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2.2: The Human Animal

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    "Cousins"

    Relative to all animals, humans and chimpanzees are "cousins." From genes to morphology to behavior, they are similar in many ways. That's because both of them are primates, and they share an evolutionary past.

    Figure \(\PageIndex{1}\): Two juvenile chimpanzees in the 

    Characteristics of Living Things

    To be classified as a living thing, most scientists agree that an object must have all seven of the following traits. These are traits that human beings share with other living things.

    1. homeostasis
    2. organization
    3. metabolism
    4. growth
    5. adaptation
    6. response to stimuli
    7. reproduction

    Homeostasis

    All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The condition in which a system is maintained in a more-or-less steady state is called homeostasis. Human beings, for example, maintain stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn't freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

    Organization

    Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in the figure below. What similarities do you see?

    thin blue cell with blue nucleus in center
    cells forming organized like bricks in a wall
    Figure \(\PageIndex{2}\): A human cell (right) is flake-shaped; the nucleus is visible as a blue sphere in the center of the cell. Onion cells (left) are organized like bricks in a wall. The nucleus of each onion cell is visible as a blue sphere on the edge of the cell.

    Metabolism

    All living things can use energy. Their cells have the "machinery" of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by converting chemicals and energy into cellular components. This form of metabolism is called anabolism. They can also break down, or decompose, organic matter, which is called catabolism. Living things require energy to maintain internal conditions (homeostasis), for growth, and other life processes.

    Growth

    All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. For example, a human infant has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?

    parent holding baby's hand
    Figure \(\PageIndex{3}\): A human infant has a lot of growing to do before adulthood.

    Adaptations and Evolution

    An adaptation is a characteristic of populations. Individuals of a population carry a variety of genes. When the environment changes, some individuals of the population can withstand the changed conditions and reproduce more than the individuals who cannot live in the given environment. A change in the allele frequencies and makeup of the populations over time is called evolution. It comes about through the process of natural selection.

    Response to Stimuli

    All living things detect changes in their environment and respond to them. A response can take many forms, from the movement of a unicellular organism in response to external chemicals (called chemotaxis), to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).

    Reproduction

    All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing into two cells. This is how bacteria reproduce. Reproduction in human beings and many other organisms is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.

    Feature: Myth vs. Reality

    Myth: Viruses are living things.

    Basic Scheme of Virus
    Figure \(\PageIndex{4}\): Diagram of a general virus. The capsid encloses the genetic material of the virus. The envelope which surrounds the capsid is typically made from portions of the host cell membranes (phospholipids and proteins). Not all viruses have a viral envelope. Some viruses contain other proteins and enzymes

    Reality: The traditional scientific view of viruses is that they originated from bits of DNA or RNA that were shed from the cells of living things but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not have most of the defining traits of living organisms. A single virus called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, called a capsid. Viruses have an organization, but they are not cells and do not possess the cellular "machinery" that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow. They do not seem to respond to their environment, and they can reproduce only by invading and using "tools" inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them. That's why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.

    Within the last decade, new discoveries in virology, the study of viruses, suggest that this traditional view about viruses may be incorrect and the "myth" that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms such as bacteria. Some of the genes code for proteins needed to build new viruses, suggesting that these giant viruses may be able — or were once able — to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells but at some point lost their cellular nature to become modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.

    How Humans Are Classified

    You probably know that modern humans belong to the species Homo sapiens. But what is our place in nature? How are our species classified? A simple classification is represented in Figure \(\PageIndex{5}\). Humans can move on their own and are placed in the animal kingdom. Further, humans belong to the animal phylum known as chordates because we have a backbone. The human animal has hair and milk glands, so we are placed in the class of mammals. Within the mammal class, humans are placed in the primate order.

    classification of humans
    Figure \(\PageIndex{5}\): This taxonomic diagram shows how our species, Homo sapiens, is classified. Our scientific name indicates that we are in the genus and species homo and sapiens. Our family is the hominid, within the order primate, within the class mammal, within the phylum chordate, within the kingdom animalia, within the domain Eukarya.

