Due partly to the fact that this took place before the written record of this region began, there have been a number of theories presented over the years to fill the gap of knowledge about how and why the end of the Cucuteni–Trypillia culture happened. These theories include invasions from various groups of people, a gradual cultural shift as more advanced societies settled in their region, and environmental collapse.

End of the Copper Age

In the larger perspective, the end of the Cucuteni-Trypillia culture marked the boundary between the Copper Age and the Bronze Age. The Copper Age, also known as the Eneolithic and Chalcolithic periods, lasted in Europe from roughly 3500 to 1700 BC. However, it ended for this culture between 3000 and 2750 BC. There is no firm point in time when this happened, since it was done over a period of many years, as first one area and then another would become integrated into the new Bronze Age civilization. Because the Cucuteni-Trypillia society was almost entirely egalitarian (with no ruling elite), there was no dramatic change of government for the whole region, as is the case when modern nations go to war and are defeated. The Cucuteni-Trypillia settlements existed independently from each other, so each experienced its own separate fate as the end of their culture swept over them, making the transition to the Bronze Age a complex and gradual process, rather than as a result of a single event.

Although there were many other Neolithic and Eneolithic cultures in eastern Europe during this time, the Cucuteni-Trypillia was probably the most advanced and influential, due to its robust settlements, highly refined ceramic art, and location. This culture was situated astride the natural "highway" between Central Asia and Europe, which may have directly contributed to its demise as other cultures from the east moved into this region following the route across the grassy plains that lie to the north and northwest of the Black Sea. Because the Cucuteni-Trypillia culture was so robust, it continued to spread into new regions as new settlements were built to accommodate the increasing population. This was especially the case in the latter period of its existence, when Cucuteni-Trypillia settlements began to pop up across the unsettled region of what is today western Ukraine.

The Old European culture and the Kurgan hypothesis In the 1950s, as a result of the cultural renaissance that was part of the Khrushchev Thaw that took place after Joseph Stalin's death in 1953, a massive program of archaeological excavations was sponsored by the Soviet Union, which included Cucuteni-Trypillia sites that are located in the now-independent nations of Ukraine and Moldova. As Soviet scholars began publishing their findings and analyses from these excavations, a new model began to emerge among some members of the international academic community that revised the way that scholars had perceived how the Cucuteni-Trypillia culture ended, among other things. This new model inspired the creation of two theories that came to be known as the Old European culture and the Kurgan hypothesis. Some of the more notable people who helped to formulate and support these theories included: Marija Gimbutas (January 23, 1921 – February 2, 1994): a Lithuanian archaeologist and author of the 1956 book The prehistory of eastern Europe. Gimbutas coined the phrase Old European culture to describe the indigenous Neolithic European peoples. According to Gimbutas, these people were peaceful and goddess- and woman-centered (matristic). She then proposed that the Bronze Age Proto-Indo-Europeans (from the Kurgan culture), who she claimed were patriarchal (androcratic) and warlike, invaded southeast Europe from the eastern steppes, and brought destruction to the formerly peaceful European societies. Joseph Campbell (March 26, 1904 – October 31, 1987): an American comparative mythologist and author of the 1959 book The masks of God. Campbell and Gimbutas worked together in formulating these theories; both of their personal collections of books and papers are today housed in The Joseph Campbell and Marija Gimbutas Libraryhttps://web.archive.org/web/20100926155553/http://www.pacifica.edu/innercontent-m.aspx?id=1750 on the Lambert campus in Carpinteria, California, as part of the OPUS Archives and Research Centerhttps://web.archive.org/web/20100324225145/http://www.opusarchives.org/individual.shtml of the Pacifica Graduate Institute. Luigi Luca Cavalli-Sforza (January 25, 1922 – August 31, 2018): an Italian population geneticist and author of the article Analysis of human evolution Cavalli-Sforza used the different human blood types as a means to create a model of human racial dispersion and historical migration patterns through the application of genetics. He attempted (among other things) to substantiate Gimbutas' claim of an Indo-European conquest of the indigenous Neolithic European peoples by comparing blood types of present-day Europeans. Although highly controversial, Cavalli-Sforza's work was undeniably groundbreaking and genuinely innovative, and lay the foundation for the subsequent science of human genetic haplogroup research. A generation of Indo-Europeanist linguists who began in the 1950s to recover a sense of historical depth to the Proto-Indo-European language, linking it with Gimbutas' theories of Indo-European conquest to explain how this family of languages was dispersed throughout Europe.

