Biologically Speaking. Who Are We? How Do We Fit In?

You are about to launch on a study of human biology. Before you begin, it is appropriate to define who humans are and how they fit into the world of living things.

Who Are We? Certain characteristics tell us who human beings are biologically speaking.

Human beings are highly organized. A cell is the basic unit of life, and human beings are multicellular since they are composed of many types of cells. Like cells form tissues, and tissues make up organs. Each type of organ is a part of an organ system. The different systems perform the specific functions listed in Table 1.1. Together, the organ systems maintain homeostasis, an internal environment for cells that varies only within certain limits. The text emphasizes how all the systems of the human body help maintain homeostasis. The digestive system takes in nutrients, and the circulatory system distributes these to the cells. The waste products are excreted by the urinary system. The work of the nervous and endocrine systems is critical because they coordinate the functions of the other systems.

Human beings reproduce and grow. Reproduction and growth are fundamental characteristics of all living things. Just as cells come only from preexisting cells, so living things have parents. When living things reproduce, they create a copy of themselves and assure the continuance of the species. (A species is a type of living thing.) Human reproduction requires that a sperm contributed by the male fertilize an egg contributed by the female. Growth occurs as the resulting cell develops into the newborn. Development includes all the changes that occur from the time the egg is fertilized until death and, therefore, all the changes that occur during childhood, adolescence, and adulthood.

Human beings have a cultural heritage. We are born without knowledge of civilized ways of behavior, and we gradually acquire these by adult instruction and imitation of role models. It is our cultural inheritance that makes us think we are separate from nature. But actually we are a product of evolution, a process of change that has resulted in the diversity of life, and we are a part of the biosphere, a network of life that spans the surface of the earth.

Like other living things, humans are composed of cells, and when they reproduce, growth and development occur. Unlike other living things, humans have a cultural heritage

How Do We Fit In? Certain characteristics tell us how human beings fit into the world of living things.

Human beings are a product of an evolutionary process. Life has a history that began with the evolution of the first cell(s) about 3.5 billion years ago. It is possible to trace human ancestry from the first cell through a series of prehistoric ancestors until the evolution of modern-day humans. The presence of the same types of chemicals tells us that human beings are related to all other living things. DNA is the genetic material, and ATP is the energy currency in all cells, including human cells. It is even possible to do research with bacteria and apply the results to human beings.

The classification of living things mirrors their evolutionary relationships. It is common practice to classify living things into five major groups called kingdoms (Fig. 1.2). Humans are vertebrates in the animal kingdom. Vertebrates have a nerve cord that is protected by a vertebral column whose repeating units (the vertebrae) indicate that we and other vertebrates are segmented animals. Among the vertebrates, we are most closely related to the apes, specifically the chimpanzee, from whom we are distinguished by our highly developed brains, completely upright stance, and the power of creative language.

Figure 1.2. Classification and evolution of humans. a. Living things are classified into five kingdoms, and humans are in the animal kingdom. b. Human beings are most closely related to the other vertebrates shown. The evolutionary tree of life has many branches; the vertebrate line of descent is just one of many

Human beings are a part of the biosphere. All living things are a part of the biosphere, where they live in the air, in the sea, and on land. In any portion of the biosphere, such as a forest or a pond, organisms of one type belong to a population. The various populations interact with one another and with the physical environment to form an ecosystem in which nutrients cycle and energy flows.

In Figure 1.3, nutrient cycling is represented by blue arrows, and energy flow is represented by yellow to red arrows. Plants use the energy of the sun and inorganic nutrients to produce organic nutrients (food) for themselves and all living things. Animals that feed on plants may be food for other animals. A major part of the interactions between populations pertains to who eats whom. Eventually decomposers make inorganic nutrients, such as carbon dioxide and nitrates, available to plants once more.

Figure 1.3. Ecosystem organization. Within an ecosystem, plants use solar energy and inorganic nutrients to produce organic nutrients (food). Organic nutrients cycle through all the populations until decomposers make inorganic nutrients available to plants once more. Energy flows from the sun through all populations. Eventually the energy of the sun is converted to heat, which dissipates

When populations feed on one another, some of the organic nutrients are broken down to acquire energy, and eventually this energy dissipates as heat. Thus, energy flows and does not cycle in ecosystems. Notice that because energy flows through an ecosystem, it is dependent on a constant supply of solar energy. However, if an ecosystem is large enough, it needs no raw materials from the outside. A big ecosystem just keeps cycling the same inorganic nutrients.

Although ecosystems constantly undergo changes, ordinarily they still endure. Certain species may die out and new ones may come, but the ecosystem remains recognizable year after year. Ecosystems can endure and even flourish when disturbances are minor; major disturbances however, can cause irreparable damage.