San Diego State University Course of Foot Evolution Analysis Paper

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[This Activity is modified from Colvard, M. and Blumenrath, S. (2015) Human Feet are Strange. HHMI BioIntractive (Links to an external site.) and Dutrow, N. (2018) New Laetoli Footprints and Hominin Body Size. HHMI BioInteractive (Links to an external site.)]

IMPORTANT: Read through the following information, and then answer all questions in a single document that you will upload using the link below the activity information.

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Introduction

Humans, along with familiar species such as orangutans, gorillas, lemurs, baboons, and chimpanzees, are primates. Among living primates, modern humans are most closely related to chimpanzees. The two species shared a common ancestor that lived about 6–7 million years ago. One of the traits that distinguish humans from all other primates, including chimpanzees, is the way we walk. Chimpanzees are primarily quadrupedal, which means that they walk on four limbs. Chimpanzees move with their hands turned under so that their knuckles make contact with the ground, which is why they are also described as knuckle-walkers. Humans, on the other hand, are bipedal, meaning that we walk on two legs.

We walk with our feet close together and directly underneath our hips, enabling us to balance on one leg while the other leg swings forward. Chimpanzees can walk on two legs only for short distances. Because their feet are not directly under their hips, they sway side to side to help maintain balance when walking on two legs.

Because humans are the only primate to walk on two legs, bipedality most likely evolved in the lineage that led to humans. But when during human ancestry did this human trait first evolve? This activity will help answer that question. In Part 1, you will make observations and inferences from a portion of the Laetoli trackway, a trail of footprints that were made in what is now East Africa about 3.6 million years ago! In Part 3, you will compare human feet and chimpanzee feet to the footprints in the Laetoli trackway, to determine whether those footprints were more human-like or chimp-like. Lastly, in Part 3, you will analyze a published scientific figure from a research study that used footprints formed millions of years ago to estimate the heights of early human ancestors.

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Part 1: Laetoli Trackway

Laetoli, in northern Tanzania, is a paleontological site made famous in the 1970s when Mary Leakey and her colleagues discovered the tracks of three bipedal hominin individuals (G1, G2, and G3) dating back 3.66 million years (see Fig. 12.1 below). Unlike the fossils of bones, which are referred to simply as fossils, fossil footprints are referred to as trace fossils. They provide indirect evidence of past life, or evidence of organismal behavior.

Fossils of Australopithecus afarensis, the same species as the famous “Lucy” fossil, have been found near the Laetoli footprints. The genus Australopithecus is a member of the clade Homininae, which includes modern humans and their extinct ancestors. The prevailing hypothesis is that A. afarensis made the footprints at Laetoli. That is what we will test today.

Take a few moments to familiarize yourself with the image of a small portion of the Laetoli trackway below. You can also download a high-resolution version download of the image for closer examination.

Figure 12.1 Photogrammetric plan of the footprints in the southern part of the Laetoli trackway.

There are two sets of prints in the trackway; one set is labeled G.1 and the other G.2/3. In each footprint, the outermost lines outline the size of the foot and each line within represents 1 millimeter of additional depth. The more lines there are encircling an area, the deeper that part of the footprint was in the ash.

Question 1. What do you observe about the footprints? a. Is the big toe pointed in the same direction as the stride? b. Where was most of the weight placed?

Question 2. What can you infer from your observations? For each of the questions below, list the observations and inferences. a. How many individuals were walking? Observation: Inference: b. What were their relative sizes—that is, how big were they compared with one another? Observation: Inference: c. Were they walking together at the same time? Observation: Inference:

Compare the Laetoli trackway to the two images below of modern human footprints. The images are followed by a link to a very short video of footprints being made in wet sand by a modern human.

Figure 12.2 Modern human footprints in wet sand.

Couple Walks Barefoot on Beach (Links to an external site.) (link to Youtube video)

Question 3. What characteristics do the modern and fossil footprints have in common? How are they structurally different from each other?

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Part 2. Chimpanzee and Human Footprint Comparison

There are many differences between chimpanzee footprints and human footprints. Chimpanzees and other nonhuman primates, like gorillas, have feet that are specialized for climbing, not walking. As stated earlier, chimps are quadrupedal, although they can walk on two feet for short distances.

As a class, watch the short video by clicking the link below, and then examine the images comparing the feet of chimps and humans. Use the information in the video and images to help answer the questions that follow.

Walking with Chimps (Links to an external site.) (link to Youtube video)

Figure 12.3 Comparison of foot characteristics of chimpanzees and modern humans.

Question 4. Using information from the video and the figure above (Fig. 12.3), did the individuals who made the Laetoli footprints millions of years ago have feet more like a human or a chimpanzee? Did they walk more like a human or a chimpanzee? Provide evidence to support your claims.

Question 5. Do you agree or disagree with this statement: “It is possible to determine when bipedalism evolved by looking only at the Laetoli footprint.” Explain your answer.

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Part 3. New Laetoli Footprints and Hominin Body Size

In 2015, scientists were excavating a site in Laetoli when they uncovered two new sets of hominin tracks (S1 and S2) located about 150 meters south of the tracks found by Leakey. These tracks are on the same surface, dated to the same time period, and oriented in the same direction as the Leakey tracks. Using previously established formulas, they inferred the body mass, stature, and walking speed of the two individuals using the size and shape of the footprints. There is some debate about the variability in body size between hominin individuals. Some paleobiologists hypothesize that as hominins increased in stature, they became better equipped to disperse from Africa to other parts of the world. We will refer to this as the “Dispersal Hypothesis”. Such a hypothesis would be supported by evidence showing a linear progression of increasing hominin stature over time. Other biologists hypothesize that variability in hominin stature is linked to sexual dimorphism or adaptation to different environments. We will refer to this as the “Dimorphism Hypothesis“. For example, in species in which males are expected to compete with each other for a chance to mate with females, there is often a large size difference between males and females, called sexual dimorphism. (We see this in living non-human great apes, such as orangutans and gorillas.) Such a hypothesis would be supported by evidence showing large variations in stature between individuals of the same species.

Examine the figure in the handout entitled NewLaetoliFootprints-Student

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• What trends do you notice in the data?
• How would you describe variations in hominin stature over time?
• Which hypothesis described above are best supported by these data? (You will be including this answer in your submission for this activity.)
• Would your answer to the previous question have been different before specimens S1 and S2 were added to the graph? Use evidence from the graph to justify your answer.
• What do the trends in the data indicate about the social behavior of early hominins?
• Is there a relationship between the number of data points in a time period and the range of stature
  measurements? How does this affect how you interpret the data?
• Each data point contains a lot of information (color, shape, filled/open, date, stature). Why do you think
  the scientists included so much data in each point? Which piece or pieces of information do you think are the most critical?

Question 6. Which hypothesis discussed above (“Dispersal” vs. “Dimorphism”) do the data support? Provide evidence from the graph.

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