What Makes a Thing “Living”? Biology is the many-faceted study of living things. But what, exactly, is a living thing? Some of our definition comes directly from the work of Pasteur, Mendel, and others. Their work demonstrated that life can come only from other life. That means that all living things must reproduce. They pass genetic information on to their offspring—and all living things do this with DNA. This genetic information helps determine the physical structure of the offspring. As Mendel and Morgan observed, an offspring’s physical structure can vary. This variation, sometimes called diversity, may have fortunate or unfortunate effects for the individual, but it helps the species survive. If a given population has numerous variations in its gene pool (all of the genes present in the population), the population is more likely to have at least some members that can survive an environmental change. Over time, these individual variations accumulate, reshaping the population in new ways. This is the nature of evolution. Because the species can only continue living via reproduction, the species is continuous over time. Living things also detect and respond to stimuli. A stimulus is a change, event, or substance that causes an organism to act. In animals and human beings, stimuli can be detected with eyes, ears, nose, touch, or taste buds. For example, you step outside and feel drops of rain on your head. You go back inside and get an umbrella. The raindrops are the stimulus. They were detected by your sense of touch. You responded by getting an umbrella. Pain is another example of a stimulus. If you stub your toe, the pain causes you to move backward or flinch. Drugs, chemicals, and electricity can also be stimuli. The important factor is that stimuli provoke responses. What else do all living things have in common? For every individual, from a bacterium to a blue whale, life begins and ends. In the stretch of time between those two phenomena, the living thing must, in essence, work to stay alive. Whether it feeds on insects or sunlight, every living thing must consume enough energy that its cells can carry out all their internal processes. These processes include building new cells for growth, removing dead cells and waste matter, helping the individual reproduce, and so forth. These processes are chemical; they depend on chemical reactions that can take place only under certain circumstances. Therefore, a living thing’s body must maintain those circumstances. These circumstances are often called the life form’s internal environment. Maintaining that environment—in other words, working to live—is called homeostasis. All living things do the following: ■ Reproduce. ■ Pass along their traits through DNA. ■ Consume energy sources and expel waste products to maintain homeostasis. ■ Respond to their environments. ■ Respond to stimulus. ■ Change over time. ■ Can differ as individuals while still being part of a species. ■ Consist of biomole ...

1. What Makes a Thing “Living”?
Biology is the many-faceted study of living things. But what,
exactly, is a living thing? Some of our definition comes directly
from the work of Pasteur, Mendel, and others. Their work
demonstrated that life can come only from other life. That
means that all living things must reproduce. They pass genetic
information on to their offspring—and all living things do this
with DNA. This genetic information helps determine the
physical structure of the offspring.
As Mendel and Morgan observed, an offspring’s physical
structure can vary. This variation, sometimes called diversity,
may have fortunate or unfortunate effects for the individual, but
it helps the species survive. If a given population has numerous
variations in its gene pool (all of the genes present in the
population), the population is more likely to have at least some
members that can survive an environmental change. Over time,
these individual variations accumulate, reshaping the population
in new ways. This is the nature of evolution. Because the
species can only continue living via reproduction, the species is
continuous over time.
Living things also detect and respond to stimuli. A stimulus is a
change, event, or substance that causes an organism to act. In
animals and human beings, stimuli can be detected with eyes,
ears, nose, touch, or taste buds. For example, you step outside
and feel drops of rain on your head. You go back inside and get
an umbrella. The raindrops are the stimulus. They were detected
by your sense of touch. You responded by getting an umbrella.
Pain is another example of a stimulus. If you stub your toe, the
pain causes you to move backward or flinch. Drugs, chemicals,
and electricity can also be stimuli. The important factor is that
stimuli provoke responses.
What else do all living things have in common? For every
individual, from a bacterium to a blue whale, life begins and

2. ends. In the stretch of time between those two phenomena, the
living thing must, in essence, work to stay alive. Whether it
feeds on insects or sunlight, every living thing must consume
enough energy that its cells can carry out all their internal
processes. These processes include building new cells for
growth, removing dead cells and waste matter, helping the
individual reproduce, and so forth. These processes are
chemical; they depend on chemical reactions
that can take place only under certain circumstances. Therefore,
a living thing’s body must maintain those circumstances. These
circumstances are often called the life form’s internal
environment. Maintaining that environment—in other words,
working to live—is called homeostasis.
All living things do the following:
■ Reproduce.
■ Pass along their traits through DNA.
■ Consume energy sources and expel waste products to maintain
homeostasis.
■ Respond to their environments.
■ Respond to stimulus.
■ Change over time.
■ Can differ as individuals while still being part of a species.
■ Consist of biomolecules arranged into cells, which have
definite internal structures and functions and in the more
complicated life forms, are organized into tissues and organs
that also have definite structures and functions.
Using these criteria, compare a rock and a snail. Both contain
large quantities of carbon. Both change over time. The rock may
be weathered by its environment, but it does not respond to
stimuli. A snail does respond to stimuli, such as the chemicals
that identify the presence of a food source. If a predator is
nearby, the rock is unaffected, but the snail’s instincts are in a
panic. Similarly, the rock may break into smaller pieces, but it
does not reproduce. It does not eat or produce waste. Its internal
molecules do not change. The snail, on the other hand, must

3. consume and expend quite a bit of energy simply to go about the
business of being a snail. Its body is constantly working to find
food, to process that food into energy and new cell tissue, to
survive the threat of predators and environmental change, and
finally to create other living copies of itself that will carry on
the snail species when this individual dies.
What if these criteria were used to compare the rock to
something else—say, a tree? Like a rock, the tree does not move
on its own. A tree does not stalk and eat its prey. Remember,
though, what Leeuwenhoek discovered: there is much more
going on than the human eye can see. A tree in the sun is eating.
It is metabolizing the sunlight into energy and releasing oxygen
as waste. Why? To maintain its internal functions. A tree grows
or withers depending on its environment. When the seasons
change, the tree responds, budding out new leaves or shedding
the old ones. The tree also reproduces. It might hide its seeds in
fruit like an apple. It might send seeds spinning down like a
maple in the form of a samara (winged fruit). It might encase its
seeds in a cone like a pine. It might simply send out a long root
to a new location like an aspen. The tree, it turns out, has far
more in common with the snail than with the rock.
Biology involves an immense variety of life forms. Because
these life forms all live on Earth, they are interrelated. During
the past couple of centuries, people have begun to recognize the
impact of environmental change on living things. Some
environmental changes—earthquakes, tsunamis, or wildfires—
are relatively easy to observe. Others are more elusive, such as
chemical changes in the ocean or shifts in the atmosphere.
Sometimes, a change in one species represents an environmental
change for another species, as when a disease in frogs causes a
drop in the population of the birds that eat those frogs.
Environmental change also includes the effects of human
activity. The extent to which people affect the world’s climate,
along with the responsibility human beings bear for attempting
to reverse some of these changes, is still a topic of major
debate. However, you can look around your neighborhood and

4. see the human effects on microclimates. The eaves of your
house provide a habitat for spiders; a building’s steam vent
creates a warm area in which lichen thrive; trash bins sustain
thriving communities of rats, birds, squirrels, raccoons,
bacteria, and bugs. Likewise, all these living things affect how
people live. The creatures in the trash might be doing us a favor
by reducing our quantity of waste. They could be carrying
diseases that might
drastically reduce our population. By catching mosquitoes and
flies, the spider might be preventing disease and keeping your
food supply safe. The lichen might eventually decompose into
soil whose nutrient patterns allow certain rare plants to grow.
These plants, in turn, attract different insects and foraging
animals and reshape a tiny part of your world.