The Y chromosome determines your sex (male/female), and it is the smallest chromosome by far. This explains why:

The Neolithic period or “New Stone Age,” developed at different times in different regions, but is generally thought to have taken place between 7,000-9,000 years ago. An important era in human development, this time period is best known for the Neolithic revolution. Here, humans began to take part in large-scale agriculture, domesticating large herds of animals, building megalithic architecture, and using polished stone tools.

Then, starting around 7,000 years ago and taking place over the next two millennia, something odd happened. The diversity of the Y-chromosome plummeted. This took place across the continents of Africa, Asia, and Europe. It’s the major reason why humans are 99.9% identical in genetic makeup today. The Neolithic Y-chromosome bottleneck (as it’s called) has stymied anthropologists and biologists since it was first discovered in 2015. Now, the mystery may have been solved.Volume 90% 

Declines in genetic diversity in a given human population aren’t unheard of. Oftentimes, a natural disaster will wipe out a large segment of a society. During the Neolithic bottleneck, curiously, only men were affected. While experts have been contemplating the bottleneck for years, this hypothesis was made by undergraduate and sociology major, Tian Chen Zeng.

Zeng scoured blog posts and over time, developed his own theory. In many societies at the time, power was organized around patrilineal kinship. A patrilineal lineage is when titles, lands, and the family name are handed down through the males of a family, from one generation to the next. Zeng surmised that intense warfare between patrilineal clans killed off so many men, only one was left for every 17 women. As a result, just a few lineages saw rapid expansion.