Teaching Human Variation

From 2005 through to 2020, every fall semester, I taught a course introducing students to the science of human biological variation within the context of American history (IB35ac Human Biological Variation, #IB35AC). This course was part of the University of California Berkeley’s American Cultures Program. 

I loved teaching this class and dedicated a lot of time and effort to update, revise, and improve the course (for example, this blog post, and the recognition of this innovation in teaching award). Enrollment grew from about 120 the first year I taught it to close to 700 in the final semester. Many graduate students, the occasional postdoc, and visiting lecturers helped out over the years, developing labs, discussion section activities, and providing critical feedback for how to improve and adjust. IB35ac was truly an intellectual family.

The last semester this course was offered was during the COVID-19 pandemic. We spent a huge amount of time and effort during the summer of 2020 turning this course into a fully on-online class with all of the main lecture content being available online and asynchronous. The format worked really well for the material, and was ultimately recognized by the campus with an Extraordinary Teaching in Extraordinary Times award.

As part of this huge effort, I made a lot of videos. Some videos were recorded in my coat closet at home, and then as some people were able to return to campus if they were desperate, I started recording in my office. All alone. For many, many hours. For many, many days between June and November 2020. 

I left UC Berkeley in January 2021 to take a research position at CENIEH in Burgos, Spain. Rather than let these lecture videos collect dust in my back-up drive, with the help of Maddie McNelis, we formatted them all and uploaded them to a playlist on YouTube. There were 14 modules (14 weeks) of the semester-long course. Not all of the modules had videos. I hope you find these interesting and helpful!

NOTE: These videos from the last semester of University of California Berkeley’s course Integrative Biology 35ac Human Biological Variation were prepared and produced by Professor Leslea J. Hlusko and she holds the copyright. If you want to reproduce any portion of these videos, refer to copyright law and contact Professor Hlusko for permission.

Overview

Module 1: The Big Picture of Biological Variation

In this module, we will explore the diversity of life on Earth and its evolutionary history. You’ll see the important role that climate change played in this, with lots of serendipity that gave mammals and primates important opportunities along the way. We hone in on our own lineage and learn about the three major stages of hominid evolution that correspond to the genera Ardipithecus, Australopithecus, and our own genus, Homo.

Humans are just one species amongst an incredible amount of diversity of life on Earth. In this video, I provide an overview of that diversity. Here’s the video of Professor Chakrabarty’s TED talk I refer to at the beginning of the video: https://youtu.be/XyTcINLKq4c
This is a whirlwind recap of evolution, from the origins of life up to the last common ancestor between humans and chimpanzees.
This video is based on HHMI’s Great Transitions in Human Evolution documentary. Because they share their content openly (https://www.biointeractive.org/classr…), I added some of my own to it. I provide an intro, and then augment the content with introductions to other scientists doing paleoanthropology fieldwork at the sites mentioned. These edits gives a more current snapshot of the diversity of scientists doing this research. Here are links to the original videos I’ve taken snapshots from: Dr. Yohannes Haile-Selassie, talking about why fossils are found in the Afar of Ethiopia: https://www.youtube.com/watch?v=ELCl4… 0.21 to 4:27 Dr. Zeresenay Alemseged, explaining why it is important to know about our evolutionary history: https://www.youtube.com/watch?v=Se0XL…

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Module 2: The Science of Evolution

There are two main topics for this module, both aimed at preparing you to think about evolutionary biology and biological variation as a science. We’ll first talk about the theory of evolution by natural selection and get you putting the science of phylogenetics into practice. Since evolution can be a controversial concept when applied to humans, we will spend a little time exploring why that is and what it looks like within the United States legal system. Our second aim is to jump into biological variation by exploring one aspect of the human body in detail: the skeleton. How does the skeleton vary? What are the forces that shape that variation? And what can we learn from that variation?

Before Charles Darwin’s theory of evolution was popularized, life was seen through the lens of religion. In this video, we will explore the breadth of perceptions of Earth’s biodiversity, from Archbishop James Ussher to Jean-Baptiste de Lamarck.
Here we will examine the life and contributions of Charles Darwin to the study of evolution.

