Why are Neanderthals considered a different species from modern humans?

Neanderthals are considered a different species from modern humans because they belong to a distinct branch of the human evolutionary tree, known as Homo neanderthalensis. They lived in different regions, had distinct physical characteristics, and had their own unique culture and tools. Additionally, genetic studies have shown that Neanderthals interbred with modern humans but are still considered a separate species due to these differences.