5.5: Tracking Genomes- Our Human Story Unfolds
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- Jennifer Hasty, David G. Lewis, & Marjorie M. Snipes
- OpenStax
Learning Objectives
By the end of this section, you will be able to:
- Describe how mtDNA sheds light on early human migrations and explain the Out of Africa model.
- Explain how studying the genomes and coevolution of lice can fill current gaps in the human fossil record.
- Describe the origin of human variation from an evolutionary perspective.
Mitochondrial Eve
Begun in 1990 and concluding in 2003, the Human Genome Project was an ambitious international effort that sequenced about 99 percent of the human genome with an accuracy of 99.99 percent. Genetics has thus far largely confirmed the Out of Africa theory , which proposes that early humans left Africa around 100,000 years ago and migrated to diverse areas of the world. When early humans left Africa and moved into Europe, they not only lived alongside but also interbred with non-African species such as the Neanderthal, who were already inhabiting the region.
Molecular anthropologists have an interest in determining when living human populations began diverging from one another. This has been difficult to do using nuclear DNA because it mutates much too slowly for measurable accumulations to occur in 200,000 years. Many of the genetic studies that have been conducted are thus based on genetic material carried in the mitochondria (mtDNA) , which are passed on maternally. There is no recombination in mtDNA, so unless the mitochondria carries a novel mutation, a child has exactly the same mitochondrial genes as its female genetic contributor (which may be its mother, egg donor, or someone in a similar genetic relationship). The mitochondria of every living person is a copy, modified only by rare mutations, of the mitochondria passed down via matrilineal descent from a population in our ancient past. This population is referred to as Mitochondrial Eve or mtMRCA (mitochondrial most recent common ancestor), believed to have lived in southern Africa 100,000–200,000 years ago.
As discussed in Chapter 4, the longer ago two populations share a common ancestor, the more time there is for mutations to occur and for adaptations and change to take place. Although genetic variation is small among the world’s human populations, it is greatest in Africa. This indicates that the human populations in Africa have the longest established genetic lineage. While multiple hypotheses exist as to human origins and new evidence could change the current views, the consensus is an Out of Africa model traced back to the matrilineal descent of a population living in Africa about 200,000 years ago.
How the Genome of Lice Can Fill in the Gaps
While perhaps not a pleasant thought, lice have long been a part of human history. Studying the coevolution relationship between humans and lice has shed much light on human story. Dr. David Reed, the Curator of Mammals and Associate Director of Research and Collections at the University of Florida Museum, has been studying the coevolution of humans and lice, an area of research that has developed only within the last 20 years. Reed’s groundbreaking research has the potential to fill in some big gaps in humans’ rather sketchy fossil record and provides important data that might have applications in medicine and biology. Two questions that this research has already begun to ask are when did we become less hairy and when did we start wearing clothes.
Figure 5.31 shows three types of lice associated with humans: the head louse ( Pediculus humanus capitis ), the body louse ( Pediculus humanus corporis ), and the crab louse or pubic louse ( Pthirus pubis ). Body lice infest clothing and lay their eggs on fibers in the fabric. Head and pubic lice infest hair, laying their eggs at the base of hair fibers. The human head and body lice (genus Pediculus ) share a common ancestor with chimpanzee lice, while crab lice (genus Pthirus ) share a common ancestor with gorilla lice. By tracking louse variations, scientists have been able to determine when the head louse and pubic louse diverged, enabling estimates as to when we lost our extra hair and when we started to wear clothes. It is interesting to note that the divergence of the genus Pediculus (head and body lice) correlates with the divergence of the human lineage from chimpanzees about six million years ago. Research on lice also provides further support for the Out of Africa model of human migration. Reed has observed that the genome of African lice shows a higher degree of genetic diversity than that of lice found elsewhere in the world, supporting the hypothesis that both humans and lice existed in Africa first.
