The extinction of the Passenger pigeon and Steller's sea cow in the last 500 years is a classic example of biodiversity loss primarily due to:

Step-by-step solution

According to the 'Evil Quartet' of biodiversity loss, over-exploitation occurs when human need turns into greed, leading to the unsustainable harvesting of natural resources. The extinction of the Steller's sea cow and the Passenger pigeon in the last 500 years was primarily due to over-exploitation by humans for meat and other resources. Answer: Over-exploitation