While it is hard to pen a clear definition of "life," most biologists agree that there are many characteristics in common among living things. If an object meets these characteristics, it is considered alive:
- Organized -Living things are made of atoms and molecules that are organized into cells. The cells in an organism can be either uniform or specialized for various functions. The cells can be further organized into tissues, organs and systems. Living things on Earth are quite diverse as to their organization and complexity.
- Homeostatic - Living things carry out functions that keep them in a constant, relatively unchanging state called homeostasis. For example, your body has systems that keep your body temperature constant -- you shiver if you're cold, sweat if you're hot.
- Reproduces - Living things make copies of themselves, either exact copies (clones) by asexual reproduction or similar copies by sexual reproduction.
- Grows/develops - Living things grow and develop from smaller and/or simpler forms. For example, a human begins life as a fertilized egg, developing into an embryo, fetus and then a baby. The baby subsequently grows into a toddler, adolescent and adult.
- Takes in energy from the environment - Staying in a relatively constant, organized state violates the second law of thermodynamics, which states that the degree of disorder (entropy) of all objects increases. For a living organism to maintain organization, it must take in, process and expend energy. The way humans and other animals do this is by eating food and extracting energy from it.
- Responds to stimuli - Living things respond to changes in their environment. For example, if a stimulus causes you pain, you respond by moving away from that object. If you place a plant near a well-lit window, the branches or shoots grow toward the light (phototropism). For protection, some animals change color to blend in with their surroundings (camouflage).
- Adapted to its environment - The characteristics of a living thing tend to be suited for its environment. For example, the fins of a dolphin are flat and adapted for swimming. The wing of a bat has the same basic structure as the bones in a dolphin's fin, but has a thin membrane that enables flight.
Now that we've got a definition of what life is, we need to look at how it changes over vast expanses of time. The basic rules governing whether species arise, live, remain unchanged or become extinct are those of evolution by natural selection as proposed by Charles Darwin. Darwin's theory of evolution has the following points to it:
- Similar organisms reproduce similar organisms -- a dog reproduces a dog, a dandelion reproduces dandelions and a fish reproduces a fish.
- Often, the number of offspring are overproduced such that the number that survive is fewer than the number reproduced.
- In any population, individuals vary with respect to any given trait, such as height, skin color, fur color or shape of beaks, and these variations can be passed on to the next generation.
- Some variations are favorable, in that they make those individuals best-suited to their environment, and some are not. Those organisms with favorable variations will survive and pass those traits on to their offspring; those individuals with unfavorable variations will die and not pass on their traits -- this is natural selection.
- Given sufficient time, natural selection will accumulate these favorable traits. The species will evolve.
Although Darwin's theory of evolution was proposed to explain changes in Earth-based species, its principles are general enough that it could be applied elsewhere in the universe as well.