The timeline is split into BC (Before Christ) and AC (Anno Domini) because it is based on the Christian calendar. The Anno Domini (AD) system was first proposed by Dionysius Exiguus, a monk in the 6th century AD. He calculated that Jesus Christ was born in the year 525 AD, and so he used that year as the starting point for his new calendar system.

The BC system was created later, in the 17th century, by the Dutch scholar Isaac Newton. He wanted to create a system that was not specifically Christian, so he used the abbreviation BC, which stands for "Before Christ."

The BC and AC system is now the most widely used system for dating events in the world. However, some people prefer to use the CE (Common Era) and BCE (Before the Common Era) system, which is religiously neutral.

Here is a table that summarizes the different dating systems:

| System | Abbreviation | Meaning |
|---|---|---|
| Anno Domini | AD | In the year of our Lord |
| Before Christ | BC | Before Christ |
| Common Era | CE | Common Era |
| Before the Common Era | BCE | Before the Common Era |

I hope this helps! Let me know if you have any other questions.
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