Look at this sentence from the story about chiropractic.

It has been observed in nature, engineering, physics and geometry that a 60 degree arc offers the strongest defence against the destructive forces of gravity and motion.

What is the difference in meaning between:

        It has been observed in nature ...
         It was observed in nature ... ?

The first sentence suggests that it was observed in the past and we can still observe it today.
(present perfect tense)
The second sentence tells us only that it was observed in the past. We don't know if it is still observed today.
(simple past tense)

The present perfect tense links the past with today. Usually something happened then and it is still happening today or it still affects us today. It is also used for the very recent past.
The past simple only tells us about the past.

Look at these examples.
1a. Mum baked many cakes during her lifetime. (past simple - maybe she is no longer alive)
1b. Mum has baked many cakes during her lifetime. (present perfect - probably she is still baking cakes)

2a. He made some tea. (past simple - he made it in the past, but we don't know if he made it 5 minutes ago or several hours ago)
2b. He has made some tea. (present perfect - he made it recently and it is still hot)

3a. There have been a lot of strange stories. (present perfect - they started in the past and are probably continuing now)
3b. There were a lot of strange stories. (past simple - they are in the past)

4a. She studied two languages at university. (past simple - she may have done it recently or it may have been a long time ago - we don't have enough information to know exactly)
4b. She has studied two languages at university. (past perfect - she probably studied them quite recently)

5a. I knew your parents for years. (past simple - suggests that I used to know them, but I don't know them now.)
5b. I have known your parents for years. (present perfect - suggests that I knew them in the past and I still know them now.)