Many hypotheses about what may have triggered the loss of hair in humans point to thermoregulation , the need to control body temperature in extreme conditions. Living in the heat of the savanna, humans needed a cooling mechanism to enable them to be better hunters. Other evidence of adaptation to the heat includes the appearance of sweat glands, which are more numerous in humans than in other primates. Another theory about the cause of the loss of hair among humans suggests that it was an adaptation to control parasites on the body. Did people immediately throw on clothes after losing all of that extra body hair? Reed’s research suggests that the wearing of clothes was not something that happened quickly. Humans lost body hair about a million years ago and didn’t start wearing clothes until around 170,000 to 190,000 years ago. That’s about 830,000 years living in their birthday suits! When humans began to wear clothes, the body louse adapted structures that enabled them to attach to clothes instead of hair.
Profiles in Anthropoogy
Molly Selba
Personal History: From the time Molly was young, she knew she wanted to be an anthropologist.
She took an archaeology class at the local community college when she was a high school student and went to field school over the summer. In college, she completed a double major in archaeology and anthropology, with a minor in Museums and Society. She later gained experience working with different museum collections and held internships at the Baltimore City Medical Examiner’s Office and the Smithsonian Museum of Natural History. After completing her undergraduate degrees, she knew that she wanted to pursue anthropology as a full-time career and began working towards her master’s and doctorate in biological anthropology.
Area of Anthropology: For Molly the most interesting thing about biological anthropology is the information that bones can tell us. Initially she was interested in what the history of disease could tell us about the lives of people in the past, but as she worked with biological anthropologists, her focus shifted to understanding how evolution can impact the shape of different bones.
She received her undergraduate degree from Johns Hopkins University in Baltimore, Maryland, and her Master’s degree from the University of Florida, where she is currently a PhD candidate. Her research interests include comparative anatomy, cranial morphology, and anatomical sciences education. She is most interested in how cranial morphology varies within and between species and how it is impacted by factors such as evolution and selective breeding practices. Her earlier research focused on the differences in cranial morphology in dogs created by artificial selection for facial reduction. Her dissertation research currently focuses on a comparative study of facial reduction across bats, primates, and dogs.
Accomplishments in the Field: For Molly her most important accomplishment in the field of anthropology has been in education and outreach. Throughout her time in graduate school, she was involved in school visits, working with teachers to facilitate the inclusion of human evolution into existing science curricula. She has specifically focused on helping educators find teaching materials that are culturally inclusive and responsive. She has led multiple professional development workshops for teachers on the same topic and has visited over two dozen classrooms and interacted with over 1,200 students in the last four years. Making science accessible to K-12 educators is an extremely important part of being a researcher, and she believes everyone in academia should strive to be effective science communicators.
“Studying biological anthropology helps us better understand our origin story as a species. It helps us recognize why our anatomy is the way that it is, how morphological changes over time can take place, and why we have such a diversity of life on earth. Just being able to recognize and identify our anatomy is only half the challenge—more important is our understanding why various traits are adaptive, how structure relates to function, or why leftover anatomical traits still persist in our body to this day.”
Natural Selection and Human Variation: Are Humans Still Evolving?
Human variability is attributed to a combination of environmental and genetic factors, including social status, ethnicity, age, nutrition, quality of life, access to healthcare, work and occupation, etc. As mentioned in Chapter 1, anthropology contributes many insights into both the social construct of race and the impacts racial categories have on people’s lives. The focus in this chapter is the role of natural selection in human variation.
A number of changes are associated with the Neolithic era and the rise of agriculture around 10,000 to 8,000 years ago. Many have noted that changes during this time period did not have positive effects on human and environmental health. The evolutionary mismatch hypothesis proposes that our bodies are best suited to the environments we have spent much of our evolutionary history in, which are very different from the environments we inhabit today (Li, van Vugt, and Colarelli 2018).
