This seems like a "well, duh," type of discovery, but it is significant because spices were valuable trading goods, and served to establish long distance trade routes. From The Christian Science Monitor:
... In fact, as early as about 6,000 ago, Neolithic people added garlic mustard, a spice, to boiling meals of meat and fish, according to a new paper published in PLOS ONE. The research identifies the earliest known, definitive example of spice use in cooking, as well as revises popular culture's take on the Neolithic peoples as unrefined, tearing at meat and fish without a care for how it tastes.
"The finding of spice use by these people implies a much more sophisticated diet than many people have imagined," says Oliver Craig, lecturer in Archaeological Science at the University of York and a co-author on the paper. "We suggest that people's perception of food also encompassed an aesthetic dimension at this time."
... In the latest research, scientists looked at charred deposits inside broken pottery that dates to about 5,800 and 6,150 years ago and was taken from three locations in Denmark and Germany. Those deposits contain what are called phytoliths, microscopic silica brewed in a plant’s leaves and that later function as ancient signatures of the plant cooked in the pot.
In these pots, the phytoliths were traced to mustard seeds. Since mustard seeds have little nutritional value, their presence, stirred up with meat and fish, suggests that Neolithic peoples used it as a spice, not as a source of calories, said Dr. Craig. And, since the seeds were found in a cooking pot, there is little doubt that the spice was used as such, he said.
The roughly 300-year-long period to which the seeds were dated corresponds to a time of critical transition in Northern Europe, when hunters and gathers turned to agriculture and seeded crops whose growth was mirrored in a sophisticating culture.
"These people were coastal hunter-gatherers and early farmers," says Craig. "This finding adds to our understanding of the complexity of these peoples life-ways and shows a rich culinary variability."