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Module 3: The Science of Human Variation

For this module, you will get your footing in the statistical language biologists use to quantify and compare phenoptypic variation within and between populations. We then add time to these concepts, as evolution occurs when the variation between populations shifts over time. There are four major forces that lead to shifts in variation over time: mutation, selection, drift, and migration. We then start to apply these concepts to the hominid fossil record, first focusing in on the shift from Homo erectus to Homo sapiens. How was a hominid, Homo erectus, that lived widely across Africa and Eurasia, essentially entirely replaced by Homo sapiens? How do the Neanderthals fit into all of this? 

In this video, I talk about the science of taxonomy. How do we name a new species? What are the rules? By investigating how Linnean taxonomy is practiced, we see that scientific names don’t encompass the dynamic process of evolution. Consequently, our scientific naming system implies that phenotypic variation is noise and time is static.
In order to study variation, we need to know how to quantitatively organize and describe it. This video goes over the fundamental concepts of histograms, means, standard deviations, variance, and p-values.
The species Homo erectus is a great example of how hard it is to put static names on the dynamic process of evolution. While evolution consists of gradual phenotypic shifts over time, scientists have to designate when that shift eventually consists of a whole new species.
In this video, I am going to take you on a close look at what happened to Homo erectus in Europe. We are going to talk about the incredible hominid fossil discoveries near Burgos, Spain, some of the most incredible excavations going on today!
Now it is time to talk about, perhaps, the most famous of all hominids, the Neanderthals. They were discovered in 1856, just years before Darwin published the Origin of Species. What were these human-like remains? What did people make of them?
When did Homo erectus, Neanderthals, and Homo sapiens become separate species? We know that a parent does not give birth to a child that is a completely different species, so where do we draw the line between different hominid species?

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Module 4: Genetic Variation

In this module, we add the genetic component to how we are thinking about evolution. In the previous module, we saw how evolution is a change in the range of variation from one population to another over time. One of the key components of the theory of evolution is that the variation has to be heritable. This means that the phenotypic variation results from genetic variation. This module introduces you to the types of genetic data we will be discussing throughout the semester. We then use population genetics to test the hypotheses about the origin of Homo sapiens introduced in the last module: Multiregionalism and Replacement.

There are a lot of different types of genetics and the people who use those different research approaches often use different languages. They also make different kinds of assumptions. In this video, we will cover quantitative genetics, developmental genetics, population genetics, and genomics and what makes each of these approaches unique.
The fossil record shows us that hominid evolution primarily occurred in Africa. However, the genus Homo was a much more geographically dispersed phenomenon. Thus, the evolutionary roots of our species, Homo sapiens, is difficult to sort out. How do you figure out when and where one species began and the older one ended when the geographic range spans across three entire continents? We are going to cover the two major hypotheses surrounding this.
Building off the previous topic of coalescence, we test the hypotheses of multi-regionalism and replacement theory using mitochondrial DNA and YDNA.
In this video, we expand on our population genetics investigation to include people from a long time ago by looking into ancient DNA.

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Module 5: Speciation

In Module 4, you were introduced to what genetic variation in humans looks like. In this module, we turn our attention to the genetic variation of other taxa for sake of comparison. Do humans have a lot of genetic variation? Are we a particularly variable species? And if we are, what might that mean in terms of taxonomy? Is there any evidence to justify subspecies within Homo sapiens? Are we a polytypic species? Let’s explore…

(I created webpages about all the great apes for this module, so there are no videos to share. I will try to transfer those webpages over at some point in time.)

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Module 6: 50,000 Years of Human Diaspora

We ended Module 5 by noting that, despite there being relatively very little genetic variation within our species, the variation that we do have reveals information about our recent past. By surveying the genomic variation across living people and the ancient genomes extracted from the bones of people who lived long ago, scientists have been able to reconstruct the movement of populations over the landscape. Add in the archaeological record, and you can get an impressive sense of how human populations diverged and coalesced, migrated into new territories, and receded from others as climates changed. For Module 6, we dive into this science. As you will see, the geographic focus is primarily placed in Europe and Asia, as that is where most of this research has been done so far. 