    Humans as Primates

    Living members of the primate order include monkeys, apes, and humans; and any member of this order of mammals is called a primate. At some point in the distant past, we shared ape-like ancestors with all these modern groups of primates. We share between 93 percent and almost 99 percent of our DNA sequences with them, providing hard evidence that we have relatively recent common ancestors. Besides genes, what traits do we share with other primates? Primates are considered generalists among mammals. A generalist is an organism that can thrive in a wide variety of environmental conditions and make use of a variety of different resources, such as consuming many different types of food. Although primates exhibit a wide range of characteristics, there are several traits that are shared by most primates.

    Primate Traits

    Primates have five digits (fingers or toes) on each extremity (hand or foot). The fingers and toes have nails instead of claws and are covered with sensitive tactile pads. The thumbs (and in many species the big toes as well) are opposable, meaning they can be brought into opposition with the other digits, allowing both a power grasp and a precision grip. You can see these features of the primate extremities in the capuchin monkey pictured below (Figure \(\PageIndex{6}\)).

    Monkey eating branches
    Figure \(\PageIndex{6}\): The five fingers, opposable thumb, and other primate features of the hand give this capuchin monkey great manual dexterity. This is the primary reason these primates are trained to assist quadriplegic human beings with daily tasks.

    The primate body is generally semi-erect or erect, and primates have one of several modes of locomotion, including walking on all four legs (quadrupedalism), vertical clinging and leaping, swinging from branch to branch in trees (brachiation), or walking on two legs (bipedalism), the last of which applies only to our own species today. The primate shoulder girdle has a collar bone (clavicle), which is associated with a wide range of motion of the upper limbs.

    Relative to other mammals, primates rely less on their sense of smell. They have a reduced snout and relatively small area in the brain for processing olfactory (odor) information. Primates rely more on their sense of vision, which shows several improvements over that of other mammals. Most primates can see in color. Primates also tend to have large eyes with forward-facing placement in a relatively flat face. This results in an overlap of the visual fields of the two eyes, allowing stereoscopic vision, or three-dimensional, vision. Other indications of the importance of vision to primates are the protection given to the eyes by a complete bony eye socket and the large size of the occipital lobe of the brain where visual information is processed.

    Primates are noted for their relatively large brains, high degree of intelligence, and complex behaviors. The part of the brain that is especially enlarged in primates is the cerebrum, which analyzes and synthesizes sensory information and transforms it into motor behaviors appropriate to the environment. Primates tend to have longer lifespans than most other mammals. In particular, there is a lengthening of the prenatal period and the postnatal period of dependency of infants on adults, providing an extended opportunity for learning in juveniles. Most primates live in social groups. In fact, primates are among the most social of animals. Depending on the species, adult nonhuman primates may live in mated pairs or in groups of up to hundreds of members.

    Life in the Trees

    Scientists think that many primate traits are adaptations to an arboreal, or tree-dwelling, lifestyle. Primates are thought to have evolved in trees, and the majority of primates still live in trees. For life in the trees, the sense of vision trumps the sense of smell, and three-dimensional vision is especially important for grasping the next branch or limb. Having mobile limbs, a good grip, and manual dexterity are matters of life and death when one lives high above the ground. While some modern primates are mainly terrestrial (ground-dwelling) rather than arboreal, all primates possess adaptations for life in the trees.

    Figure \(\PageIndex{7}\) shows the present distribution of nonhuman primates around the world. Tropical forests in Central and South America are home to many species of monkeys, including the capuchin monkey pictured above. Old World tropical forests in Africa and Asia are home to many other species of monkeys, including the crab-eating macaque pictured above, as well as all modern apes.

    Nonhuman primate range shown on world map
    Figure \(\PageIndex{7}\): This map shows the present worldwide distribution of nonhuman primates.

    Humans as Hominids

    Who are our closest relatives in the primate order? We are placed in the family called Hominidae. Any member of this family is called a hominid. Hominids include four living genera: chimpanzees, bonobos, gorillas, orangutans, and humans. Among these five genera are just seven living species: two in each genera except humans, with our sole living species, Homo sapiens. The Orangutan mother pictured in figure \(\PageIndex{8}\) cradling her child shows how similar these hominids are to us.

