There are many biological reasons, also, why the Theory of Evolution is valid. I will attempt to cover the current evidence biologists use to support the theory, adding my own attempt to stand up for sanity. As for bol's mention of religion; many religions have come to accept that creation is not a valid theory.

Biologically, there are 6 major areas that support Evolution: Fossil Record, Embryology, Comparitive Anatomy, Study of Intermediate Forms, Biogeography and Molecular Biology. These are discussed briefly below.
  1. Fossil record

  2. Embryology
    • During the embryonic development organisms often develop structures that resemble those seen in ancestral adults.
      e.g. Human embryos with gills, gills are lost later in development

  3. Comparitive Anatomy
    • Organisms may share basic structural forms that suggests common ancestry (Homologolous structure).
      e.g. Forelimb structures

  4. Study of Intermediate Forms

  5. Biogeography
    (the study of the distribution of organisms)
    • Evidence of modern animals being decended from ancestors in the same area.

  6. Molecular Biology

As with bol's write-up, these facts do not prove the theory of evolution without doubt, but they do show it to be a very valid theory.
This is NOT a cut-and-paste write-up.