What does human evolution and fire coral have in common? We are going to be exploring the various populations of humans that diverged and coalesced over time, starting with the Ghost Eurasians.
Continuing our discussion of ancient human populations, we will talk about the basal Eurasians. They were a stable subpopulation in Europe for about 50 thousand years, about 39 thousand years ago.
What do volcanoes have to do with ancient human populations? Turns out… a lot! This leads us into our third sub-population, the Aurignacians, a group of hunter-gatherers, as we track through the populations moving and replacing each other in ancient Eurasia.
Time After Time… We see new populations replacing and emerging. This one is the Gravettians, who set themselves apart by having meticulous burial practices and being intense mammoth hunters!
Now, we are moving into the Magdalenians! During the last ice age there was a huge shift in the human populations. About 19 thousand years ago to 14 thousand years ago, we see the Magdalenian expansion, out from the Iberian Peninsula where a subset of humans weathered the ice age.
To put the advent of farming in perspective, we are going to be pulling in analytical results from two ancient DNA studies that come from the research group led by David Reich at Harvard and intertwining it with archaeological evidence.
Today we are going to address two questions testing hypotheses related to agriculture, one revolving around religion and the other revolving around the spread of agriculture by cultural dispersal.
In this video we are going to talk about the domestication of horses and the huge impact it had across Eurasia.
I introduced the Yamnaya at the end of the previous video. In this video we will be talking about agriculturalists mixed with hunter-gatherer practices. We are going to talk about how the Yamnaya took horses and the invention of early wagons and really pushed it to the next level.
Now that we’ve talked about the populations of people living in Eurasia, we are going to take some of what we have learned and put it into a more modern perspective with the DNA and genetic variation of current European peoples.
Today we are talking about human population movement into the Western Hemisphere, in three proposed waves, through coastal routes and the Bering Strait.
To explore the story of the ancient North Eurasians, we will be starting with genetic evidence from Lake Baikal in Siberia and looking into subterranean houses made of bones and antlers.
The story of the Ancient Beringians starts with the last glacial maximum and the large swath of land exposed during sea level drops.

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Module 7: Variation in What We Look Like

This module covers the variation that likely first comes to mind when you hear the words “human biological variation.” Why is there so much variation in skin tone? Why are some people tall and others short? What’s up with body shape and the variation in weight from one person to the next? In this module, you’ll learn our current understanding of the genetic and non-genetic influences on these phenotypes. We’ll explore the evolutionary context in which that variation evolved. And, you’ll spend some time exploring the health and well-being implications of our culture’s perceptions on adiposity. As for the health and well-being implications of skin color in the United States, we will explore that in-depth in Module 11 and Module 12.

(I created webpages for the content in this module, so, no videos to share.)

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Module 8: Adapting to Our Environment, Part 1

In Module 7 you learned how variation in skin pigmentation is selected on by variation in ultraviolet radiation. As people moved into different latitudes, the selective pressures changed. This is an excellent example of human environmental adaptation. In this module and the next, we will explore even more ways that our environments have shaped the variation we see in our species. This module focuses on our most mammal of traits, milk. We will explore the evolution of milk production in mammals and why it is that some humans have the very peculiar ability to digest milk as adults, in contrast to all other mammals.

We’ll continue the conversation by looking at another phenotype—fatty acid synthesis—that has been hypothesized as being under dietary selection. While at first glance, this may seem unrelated to milk, you’ll see how milk challenges the dietary hypothesis for variation in fatty acid synthesis. We will also take a brief side road to consider the development of a drug that relates to one of the genes involved in fatty acid synthesis.

