[TVV]

I am just an ordinary collector, not in academia and far from someone who can claim detailed knowledge. My thoughts on the origin of yataghans are below - it is mostly conjecture, and much of it may be impossible to ever prove, but more on that further down.

When it comes to yataghans, it is a weapon form that appears almost immediately following the Ottoman conquest of the Balkans, and I do not think this is a coincidence. Since there are no yataghans in Central Asia and there are no records of yataghans used by the Seljuks of Rum the form was not brought by the Ottomans, but rather appeared as a result of their expansion.

Machairas and sikas (forward curved blades) were used extensively in the Balkans prior to the Roman conquest, but obviously lost their military significance as they were replaced by the weapons brought by successive waves of invaders during the Migrations Period - Goths, Huns, Slavs, Avars, Bulgars, etc. However, the ethnic Thracian, Illyrian and Dacian (north of the Danube) element did not disappear, and probably kept a part of its material culture. It is possible the blade form survived in a scaled down, utilitarian version - an every day knife not much different from the more modern shepherds' knives.

The Ottomans banned peasants, both Christian and Muslim from wearing certain arms, such as swords and maces. Knives were not subject to the bans. As one of few self defense options available to peasants in the Ottoman Empire, it is easy to imagine how the size of every day knives grew to where they became of short sword proportions. There are parallels in other places - the bauerwehr in Central Europe, the arm daggers in the Sahel, etc.

Getting strong evidence to support the above theory is complicated, due to the following reasons:
1. Archaeological - following the Christianization of the Balkans, which started in the 4th century AD, there are no more burials of nobles and warriors with their entire military gear, or any military gear for that matter. This limits any finds to random ones and as a general rule finds of swords and daggers from the Middle Ages in the Balkans are far fewer than those from Antiquity. On top of the smaller sample size, there is the practical problem of dating the rusty remains of a forward curved blade pulled out of the ground - the organic materials are gone, so no carbon dating and there is hardly any archaeological context. How can one determine whether it is an ancient blade, a late medieval one or simply a poorly preserved 19th century one?
2. Regulatory - Bulgaria for example has extremely restrictive laws on treasure hunting (not allowed) and objects that are pre-1800 have to be registered with museums and certified by Museum experts as not being part of Bulgarian cultural heritage. This makes collecting the early Ottoman period either very complicated, or clandestine even, and so treasure hunter finds or objects in private collections are never published or even shared on the internet. I suspect other Balkan countries have similar regulations, which are based on good intentions but ultimately do not eliminate the looting problem and actually contribute to loss of knowledge and history, rather than their preservation.
3. Bias in what specimens get preserved, resulting in a general lack of early Ottoman Arms, especially munition grade ones. The earliest yataghans we have are those made in court workshops in Constantinople, for sultans and grand viziers. Whatever the rank and file carried has not survived, and generally only very special and important swords from that period were preserved, the more mundane examples discarded and recycled. When it comes to late 17th, early 18th century Ottoman arms there are possibly more of them in European collections, taken as trophies after the Second Siege of Vienna and the subsequent Holy League advance than in the collection of the Askeri for example. Even when it comes to sultans' swords, the ones that remain date from the mid 15th century at the earliest, the previous ones lost. It is not dissimilar to the way a lot of late medieval European swords were preserved in the Alexandria arsenal as opposed to European collections. Collecting was mostly limited to trophies, and during the 15th and early 16th centuries when the Ottomans were winning the battles, there were not any Ottoman trophies to collect. When it comes to Balkan museums, those collections tend to have a lot of 19th and 18th century material, but very little Ottoman arms from the prior centuries.

TL;DR: The yataghan is a long knife, which may have developed from antique forward curved blade forms in the Balkans as a result of Ottoman restrictions on other arms. Due to a variety of factors, there are very few 15th and 16th century examples in existence, and finding archaeological material to support and illustrate that theory is going to be very difficult.