Evolution is a scientific fact.
We don't know exactly how life originated on earth. We think that it arose from non-life (this idea is called "abiogenesis"), but we don't know. And the fact that science doesn't yet know for certain doesn't mean that the religionists are right ("Allah created everything").
Life forms on earth reproduce. Any life form that didn't reproduce is extinct.
Approximately 99% of all the species that ever lived on earth are now extinct. The ones that are still here on earth are the survivors. They (or their ancient ancestors) outlived all the weird stuff that Mother Earth could throw at them — floods, ice ages, meteor hits, climate change.
Reproduction does not produce an exact copy of the parent; my children are not exactly like me. Their children will not be exactly like their parents.
The subtle differences in my offspring will either help them survive more successfully in the environment or will hinder their survival. Nature "selects" the individual that is best able to survive and reproduce.
The survivors, those who get to be old enough to reproduce, will reproduce, and most likely will pass on their survival/survivability characteristics to the next generation.
None of my ancestors (and none of your ancestors) died of childhood illnesses or childhood accidents. The one thing that we absolutely know about all of your ancestors and all of my ancestors is that they lived long enough to be old enough to reproduce.
There are a thousand different factors that can make a difference in the survivability of an individual, depending on (1) the type of animal (or plant) and (2) the environment that it lives in. There are many different ecosystems on the earth; some of them are conducive to the survival of certain species, and some are not.
Some of the "survival factors" are:
For the life form itself:
Sexual attractiveness
Size (larger, or maybe smaller)
Speed and strength
Color
Keen eyesight
Intelligence
The ability to regulate body temperature
The ability to cooperate with other members of the species
For the environment:
Temperature
Rainfall
Terrain: flat, rocky, mountainous, etc.
"Survival of the fittest" doesn't that mean you're the strongest. It just means that you are best able to fulfill the requirements of your environment.
The terminology of evolution is important: Species emerge; an individual is selected; populations evolve.
Individuals do not evolve.
Remember that our very first ecosystem was the oceans, which cover 70% of the planet.
Every living creature on earth, the flowers and the monkeys and the viruses and the bacteria and the parakeets, are all equally evolved. There's no such thing as one species being "more highly evolved" than any other.
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