same fossils. The students will use the class framework to compare their observations about the fossils.  

1. Find a partner who looked at the same two fossils you observed. If your partner looked at the same species but different specimens, record the exhibit name or number for the fossils your partner observed and identify the location or site where the fossils were found, the fossils’ estimated ages and the species name.

My partner observed the same species and specimens that I did.

2. Compare and contrast your fossil observations with those of your partner.

Students should identify where they used the same or similar descriptions for features on each specimen. They should also note where their observations or descriptions differed.

3. What do you think caused any differences between your observations and those of your partner? Do you see any evidence of biases that may have affected the observations?

I think I paid more attention to details of certain features, such as the eyes and the teeth, but my partner focused more on general impressions of the shape of the skull and face. I may have been too focused on looking for small details that would let me differentiate between the different specimens, which made it difficult for me to see how similar some of the specimens were to each other.

4. What do the differences indicate about inferring explanations of evolutionary history from fossil evidence alone?

It’s important for observations about fossils to be standardized and quantified, if possible, so that people can easily tell the difference between specimens of different species. However, it’s important for multiple people to observe and describe the fossils so that some features and details are not missed. I understand how scientists struggle to use gross anatomical features to infer lineages because the characteristics of the specimens don’t fit within neat categories or follow expected orders. Some older fossils have features that appear more humanlike than younger fossils do, and vice versa.

Potential Extension: Encourage students to try the “Mystery Skull Interactive” feature on the Smithsonian National Museum of Natural History’s “What does it mean to be human?” site.

Additional resources

Science News articles:

T. Hesman Saey. “How scientists can get a better picture of our extinct relatives.” Science News for Students. Published online May 26, 2021.

B. Bower. “Oldest known Homo sapiens fossils come from northern Africa, studies claim.” Science News. Published online June 7, 2017.

B. Bower. “Ancient DNA suggests new hominid line.” Science News. Published online March 24, 2010.

Other resources:

 “The science of studying fossils” Maropeng and Sterkfontein Caves Official Visitor Centres for the Cradle of Humankind World Heritage Site

 H. Herrero. “What Makes Us Human?” National Geographic Resource Library
Smithsonian Journeys. “Human Origins: Misconceptions about Evolution.”

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