Most instruments have understandable shapes.
A lute or balalaika has an obvious shape to try, probably based on something simple like a gourd or a triangle.
But the violin has that “waist” area and the curly slits.
Did those arise from some key instrument maker?
Or are they related to some dumbbell-shaped gourd?
Or were there many, many refinements leading to the shape?
The inward-curving sections are the ‘centre bouts’. They have a very practical purpose - they make room for the bow when the outer string are being played. If the gaps weren’t there, the body of the instrument would be in the way. The exact shape is partly to make construction possible (given that the curved ribs are made from individual pieces of wood), partly for strength, and partly for tradition. I’ve seen less-normal instruments, but beyond the ‘oooh, that looks odd’ factor, they’re not nice.
There are various instruments like the banjo, fiddle, ukulele, etc. which appear to be ‘primitive’ forms of the violin, having a more regular shape instead of the finely tuned shape and soundings of the violin.
I suspect that these are older instruments, and refinements of older stringed instruments created the current violin shape.
“Fiddle” is just another name for the instrument also known as the violin. Many classical-trained violinists are happy to refer to their instrument as “the fiddle.” No, you probably won’t find someone playing a Stradivarius at your local barn dance (You won’t find too many Strads at your next symphony orchestra concert, either - cost and scarcity see to that).
The banjo is not a “primitive” form of anything. It’s a unique American instrument which probably developed from similar instruments (long neck, round resonator) which would have been familiar to African-born slaves. Most modern banjos have about as much fancy hardware (in the form of tuning machinery) as a violin.
The 'ukulele appears to have developed from the Portuguese braguinha, which was brought to the Hawai’ian islands by Portuguese immigrants (Portuguese men had been encouraged to emigrate to Hawai’i to work in the sugarcane fields there). The Portuguese musical armamentarium included several guitar-like instruments, of which the *braguinha * was one. There are a couple of stories regarding the origin of the name “'ukulele,” but all agree on the Portuguese origin. In any case, the 'ukulele isn’t a “primitive” anything. The 'ukulele, *braguinha[/], and violin are related in that they are all ultimately derived from the older European viol, which looked a bit like a moder cello but had frets.
While this is some ways correct, it’s also true that ‘fiddle’ is often used to mean ‘non-classical violin’. Find real Irish or Scottish traditional musicians, and they’ll uniformly call their instrument by that name. It also can be used to refer to the different techniques used by the musicians. Plus, there’s all sorts of etymological connections to other ‘fiddles’ in Europe.
But this is all just differences in terminology and usage, right? Not a difference in instruments themselves? I’ve always understood that the same instrument (the actual piece of wood) could/would be called either violin or fiddle dependant more on the technique and/or style of music than the instrument.
A classical violin is certainly the most common ‘fiddle’, although it’s impossible to determine the popularity of instruments more than a century or two ago. However, there’s other instruments, such as the Hardanger Fiddle and the east European Suka, which are fiddles not comparable to the modern violin.
And if you mean “f holes” by “curly slits” you tread into deep water.
The difference in front and back on a violin lets it resonate in a huge number of modes. Get a supercomputer and go wild. You’ll develop the next super-violin.
Toby Faber’s book, Stradivari’s Genius : Five Violins, One Cello, and Three Centuries of Enduring Perfection gives not just a fascinating history of how the modern violin evolved from earlier instruments but also the various bits and pieces that go into making a violin and what each do and why they are necessary. And he makes it readable for someone like me who can’t tell a symphony from a sonata.
There was an interesting documentary maybe 10 years ago (History Channel?) about an engineering professor from the University of Minnesota who was trying to develop a computer program to design violins. His program took into account the arch of the top, the ƒ shaped cutouts, countouring of the thickness of the back and stuff like that there.
The first violinist from the Minneapolis, or maybe it was the Twin Cities, symphony was there to help him put the final touches on one of his designs. They spent a lot of time on the sound post. Its location seemed to be the one critical item and just a little movement had quite an effect on the “voice” of the instrument as judged by the violinist.
It’s the Minnesota Orchestra now, but it used to be the Minneapolis Symphony Orchestra.
Yep, along with the bridge, the soundpost has far and away the biggest effect on the sound of a violin. No computer needed to tell you that 
Unfortunately, I’ve never seen a followup to the documentary. I believe his purpose was to develop the program so that the manufacture of parts for good violins could be mechanized via numerically controlled machines.
Possibly he decided that it was not feasible because of the additional labor required for assembly.
Or his grant from the fiddle maker ran out before he could finish so he wrote a report and went on to other things.
There’s plenty of mechanisation of manufacture of low-quality instruments. The trouble with trying to do this for good violins is that every piece of wood responds in a slightly different way - before you even add personal judgement & preference about the setup of the instrument.
And the varnish or other finish, and the glue and on and on. And I would assume that the way the instrument ages has an effect also.
And he wasn’t interested in routine violins. He was after quality stuff.