Scientists have known that monkeys and other mammals are capable of basic arithmetic. But how far can mathematical reasoning skills can be traced back in human evolution?

Scientists recently attempted to find out by successfully teaching rhesus macaques to identify symbols -- the 10 Arabic numerals and 16 letters -- representing the numbers zero to 25.

Each symbol was associated with varying levels of rewards for the test monkeys. The larger the symbol or number, the bigger and better the reward -- drops of water, juice or orange soda. The macaques were able to use their newly-acquired knowledge to distinguish which of two symbols represented the larger quantity, and earn the superior reward. The monkeys chose the larger symbol with 90 percent accuracy.

"The monkeys want the most of whatever is out there, and this is just one of many ways to figure out the best way to get the most," explained lead researcher and neurobiologist Margaret Livingstone of Harvard University Medical School.

Livingstone's work was published this week in the journal the Proceedings of the National Academy of Sciences.

Understanding mathematical symbols are an important indicator of advanced mathematical reasoning, as humans can much more efficiently distinguish between the symbolic numbers of 21 and 20 than they can differentiate a group 20 dots and 21 dots, or a pile 20 pebbles and 21 pebbles.

"They turned out to be like us -- more accurate when values were represented by symbols than by the number of dots," Livingstone said. "It tells us what good symbols are."

Scientists have previously shown chimpanzees, human's closest monkey relative, to possess math skills. Chimps and humans diverged on the evolutionary timeline about 6 million years ago. Human genetics split from rhesus monkeys roughly 25 million years ago, making them a more ancient ancestor -- and now, verifiably, one of the most primitive possessors of mathematic reasoning.