I then link milk and fatty acid synthesis together by sharing with you some of my own research in which I identified an episode of human adaptation to the Arctic during the Last Glacial Maximum. This selective event looks to have acted on variation in nutrient transfer from mama to baby through breast milk. As the grad students in my lab like to joke, it *always* comes back to milk. 🙂

Today we are going to talk about the roles of lipids and fatty acids in your body. Adipose tissue is important to your body for many reasons, from energy storage, thermal regulation, and even cushioning your organs and nerves.
In the previous video, we talked about how essential short-chain PUFA (poly-unsaturated fatty acids) are, as our body cannot synthesize them. We have to eat them. But how does what we eat become useful in our bodies?
What is the FADS1 gene and why is it so highly represented in Native American and indigenous people from Greenland? What does that mean for human evolution?
This video is going to explore the creation and approval of the first race-based drug approved by the FDA. Bidil was developed to deal with the high percentage of Black Americans dying of heart disease, but was it actually successful?

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Module 9: Adapting to Our Environment, Part 2

In Module 8, we used milk to orient ourselves on a few examples of human adaptation. We saw, from lactase persistence to fatty acid synthesis to adaptation to the Arctic, that human biological variation is selected on by variation in our geographic environments as well as variation in our cultural environments. Adaptation is context-specific, and what is beneficial in one setting is not necessarily beneficial in another. 

We take our exploration of this concept a step further in this module. Instead of milk, we’ll use blood as our focal point. This journey goes from the structure of the hemoglobin protein inside a red blood cell to the proteins on its surface, and from the heights of the Himalaya Mountains to the puddles that form in our agricultural fields, all bearing witness to the interplay between environmental variation and the variation in our blood. 

(I used other resources for the content in this module.)

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Module 10: Creative Discovery Project in Science Communication

With this module, we are going to shift gears. Modules 1 through 9 introduced you to evolution, the science of biological variation, and an overview of the current scientific understanding of human genetic variation and adaptation. Over the last few weeks in your discussion section, you explored the entanglement of science and society through the history of Henrietta Lacks and what happened to her family and to her cells. A thread that kept appearing in the book The Immortal Life of Henrietta Lacks is the break-down in communication between scientists and non-scientists. As we move into the history of the science of human biological variation, you will see more examples of how science, and society’s understanding and utilization of that science are much more intertwined, and much less objective than any of us would like. 

As part of UC Berkeley’s American Cultures Creative Discovery program, for the rest of your discussion sections this semester, you will explore science communication and give it a try yourself. There is no better way to understand a process than to experience it first-hand!

(This module prepared students for this project. There are no publicly available videos.)

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Module 11: The Science of Race in US History

The city of Cahokia in AD 1250 was a spectacularly large city for the time. Archaeologists estimate that there were between 10,000 to 20,000 residents; more people lived in Cahokia than in the city of London. Located near where you find St. Louis today, Cahokia was the center of a large cultural group referred to as the Mississippians (the Hopewell and Adena cultures), with an extensive network that spanned across almost half of the continental United States. 

Have you ever heard of it? In all the years I have taught this class, a total of only about a dozen people have ever replied, “yes”. Why is this larger-than-London-sized ancient city in North America mostly unknown to the people who live in North America today?

In this module, we will explore the interplay between the science of human biological variation and the cultural context in which it is done. Given that this is an American Cultures course, we will focus our attention on the United States. Let’s begin at the beginning.

In Module 6 you were introduced to the first people who came to live in the Western Hemisphere. The  Beringian Standstill people moved to the southwest, back into eastern Asia, and also to the southeast as the ocean level rose and the ice sheets receded. The latter are the ancestors of the Indigenous people living in what we now call North and South America. Starting about 15,000 year ago, those ancestral people spread out geographically all across the two continents and flourished until the 15th century AD.

When British and Spanish colonialists first set foot in the Western Hemisphere, there were people already here. When the European colonists started to establish massive agricultural endeavors, they needed labor. As you know from your reading in Module 2, the Indigenous people who lived in the 16th century missions in Florida carried out a significant amount of manual labor that is recorded in the morphology of their skeleton, but this did not provide as much cheap labor as was desired (plus, Queen Isabella of Spain had deemed that no Spanish subjects would be forced into slavery). From the 17th to 19th centuries, slaves were brought from western Africa to work on plantations, enabling the economic power of the early United States. As the economy grew, immigrants from Asia made their way here across the Pacific ocean. And as European countries became politically unstable in the early 20th century, even more immigrants made their way across the Atlantic.

