I define “biological evolution” as change in the genetic characteristics of a population of organisms over time. A more formal definition would be:
"In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next."
There are some important points of emphasis to make regarding this definition.
Individual organisms do not evolve. Instead populations of organisms evolve. Every individual organism has the same DNA when they die that they had when they were born (within some minor constraints that aren’t worth discussing). But within populations, the relative frequency of genetic characteristics can – and will – change over time. For example, the average size of all of the individuals in the population may become smaller – or larger – over time. But they will very often change.
The actual direction of those changes is somewhat unpredictable. In regards to complexity, organisms may become more complex or they may become less complex over time.
Some are surprised by this point. They have been led to believe that evolution only works to make things more complex – bacteria being less complex than humans but humans evolving from bacteria. But actually it works in both directions. Given a long enough period of time, any “random walk” will take some populations further and further away from their starting point, some in the direction of more complexity and some in the direction of less complexity. For example, cave fish and cave salamanders have less complex eyes that their ancestors. In fact they have lost sufficient complexity so that they don’t work at all!
Moreover, by any measure bacteria, which are similar to the first life on Earth, are still the dominant type of life. There are without doubt many more individual bacteria cells than any other type of life, but even the total bio-mass of all bacteria also surely exceeds that of any other species (including humans).
Stephen Gould provided a wonderful analogy, which I quote below. In his analogy, a gutter represents fully-complex and thinking beings (i.e. humans) and the wall represents minimal complexity (a single cell). The walk is the path of evolution.
“It's an old statistical paradigm called the drunkard's walk, which is a wonderful way of illustrating how you can get directional and predictable motion within a totally random system. All right. Here's the story. A drunk staggers out of a bar. Here's the bar, and he's leaning right against the wall of the bar. Now, he's staggering completely at random, back and forth. There's a gutter 30 feet away. He staggers five feet every time he staggers, completely at random, goes towards the bar as often as he goes away, except if he hits the bar wall, he can't go through it, so he just stands there until he staggers away. Now, where does he end up every time? Of course, he ends up in the gutter. He falls down in the gutter, the thing's over. We understand that very easily.”
All life on Earth evolves, not just humans.
One creationist argued that human evolution is falsified by the fact that humans still suffer from diseases that are caused by microscopic life forms. She contended that if we were becoming continuously better adapted to our environment over time, then we should be developing immunities to these diseases. I pointed out that we have indeed evolved immunities to some diseases. But she was a bit startled when I pointed out that those microscopic organisms such as bacteria are evolving right along with humans. Those pesky “germs” are also becoming better adapted (particular becoming more immune to medications) over time.
The origin of life on Earth is not relevant to biological evolution. Instead, evolution is the theory that best explains how the diversity of life we see on Earth came about after the first living thing appeared.
As I write this, there is no established explanation for how life first came about on Earth, and creationists are fond of pointing that out. But that is not a problem for evolution itself. Life could have come from outer space or planted here by a space alien, but in any of those cases it wouldn’t make any difference. After life was here, it evolved. (Nonetheless, I discuss this subject in Chapter 11alongwith the Bog Bang.)
The bottom line: all life shares a common ancestor. All life on Earth is related to all other life on Earth.
 Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
 http://www.pbs.org/newshour/bb/science/gould_11-26.html, Nov. 26, 1996 interview "Spinning Evolution".