    Orangutan mother and baby
    Figure \(\PageIndex{8}\): Orangutan mother and child

    Hominids are relatively large, tailless primates, ranging in size from the bonobo, or pygmy chimpanzee, which may weigh as little as 30 kg (66 lb), to the eastern gorilla, which may weigh over 200 kg (440 lb). Most modern humans fall somewhere in between that range. In all species of hominids, males are somewhat larger and stronger, on average, than females, but the differences may not be great. Except for humans, hominids are mainly quadrupedal, although they can get around bipedally if need be to gather food or nesting materials. Humans are the only habitually bipedal species of living hominids.

    The Human Genus

    Within the hominid family, our species is placed in the genus Homo. Our species, Homo sapiens, is the only living species in this genus. Several earlier species of Homo existed, some at the same time as Homo sapiens, but have since gone extinct, including the species Homo erectus. An artist's reconstruction of a Homo erectus individual is shown in figure \(\PageIndex{9}\).

    Homo erectus
    Figure \(\PageIndex{9}\): A Homo erectus individual, reconstructed here, shows both similarities and differences with modern Homo sapiens.

    By about 2.8 million years ago, early Homo species such as Homo erectus were probably nearly as efficient at bipedal locomotion as modern humans. Relative to quadrupedal primates, they had a broader pelvis, longer legs, and arched feet. However, from the neck up, they were still quite different from us. They typically had bigger jaws and teeth, a sloping forehead, and a relatively small brain.

    Video

    The story of human evolution began about 7 million years ago, when the lineages that lead to Homo sapiens and chimpanzees separated.
    Question after watching: How does the natural selection of certain physical and behavioral traits define what it means to be human?

    Homo sapiens

    During the roughly 2.8 million years of the evolution of the Homo genus, the remaining features of Homo sapiens evolved. These features include:

    • small front teeth (incisors and canines) with relatively large molars, at least compared to other primates.
    • a decrease in the size of the jaws and face, and an increase in the size of the cranium, forming a nearly vertical forehead.
    • a tremendous enlargement of the brain, especially in the cerebrum, which is the site of higher intellectual functions.

    The increase in brain size occurred very rapidly as far as evolutionary change goes, between about 800,000 and 100,000 years ago. During this period, the size of the brain increased from about 600 cm3 to about 1400 cm3 and the earliest Homo sapiens appeared. This was also a period of rapid climate change, and many scientists think that climate change was a major impetus for the evolution of a larger, more complex brain. In this view, as the environment became more unpredictable, bigger, "smarter" brains helped our ancestors survive. Paralleling the biological evolution of the brain was the development of culture and technology as behavioral adaptations for exploiting the environment. These developments, made possible by a big brain, allowed modern humans and their recent ancestors to occupy virtually the entire world and become the dominant land animals.

    Our species Homo sapiens is the most recent iteration of the basic primate body plan. Because of our big, complex brain, we clearly have a much greater capacity for abstract thought and technological advances than any other primate, even chimpanzees who are our closest living relatives. However, it is important to recognize that in other ways, we are not as adept as other living hominids around the world. We are physically weaker than gorillas, far less agile orangutans, and arguably less well-mannered than bonobos.

    Feature: Human Biology in the News

    Imagine squeezing through a seven-inch slit in rock to enter a completely dark cave full of lots and lots of old bones. It might sound like a nightmare to most people, but it was a necessary part of a recent exploration of human origins in South Africa as reported in the New York Times in September 2015. The cave and its bones were actually first discovered by spelunkers in 2013, who reported it to paleontologists. An international research project was soon launched to explore the cave. The researchers would eventually conclude that the cave was a hiding place for the dead of a previously unknown early species of Homo, whom they gave the name Homo naledi (Figure \(\PageIndex{10}\)). Members of this species lived in South Africa around 2.5 to 2.8 million years ago.

    Homo naledi individuals were about 5 feet tall and weighed around 100 pounds, so they probably had no trouble squeezing into the cave. Modern humans are considerably larger on average. In order to retrieve the fossilized bones from the cave, six very slender female researchers had to be found on social media. They were the only ones who could fit through the crack to access the cave. The work was difficult and dangerous but also incredibly exciting. The site constitutes one of the largest samples for any extinct early Homo species anywhere in the world, and the fossils represent a completely new species of that genus. The site also suggests that early members of our genus were intentionally depositing their dead in a remote place. This behavior was previously thought to be limited to later humans.

