Your cousin's Facebook friends are probably going nuts over this image that claims to show how the early history of Arabic geometric design informs how we write numerals today. "Each figure contains its own number of corners and angles," reads the text. That's half-true of the drawings in the image. The rest is patently false.
The design we commonly refer to as Arabic numerals today actually derives from Indian mathematics between the 2nd century BC and 3rd century AD. That's when scholars believe that the Bakhshali manuscript, the oldest extant document in Indian mathematics, was written. Aside from it's age, the manuscript is notable for expressing the early examples of algorithms as well as the foundational concept of the mathematical zero. It's also pretty easy to see the similarities between the ancient numeral design (also known as Brahmi numerals) and the modern day Arabic numeral system (also known as the Hindu-Arabic numeral system).
Numerals from the Bakhshali manuscript, which was discovered by Augustus Hoernle in 1887 and believed to have been written between the 2nd century BC and 3rd century AD.
Obviously, the design evolved over the course of many centuries. The system spread to ancient Persia in the Middle Ages, and the Europeans eventually adopted it from the Arabs. The first known use of Arabic numerals in the West was in the Codex Vigilanus, compiled by three monks in northern Spain between 881 AD when it was first compiled and 976 AD when it was last updated. Note how the series of numerals at the bottom of the snippet below is written from right to left:
The Codex Vigilanus contained a variety of historical documents, from early canonical and civil law to a calender.
Look familiar? With the exception of the "4" and "5" figures, almost all of the numerals are perfectly recognisable to the modern Westerner. But the evolution doesn't stop there. The Hindu-Arabic system was locked into place a couple centuries later by none other than Leonardo Fibonacci. In 1202, his Liber Abaci (or Book of Calculation in English) popularised the system amongst mathematicians across Europe. That's the same book that contains the famous Fibonacci sequence.
Over the course of the next couple centuries, the Hindu-Arabic system gained widespread adoption in mainstream European society, spreading north from its Arab roots in Spain. By the 15th and 16th centuries, the numerals appeared on clocks and inscriptions in Britain and were detailed in German teaching manuscripts, like the fencing manual below:
Looking ahead to the Renaissance years, more academic reviews of the history of numerals start to appear. These studies would show that the Hindu-Arabic system that had become popular in Europe was both derivative of the ancient Brahmi numeral and influenced by typographical systems that evolved in Europe. French historian Jean-Étienne Montcula published his Histoire de la Mathematique in 1757, along with some helpful diagrams charting the roots of Arabic numerals. You'll notice that you don't see that X-shaped number eight or reverse curly-cue number nine anywhere on the chart:
Check out a more detailed chart from Montcula here.
Row seven contains 10 very familiar characters. Labelled Chiffre Modernes ("modern digits"), the numeral system in common usage throughout Europe by the mid-18th century is more or less identical to what we use today. No, you can't count the number of angles and figure out which number each symbol represents. But you can depend on history to provide complicated, however reasonable explanations behind the facts. Your Facebook News Feed is not so dependable at providing the whole story.