Simultaneous to all of this mixing of people from around the world was a young country trying to prove its mettle. As American scientists strove to make international names for themselves, the earliest scientific idea they came up with that was widely lauded across Europe concerned the science of race.

In this module, we will use the hindsight of history to explore how the American science of human biological variation fed into the huge power imbalances between the five major groups of people living in the United States (the descendants of the early colonialists, the Indigenous peoples, the African slaves, the Asian immigrants, and the later European immigrants). We will look at the lingering effects of this today in Module 12.

Where did the concept of race come from? Where did colonization begin? To figure that out let’s go back into European history, starting with the Iberian peninsula. (Please forgive the mislabeled first slide. I was getting really tired by the time I was recording this one…)
In this video we will go back to the very first United States Census to track some of the groups of people present during the “birth” of the country. (Please forgive the mislabeled first slide. I was getting really tired by the time I was recording this one…)
Early colonialists thought mounds were built by gods and while they were wrong, they can still give us exceptional details about the culture and people who lived in North America before us.
From measuring cranial capacity and head size to a test that quantifies intelligence in individuals, we are going to talk about the ways that humans have tried to define intelligence and ultimately failed.

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Module 12: Race & Ethnicity in the United States Today

You have learned over the course of the semester that human biological variation does not fall into racial categories. Genetic variation does not reveal distinct clusters that correspond to deep biological divisions between racial groups. Rather, genomic studies show that human populations have been migrating, separating, converging, diverging, and intermixing with each other for hundreds of thousands of years. Each of our genomes is a composite of intermixed populations rather than a direct lineage from any one of them.

We have also seen that phenotpyic variation does not cluster into races either. Variation in skin pigmentation is clinal, varying by latitude and exposure to ultra violet radiation from the sun. Variation in body shape is related to variation in temperature, but migration makes this an imperfect correlation. Blood types, lactase persistence, adiposity, adaptation to hypoxia or to the Arctic, immunity, all of these phenotypes vary but do not do so along the racial categories that are embedded in the United States’ approach to human variation. No phenotypic variant is found only in Africans, Asians, Caucasians, Native Americans, or Polynesians. 

By looking at history, we’ve seen that the racial categories used in the United States began as concepts to understand, to explain, and, in many instances, to justify geopolitical relationships. Throughout the 18th, 19th, and 20th centuries, scientists in the United States went in search of evidence that would explain what they could see before them: people with dark skin were enslaved, the Indigenous people in North America had been severely and negatively affected by European colonization, recent immigrants did not perform as well on IQ tests, people from northern European countries had most of the wealth. When you go in search of finding differences between categories of people in an attempt to explain the existence of an economic hierarchy, everything you find is viewed through the lens of biological determinism. The differences are then interpreted as “biological”, as “genetic”, as, consequently, as inevitable rather than as the outcome of particular sequence of historical events. 

Science has long moved on from this framework. We have tested the hypothesis of human races and rejected it over and over again. But the general public’s understanding of human variation hasn’t caught up. The momentum of racialized biology is hard to push off course, despite the fact that it is scientifically incorrect.

This is where you come in. You are an important change-maker in the effort to update our culture’s understanding of human biological variation. Now that you know the science, you can bring this updated scientific knowledge with you to your every day life. You can push against the historical momentum of racism whenever you have a chance.

In this module, we will explore what those every-day life experiences look like, how the history of racialized human biology affects everything from the census to your SAT scores, from the medicines your doctor prescribes to the pay-out you may receive in a legal settlement. We’ll mostly focus on healthcare and medicine, but not exclusively.