    Like other early Homo species, Homo naledi exhibits a mosaic of old and modern traits. From the neck down, these early hominins were well adapted for upright walking. Their feet were virtually indistinguishable from modern human feet (see image below), and their legs were also long like ours. Homo naledi had relatively small front teeth but also a small brain, no larger than an average orange. Clearly, the spurt in brain growth in Homo did not occur in this species.

    Digital reconstruction of the foot of Homo naledi
    Figure \(\PageIndex{10}\). The reconstructed foot bones of H. naledi are virtually the same as our own

    Watch the news for more exciting updates about this early species of our genus. Paleotontolgists researching the cave site estimate that there are hundreds if not thousands of fossilized bones still remaining in the cave. There are sure to be many more discoveries reported in the news media about this extinct Homo species.

    Review

    1. Identify seven traits that most scientists agree are shared by all living things.
    2. What is homeostasis? What is one way humans fulfill this criterion of living things?
    3. Define reproduction, and describe an example.
    4. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?
    5. Describe viruses and what traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
    6. People who are biologically unable to reproduce are certainly still considered to be alive! Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
    7. What are the two types of metabolism described here and what are their differences?
    8. What are some similarities between cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
    9. What are two processes that use energy in a living thing?
    10. Give an example of a response to stimuli in humans.
    11. Do unicellular organisms, such as bacteria, have an internal environment that they maintain through homeostasis?
    12. Evolution occurs through ___________ ____________ .
    13. If alien life is found on other planets, do you think they will necessarily have cells? Discuss your answer.
    14. Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .
    15. Outline how humans are classified. Name their taxa, starting with the kingdom and ending with the species.
    16. List several primate traits. Explain how they are related to life in the trees.
    17. What are hominids? Describe how living hominids are classified.
    18. Discuss species in the genus Homo.
    19. Relate climatic changes to the evolution of the genus Homo within the last million years.
    20. What is the significance of the fact that we share 93 to 99 percent of our DNA sequence with other primates?
    21. Which species do you think we are more likely to share a greater amount of DNA sequence with — nonprimate mammals or nonmammalian chordates? Explain your answer.
    22. What is the relationship between shared DNA and shared traits?
    23. Compared to other mammals, primates have a relatively small area of their brain dedicated to olfactory processing. What does this tell you about the sense of smell in primates compared to other mammals? Why?
    24. The part of the brain in primates that is specially enlarged is the:
      1. cerebrum
      2. cerebellum
      3. clavicle
      4. brainstem
    25. Why do you think it is interesting that nonhuman primates can use tools?
    26. True or False. All primates are primarily quadrupedal.
    27. True or False. Homo erectus was in the same family as modern humans.
    28. True or False. Humans are superior in all ways to other primates.
    29. Explain why the discovery of Homo naledi was exciting.

    Attributions

    1. Child and monkey, public domain via piqsels
    2. Human taxonomy by Suzanne Wakim dedicated to the public domain is based on biological classification by Peter Halasz, public domain via Wikimedia Commons
    3. White-fronted Capuchi Monkey by WolfmanSF, licensed CC BY 2.5 via Wikimedia Commons
    4. Non-human primate range by Jackhynes dedicated to the public domain via Wikimedia Commons
    5. Orangutan mother and baby by Bonnie U. Gruenberg, CC BY-SA 3.0 via Wikimedia Commons
    6. Homo erectus by Ryan Somma, CC BY-SA 2.0 via Flickr
    7. Foot of Homo naledi by W. E. H. Harcourt-Smith, Z. Throckmorton, K. A. Congdon, B. Zipfel, A. S. Deane, M. S. M. Drapeau, S. E. Churchill, L. R. Berger & J. M. DeSilva,
    8. Licensed CC BY 4.0 via Wikimedia Commons
    9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

    This page titled 2.2: The Human Animal is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Tara Jo Holmberg via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.