Case Study Conclusion: Our Invisible Inhabitants
As you may recall, Julia’s strep throat was caused by Streptococcus pyogenes bacteria, the species shown in the photomicrograph above. She took antibiotics to kill the S. pyogenes, but this also killed her “good” bacteria, throwing off the balance of microorganisms living inside of her, which resulted in diarrhea and a yeast infection.
After reading this chapter, you should now know that microorganisms such as the bacteria and yeast that live in humans are also similar to us in many ways. They are living organisms and therefore share the traits of homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction with us. They, like us, contain genes, consist of cells, and have the ability to evolve. Julia’s beneficial gut bacteria help digest her food as part of their metabolic processes. Julia got a yeast infection likely because the growth and reproductive rates of the yeast living on her body were not held in check by beneficial bacteria after she took antibiotics. You can see that there are many ways in which an understanding of the basic characteristics of life can directly apply to your own.
You also learned how living organisms are classified, from bacteria that are in the Bacteria domain, to yeast (fungus kingdom) and humans (animal kingdom) that are both in the Eukarya domain. You probably now recognize that Streptococcus pyogenes is the binomial nomenclature for this species, and the fact that Streptococcus refers to the genus name.
As Julia’s doctor told her, there are many different species of microorganisms living in the human digestive system. You should recognize this as a type of biodiversity called species diversity. This diversity is maintained in a balance, or homeostasis, that can be upset when one type of organism is killed — for instance, by antibiotics.
Julia’s doctor advised her to complete the entire course of antibiotics because stopping too early would kill the bacteria that are most susceptible to the antibiotic, while leaving the bacteria that are more resistant to the antibiotic alive. This difference in susceptibility to antibiotics is an example of genetic diversity. Over time, the surviving antibiotic-resistant bacteria will have increased survival and reproductive rates compared to the more susceptible bacteria, and the trait of antibiotic resistance will become more common in the population. In this way, the bacteria can evolve and become better adapted to their environment — at a major cost to our health because our antibiotics will no longer be effective. This issue of improper use of antibiotics leading to increased antibiotic resistance is a major concern of public health experts.
After reading the last section of this chapter, you know how humans are classified and some characteristics of humans and our near relatives. Beyond our more obvious features of big brains, intelligence, and the ability to walk upright, we also serve as a home to many different organisms that may be invisible to the naked eye, but play a big role in maintaining our health.
In this chapter, you learned about the basic principles of biology and how humans are situated among other living organisms. Specifically, you learned:
- To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics, including:
- Maintaining a more-or-less constant internal environment, which is called homeostasis.
- Having multiple levels of organization and consisting of one or more cells.
- Using energy and being capable of metabolism.
- The ability to grow and develop.
- The ability to evolve adaptations to the environment.
- The ability to detect and respond to environmental stimuli.
- The ability to reproduce, which is the process by which living things give rise to offspring.
- Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity within species, and ecosystem diversity.
- The formal biological definition of species is a group of actually or potentially interbreeding organisms. In reality, organisms are often classified into species on the basis of morphology.
- A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, called binomial nomenclature.
- The domain, a taxon higher than the kingdom, was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. Humans and other animal species are placed in the Eukarya domain.
- Modern systems of classification take into account phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams.
- The human species, Homo sapiens, is placed in the primate order of the class of mammals, which are chordates in the animal kingdom.
- Traits humans share with other primates include five digits with nails and opposable thumbs; an excellent sense of vision including the ability to see in color and stereoscopic vision; a large brain, high degree of intelligence, and complex behaviors. Like most other primates, we also live in social groups. Many of our primate traits are adaptations to a life in the trees.
- Within the primate order, our species is placed in the hominid family, which also includes chimpanzees, gorillas, and orangutans.
- The genus Homo first evolved about 2.8 million years ago. Early Homo species were fully bipedal but had small brains. All are now extinct.
- During the last 800,000 years, Homo sapiens evolved, with smaller faces, jaws, and front teeth but much bigger brains than earlier Homo species.
Now that you understand the basic principles of biology and some of the characteristics of living organisms, in the next chapter, you will learn about the molecules that make up living organisms and the chemistry that allows organisms to exist and function.
Chapter Summary Review
1. What are the four basic unifying principles of biology?
2. A scientist is exploring in a remote area with many unidentified species. He finds an unknown object that does not appear to be living. What is one way he could tell whether it is a dead organism that was once alive, versus an inanimate object that was never living?
3. Cows are dependent on bacteria living in their digestive systems to help break down cellulose in the plant material that the cows eats. Explain what characteristics these bacteria must have to be considered living organisms themselves, and not just part of the cow.
4. What is the basic unit of structure and function in living things?
5. Give one example of homeostasis that occurs in humans.
6. Can a living thing exist without using energy? Why or why not?
7. True or False. Evolution is a change in the characteristics of living things over time.
8. True or False. Only some living things have genes.
9. Give an example of a response to stimuli that occurs in a unicellular organism.
10. A scientist discovers two types of similar looking insects that have not been previously identified. Answer the following questions about this discovery.
a. What is one way she can try to determine whether the two types are the same species?
b. If they are not the same species, what are some ways she can try to determine how closely related they are to each other?
c. What is the name for a type of diagram she can create to demonstrate their evolutionary relationship to each other and to other insects?
d. If she determines that the two types are different species but the same genus, create your own names for them using binomial nomenclature. You can be creative and make up the genus and species names, but be sure to put them in the format of binomial nomenclature.
e. If they are the same species but have different colors, what kind of biodiversity does this most likely reflect?
f. If they are the same species, but one type of insect has a better sense of smell for their limited food source than the other type, what do you think will happen over time? Assume the insects will experience natural selection.
11. Put the following taxa in order from the most specific to the most inclusive:
phylum; species; kingdom; genus; family; domain; class; order
12. Humans are in the _______________ domain.
13. Monkeys, apes, and humans are all in the:
A. Same genus
B. Same order
C. Same class
D. Both B and C
14. Amphibians, such as frogs, have a backbone but no hair. What is the most specific taxon that they share with humans?
15. Arboreal means:
A. Living on the ground
B. Living in the ocean
C. Living in trees
D. Living on grasslands
16. What is one characteristic of extinct Homo species that was larger than that of modern humans?
17. What is one characteristic of modern humans that is larger than that of extinct Homospecies?
18. True or False. Most primates live in social groups.
19. True or False. Most other mammals have longer lifespans than primates.
20. True or False. Archaea are classified into the Bacteria domain.
21. How is the long period of dependency of infants on adults in primates related to learning?
22. Name one type of primate in the hominid family, other than humans.
23. Why do you think that scientists compare the bones of structures (such as the feet) of extinct Homo species to ours?
24. Some mammals other than primates also have their eyes placed in the front of their face, such as cats. How do you think the vision of a cat compares to that of a mouse, where the eyes are placed more at the sides?
25. Living sponges are animals. Are we in the same kingdom as sponges? Explain your answer.
Credit: Public domain. This media comes from the Centers for Disease Control and Prevention's Public Health Image Library (PHIL), with identification number #2110.;
License: CC BY-NC 3.0