We are constantly asked to check a box indicating our race and ethnicity, but what exactly do the terms race and ethnicity refer to?
One of the most obvious ways to see that the racial and ethnic categories that we use in the United States are social constructs rather than biological realities, is to look at the evolution of the US census.
Should we move on from using the concepts of race? Should we just stop thinking about them? I am going to present some scientific evidence that both suggests we should and then also why we really cannot.
What do you think about the SAT exam? Do you think it accurately portrays your abilities and potential? I would guess that only a handful of you might feel like that. Let’s talk about the intersection between harmful stereotypes and standardized tests.

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Module 13: Human Variation in Sex and Gender

We have covered so much information about human biological variation this semester! You should be proud of all your hard work and how much you know. But before we let the semester come to a close, there is one glaringly huge way that people vary that we haven’t spent anywhere near enough time talking about: sex and gender. In this module, we’ll clear up any confusion you may have about the difference between sex and gender, and then we’ll add nuance to what we mean by the word “sex”. The English language divides us into female and male, girl and boy, woman and man. This use of the language implies that these are discrete categories. But, like we saw with taxonomic nomenclature, these categories do not accurate reflect the range of variation that actually exists. Male and female are dichotomous terms superimposed on a bimodal but continuous distribution. Let’s take a look at the evidence.

In this module, we are going to talk about sex. But what do I mean when I say sex? It can mean so many things, but in this video we are talking about sex chromosomes, specifically the X chromosome.
Now, we are going to focus on the Y chromosome.
Here we explore the development of sex differentiation in the gonads, the ovaries, the testes, and also in the external genitalia.
Welcome to a discussion of typical male and female anatomy!
In the last video, we talked about typical male and female external genitalia. Now, I want to talk about the intermediate, about variation in sexual dimorphism.
We are almost at the end of our exploration of human biological variation in regards to sex. Now we are going to talk about sexual orientation.

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Module 14: Fertility

We began the semester talking about evolution. From an evolutionary perspective, what matters is how many offspring an individual has that make it to adulthood and successfully reproduce themselves. The number of offspring an individual has is referred to as that individual’s fertility. And therefore, evolutionary success is measured by fertility. In this last module of the semester, we’ll tie together pieces from across the previous modules by talking about this most fundamental part of human evolutionary biology. There is so much variation in how many children people have. Some people have none, some have a dozen. Some people have a strong opinion about how many children they want and others let children happen. As you work through the experiences in this module, pause from time to time and let your mind wander through all of the past modules. How might phenotypic variation, adaptation, and selection have influenced fertility in past populations? We’ll give you a chance to tie those all together in the final quiz. Let’s get started!

In the previous module, we talked about the fundamentals of spermatogenesis. In this video, we’ll dive into that process for females, what is called oogenesis.
As you’ve seen, there are many moving parts to work together in concert for reproduction to occur. Timing is everything. This is where hormones come in, they basically drive it all.
Now that you have a good handle on the biology of human reproduction, we will take a look at the interface between that biology and culture.
Now that we have a handle on some important aspects of fertility, we need to think about how fertility varies from population to population. We are going to explore some of these natural populations and the culture that is essentially tinkering with fertility outcomes.
While there is a lot of variation in our fertility today, there has been even more over the course of our evolution. Let me take you on an evolutionary journey.
Unlike giraffes, human babies cannot stand and walk with their herds almost directly after birth. Needless to say they are helpless and therefore classified as altricial young. In this video, we are going to explore the needs of human infants and how the extended period of care has shaped a lot of human life and culture.
We have talked about plant and animal domestication in these videos, now let’s pull some of those threads together and think about how the agriculture that defined the Neolithic also affected fertility.
The main demographic transition acknowledged by most is the one that started about 200 years ago with the Industrial Revolution, with advances like personal hygiene and improved nutrition.
Before we start to think that we have this whole fertility thing understood, I want to add a bit of the scientific process into our conversation, specifically for the concept of the human estrus.
We saw that controlling fertility was and still is a key component to the demographic transition. For this video, I want to share just a little of what is known about the history of contraception and how technology is overcoming infertility.

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