3.9 billion years.
That is the predicted amount of time life has been alive.
That, to the human mind, is not a comprehensible number. We, as human beings, cannot even begin to know how large that number is. Its impossible.
The first eye was a light sensor to let single celled aquatic creatures know if they were close to the surface, or deeper underwater.
Our DNA replication is a very specific process. First, the DNA is split. Then, RNA polymerase makes an rna copy of the split section. This rna copy is used to make the second half of the original split DNA piece, resulting in two daughter DNA strands. Because of this complex process, the nitrogenous bases in DNA can be messed up sometimes. These are called mutations. Sometimes they do nothing, sometimes they kill. However, when they are helpful, they pass on.
You see, a helpful mutation allows an organism to have a higher survival chance. With this, it can find more mates, or reproduce longer. Even though they have a mutation, that mutation will then be copied to their offspring. Therefore, organisms that have helpful mutations reproduce more than those without, causing it to spread without the species.
Imagine the first organism to be able to distinguish forms, no matter how basic, instead of just using chemical sensory like bacteria and cells. It would get more food, from finding more prey. That means more offspring, and they beat the competition. Further down the line, imagine a mutation that allows for harder skin that predators have a tougher time getting through. They will outlive competition in their species. For the eye, do this over billions of years, with countless failures, mutations, among other things.
For evolution evidence, here is one big thing:
Every. Single. Limbed animal has the same limb structure:
one strong bone, two bones below it, and then a cluster of smaller bones for the wrist and hand. Every single limbed land animal on Earth has this. Why?
The fossil of a transitional species called Tiktallik was discovered. It was a fish. However, it had one thing. It has bony fins, that allowed it to host itself up on land, and move just a little bit. And what was the bone structure, you ask?
One stronger bone, two bones, then a cluster of bones for the wrist. Same that every limbed animal has today.
This ablity to go on land just for a little bit made a huge advantage. Predator? No problem. Hide for a couple seconds above water and it will go away. Over 400 million years, countless mutations occurred, including longer bones, for more movement, as well as better suited breathing process for land. Transitional fish have lead to every land animal with bones today.