The complexity of life is often used as an arguement for intelligent design. However, the complexity of life can be explain by evolution, fossil records detail how species evolved over time gradually from simple single cellular organisms to complex multicellular organisms.
Usually in evolution you find that a protein used for doing one job in the body is adapted for use in something else. This can happen by a gene being copied, so it appears twice in a genome (the DNA sequence). Once this happens only one copy is needed, so the other one can change in random ways – undergo mutation. Genes lead to proteins, so making the gene appear twice makes the protein appear twice.
With lots of time and individuals who are descendants of the individual with the mutation, the second copy of the gene can become useful for a totally different purpose.
This happens with limbs as well: this is how dinosaurs changed some of their legs into wings, and became birds. Birds are also dinosaurs, so dinosaurs are not extinct.