Humans evolved for one million years as hunter-gatherers. Today, human bodies are still trying to adapt to the largely grain-based diet brought about by agriculture, a diet characterized by less diversity and lower levels of nutrition than that of a typical hunter-gatherer. Incomplete adaptation to this change has made people susceptible to a number of diseases and nutritional deficiencies. Lactose intolerance is a prime example. The domestication of cattle and the drinking of cow’s milk began during the agricultural age, not very long ago in evolutionary history. Currently 65 percent of humans are unable to digest cow’s milk. Dental caries (cavities) are another problem linked to the change in diet associated with agriculture. The grain-based and high-sugar diets associated with agriculture are very different from the diet of hunter-gatherers. Neither our bodies nor the bacteria in our mouths have had time to fully adapt to this change.
Another adaptation that took place during the Neolithic era is related to variation in skin pigmentation. Humans who left Africa and settled in Europe about 40,000 years most likely had dark skin with high levels of melanin , which provides protection against ultraviolet radiation New data confirms that about 8,500 years ago, early hunter-gatherers in Spain, Luxembourg, and Hungary also had darker skin. Skin pigmentation is an adaptation to ultraviolet radiation, with different tones offering different advantages, depending on one’s distance from the equator. As humans migrated to the Northern Hemisphere, they were exposed to less ultraviolet radiation, which also meant less absorption of the Vitamin D needed for strong bones and other important immune functions. In order to compensate for this loss and to allow for greater exposure to ultraviolet radiation, skin pigmentation became lighter.
Another example of human variation as a result of adaptation to the environment can be seen in Indigenous populations in the Andes, Tibet, and the Ethiopian highlands. Each of these three groups faces the same environmental challenge, living in a low-oxygen environment, and they have responded with unique adaptations. Tibetans compensate for low oxygen levels by taking more breaths per minute than people who live at sea level. Those living at high altitudes in the Andes have been found to have higher concentrations of hemoglobin in their blood than other people. Ethiopians living at altitudes of 9,800 to 11,580 feet have neither of these adaptations. The explanation as to how the Ethiopian highlanders thrive in their environment is still a mystery.
This chapter has explored just some of the immense biological and cultural diversity of the genus Homo . This diversity has emerged in response to highly complex and variable environments connected to factors such as exposure to UV radiation, low oxygen levels at high altitude, changes in diet as a result of hunting or agricultural practices, geographic isolation in island populations, and climate variability and temperature. The genus Homo has proven to be resilient and adaptive in response to whatever environment or challenge it has faced. Variation is the key to survival. While scientists recognize that biological and cultural variation has greatly contributed to our human evolution, the human species is now facing a moment in which we must contemplate a difficult question: To what extent has our success as a species jeopardized the survival of other species and the health of the planet we all call home?
Mini-Fieldwork Activity
Identify the Fossil
Imagine that you have just discovered a hominin fossil with some of the characteristics listed below. Write each of the characteristics on a card and shuffle them together. Then, working in a group, decide which characteristics belong in the Homo group and which belong in the Australopithecus group. What scientific name (genus/species) would you give it, and what criteria did you use? (Note: This is an actual hominin fossil!)
- Brain similar in shape and structure to modern human brains
- Hands suited for tool use
- Small jaws and teeth
- Third molar larger than other molars (found in australopithecines and some early Homo species)
- Skull shaped more like H. erectus or H. habilis
- A sagittal keel (as seen in H. erectus )—a small raised ridge on top of the skull
- Bipedal and walked with a human gait
- Humanlike feet with arches and ankles
- Flaring blades of the pelvis (primitive)
- Broad rib cage
- Lower part of pelvis like modern humans’
- Small braincase (EQ 4.5)
- Skull shows prognathism (protruding face)
- Primitive shoulder position suggests suitability for climbing and swinging
- Curved fingers (What would that suggest?)
Additional Resources
Visual timelines and maps
The Atlas of Human Evolution has put together a user friendly interactive map on the development of Homo sapiens.
The Smithsonian Institute's Human Evolution Interactive Timeline provides an interactive overview of major milestones and species ranges.
Coevolution of lice and humans
David Reed, associate curator of mammals at the Florida Museum of Natural History, offers an explanation of his research for a general audience.
The Smithsonian Magazine explores possible causes and benefits of the human loss of hair.