Whether an amoeba or a baboon, the biological concepts governing them consist of a handful of attributes. These unifying themes of biology define what is living and what is not; if any of the themes don't apply to a certain individual, then it's not alive. Although viruses seem to be living things, many biologists do not consider them organisms because they lack one or more of these unifying characteristics. Organisms, no matter how strange or rare, must obey certain laws compatible with life.
Structure and Function of Cells
All forms of life consist of at least one cell. Cells provide structure to organisms and perform all functions. In the 17th century, scientists Robert Hooke and Anton von Leeuwenhoek observed cells and noted their characteristics under microscopes. These and subsequent observations led to formation of the cell theory, stating that cells make up all life, carry out all biological processes and can only come from other cells. All cells contain genetic material and other structures floating in a jelly-like matrix, acquire energy from their surroundings and are encompassed in an envelope protecting them from the external environment.
Interactions Between Organisms
Organisms don't exist in vacuums. Each living thing has uniquely adapted to a particular habitat and developed specific relationships with other organisms in its vicinity. In ecosystems, energy flows from the sun, which generates the initial energy. Plants use this light energy to make their own food, which becomes the source of energy. As subsequent organisms consume the plants, the energy transfers, too. Other creatures eat these plant-eating organisms and receive the energy. When an organism dies, its energy flow doesn't cease; instead, the energy travels to the soil and back into the environment through the actions of scavengers and decomposers that break down dead organisms.
Ecosystems also define relationships between organisms. Some organisms eat others in predator-prey associations. Parasites derive nutrients and shelter at the expense of others. Some organisms form close-knit connections with others in mutually beneficial relationships. As a result, changes affecting one species will influence the survival of other species within the ecosystem.
Organisms don't appreciate change. Any alteration in internal or external environments could spell death to a living thing. Much of the energy used by an organism maintains a consistent internal environment. Single-cell organisms keep their internal fluids, acidity and temperature relatively stable.
In multi-cellular creatures, all organ systems work in conjunction to balance substances such as fluids, ions, acidity, gases and wastes. Each species can tolerate only specific environmental conditions within its range of tolerance. Outside of this range lies the zone of intolerance where all members of a species die. Although the external environment changes, individuals have to maintain a constant internal environment through constant adaptation. Otherwise, they perish.
Reproduction and Genetics
All organisms reproduce to pass on characteristics to their offspring. Living things reproduce in a number of ways. In asexual reproduction, offspring are exact replicas of their parents. More complex lifeforms lean toward sexual reproduction, which involves the coming together of two individuals to produce offspring; thus, the offspring share half of the characteristics of each parent.
In the mid 1800s, an Austrian monk, Gregor Mendel, conducted a series of famous experiments in which he explored the relationship between sexual reproduction and heredity. Mendel realized that units called genes determined heredity and could be passed from parent to offspring.
In the early 1800s, Jean Baptiste de Lamarck, a French biologist, hypothesized that the use of certain structures would strengthen their existence whereas non-use of other structures would cause them to eventually disappear in subsequent generations. This would explain how snakes evolved from lizards when legs became unused and how giraffe necks grew longer with stretching, according to Lamarck.
Charles Darwin constructed his own theory of evolution of species and called it “natural selection.” Following his stint as naturalist on the ship, the HMS Beagle, Darwin formulated a theory that claimed all individuals possessed differences that allowed them to survive in a particular environment, reproduce and pass on their genes to their descendants. Those individuals who adapted poorly to their environments would have fewer opportunities to mate and pass on their genes. Eventually, the genes of the stronger individuals would be better represented in subsequent populations. Darwin’s theory has become the most accepted theory for evolution.