In terms of Roman numerals they kept the I and the X from the Etruscan alphabet, and inverted the ? to make V. It is thought the Romans evolved the Roman numerals from the Etruscan tally stick method of counting, however it’s not known for sure how the other symbols evolved as they diverge from the Etruscan method which was still in use during the 19th century, long after the Roman system was abandoned. They may well have begun as Etruscan symbols (around the 4th century, BCE), but were eventually swayed by Latin symbols, such as C for centum (hundred) and M for mille (thousand).
The Etruscan tally stick method of counting replaces every fifth I with a ?, and every 10th with an X, such as IIII?IIIIXIIII?IIIIXIIII?IIIIX. Under the Romans, it is thought that every 10th V and X was scored with an I to denote multiples of 50 and 100. Thus the 10th X (100) would have begun as something like Æ. This later became >I< and then ?IC before finally settling upon C. L (50) is thought to have begun as a superimposed I and V, a bit like a down pointing arrow, that later got flattened to ? before finally become L. However 50 was also variously denoted with N, È, K, ? and ?, so it's not exactly clear where the L really comes from. It's certainly not an abbreviation for either "fifty" or "fiftieth". D and M are thought to derive from the hundredth V (500) and hundredth X (1000) which were both circled. Thus 1000 was denoted with ? or ? which evolved into ?, ? and ? before finally settling on M. ? is a clear influence upon D (half the symbol, hence half the value). This seems plausible because ? was also written as CI? and 500 became I?. This type of notation was used in the Middle Ages to denote large numbers, by extending the symbols on either side. However, other influences for D may stem from a circled V which somehow evolved into something like ? or ? during the time of Augustus.
The Etruscan tally stick method of counting replaces every fifth I with a ?, and every 10th with an X, such as IIII?IIIIXIIII?IIIIXIIII?IIIIX. Under the Romans, it is thought that every 10th V and X was scored with an I to denote multiples of 50 and 100. Thus the 10th X (100) would have begun as something like Æ. This later became >I< and then ?IC before finally settling upon C. L (50) is thought to have begun as a superimposed I and V, a bit like a down pointing arrow, that later got flattened to ? before finally become L. However 50 was also variously denoted with N, È, K, ? and ?, so it's not exactly clear where the L really comes from. It's certainly not an abbreviation for either "fifty" or "fiftieth". D and M are thought to derive from the hundredth V (500) and hundredth X (1000) which were both circled. Thus 1000 was denoted with ? or ? which evolved into ?, ? and ? before finally settling on M. ? is a clear influence upon D (half the symbol, hence half the value). This seems plausible because ? was also written as CI? and 500 became I?. This type of notation was used in the Middle Ages to denote large numbers, by extending the symbols on either side. However, other influences for D may stem from a circled V which somehow evolved into something like ? or ? during the time of Augustus.
