In the first and early centuries of the 1st millennium, the territory of present-day Poland was inhabited by a tribe known as the Lugians. They are mentioned in various sources. However, during the middle of the 1st millennium AD, the territory was largely depopulated, with the indigenous theory attributing this to various factors such as the arrival of the Slavs. The people inhabiting the southern territories of present-day Poland over 4,000 years ago were closely related genetically to earlier groups. Mieszko I assumed power somewhere between 950-963 and ruled over a population of 1 million people, most of whom lived along river valleys. During the tribal period, the main occupation of the Slavic population in these lands was agriculture. They were considered direct ancestors of the Slavs due to their proximity to southern Poland. The first Slavs from the east arrived in the territory of modern-day Poland in the second half of the 5th century. In the 10th century, the Polans dominated other tribes and expanded their influence. In the 11th century, the settlement and political rule of Mazovia extended up the Vistula and Biebrza rivers, reaching close to Grodno