Can you explain how evolution occurs?
Try the Wiki entry on this one - there are whole courses on this topic!
Basically, evolutionary basics are:
1) resources (food, water, etc) in any ecosystem are limited.
2) members of an individual species of animal are different from each other (slightly different genes)
3) these differences make some individuals better able to complete for resources.
4) genetic traits are passed on to one's offspring/children (e.g. hair and eye color in humans).
5) heritable traits of better competitors will become more common in a given species if they help the organism thrive and have more children.
6) organisms will therefore change over time, either because different conditions select for particular traits or because genetic changes keep occurring, generations become better adapted to their environment
For example, giraffes 20 million years ago has short necks and food was abundant near the ground. The climate might have changed so that low lying food was no longer available except in tall trees. There may have been a random genetic mutation change that allowed some giraffes to have slightly longer necks and this allowed them to get high food and they had more babies and did better overall. Those giraffes that didn't have this mutation didn't do so well and perhaps died off. Then the longer neck giraffe became more successful and perhaps an even longer neck evolved that made them even more successful and had more babies, and so and so on.