To illustrate these two theories, the table below juxtaposes the Cucuteni-Trypillia culture, which Gimbutas included as one of the Old European culture societies, against the Yamna culture (also known as the Pit grave culture), which was the society that Gimbutas suggested was the most likely candidate for being the Proto-Indo-European group that was active in the first wave of the Kurgan conquests of Old European cultures. Here, then, are the basic details about these two cultures, according to Gimbutas:

+ Comparison of Cucuteni-Trypillia and Yamna cultures Comparison Cucuteni-Trypillia culture Yamna culture Origins Blending of the Boian culture, with some traces of the Hamangia culture (both originally from Anatolia), and the Musical note culture (also known as the Middle Linear Pottery culture, or "LBK"), from the northern Subcarpathian region of southeastern Poland and western Ukraine; all of which were Neolithic and non-Indo-European. An amalgam of Eneolithic Proto-Indo-European tribes from the southern region of the great Pontic steppe, mostly along river valleys, including (from west to east) the Dniester, the Bug, the Dnieper, the Donetz, the Don, West Manych, and the middle Volga rivers. Agricultural model Sedentistic subsistence agriculture Pastoral nomadism Social stratification Egalitarian acephalous society Tribal chiefdom with social hierarchical levels Economic model Generalized reciprocity or gift economy Traditional economy featuring trade bartering Division of labour No occupational specialization, each household produced all necessary goods and services independently. Many specialized occupations, including priests, warriors, healers, metalsmiths, traders, herders, and slaves. Technological Sophistication Superior work in agricultural techniques, as well as in ceramics, compared to the Yamna. Cucuteni-Trypillia ceramics have been found in Yamna sites. Superior work in copper metalworking than the Cucuteni-Trypillia during the Eneolithic. Later, the Yamna worked in brass, and some of their brass artifacts have been found in Cucuteni-Trypillia sites. The Yamna also used domestic horses for travel, which the Cucuteni-Trypillia culture most likely did not have. Militarization Almost no artifacts have been found that would have been meant for defense against a human enemy. No skeletal remains have been found that would indicate the person had been killed with a weapon. Only at the end of their culture did they begin to build walls and ditches around their settlements, yet still no weapons have been found. The Yamna perfected military weapons, rode domesticated horses, and probably conducted raids against other peoples regularly. Many weapons have been found in their grave sites. In addition, they also constructed hill-top fortresses, similar to the Medieval Motte-and-bailey design. Religion The archaeological record indicates the worship of a female fertility goddess. There is also evidence to indicate that they used clay fetishes in various ritualistic purposes, ranging from fertility to sigils for protection against evil spirits or human enemies. There is evidence to indicate that they probably participated in ritual human sacrifice of captured enemies. They worshipped a warlike male deity. Trade network Very rudimentary trade network involving only a handful of goods, the most important of which was salt. No indication of traders or merchants as a profession. Some evidence does indicate the possible use of barter tokens as an early form of exchange. An extensive trade network spanning a large region from central and southeast Europe to modern-day Kazakhstan and Russia, involving many trade goods, and indication of a class of merchants and traders. Encounters with each other Starting around 4500 BC, Cucuteni-Trypillia settlements began to appear in western Ukraine, where they encountered Yamna tribes. Some scholars hold that this is partly the cause for the creation of very large settlements in this region, to aid in defense against Yamna raids. Also beginning around 4500 BC, the Yamna culture began to establish settlements as far west as Transylvania, which existed side-by-side with Cucuteni-Trypillia settlements.

Gimbutas believed that the expansion of the Kurgan culture was conducted as a series of hostile, military conquests. Gimbutas wrote:The process of Indo-Europeanization was a cultural, not a physical, transformation. It must be understood as a military victory in terms of successfully imposing a new administrative system, language, and religion upon the indigenous groups. Gimbutas pointed out how the extinction of the Cucuteni-Trypillia culture coincided with the 3rd Wave of Kurgan expansion, c. 3000–2800 BC, which saw the Kurgans move en masse into modern-day Romania, Bulgaria, and eastern Hungary. This matched the archaeological evidence that showed that the Cucuteni-Trypillia settlements (some of which were the largest in the world at the time), were all abandoned by 2750 BC, marking the end of the culture.

Taken together, the Kurgan hypothesis and the Old European culture theories presented a compelling story that directly went against the prevalent image of prehistoric cultures (especially cultures that existed outside of the traditional "Cradle of Civilization" in the Fertile Crescent) as "primitive", wild, half-starved savages. These theories were more fully developed during the cultural upheavals of the 1960s, and were received with tremendous support by many people, especially by many of the younger college students who were exposed to them. Additionally, the theories were greatly acclaimed by supporters of the anti-war peace movement in the U.S. and the burgeoning feminist movement, who saw in these ancient Neolithic cultures a model of how human society could exist without war, and how women could be treated with equal status as men. Moreover, the other image of the warlike, patriarchal Indo-European invaders who brought death and destruction to a peaceful, egalitarian people resonated as well with the members of these movements, during a time when the U.S. was involved in an unpopular war in Vietnam. So strongly did these theories affect some people, that they provided much of the foundation for the creation of the Neopagan religious movement, that still views the writings of Gimbutas and Campbell with high regard.