Why South Indians are black?

I came across a post in famous forum Quora where people were debating on dark skin people of South Indian. There were many diverse answers. Some answers were hilarious and sounded foolish. 

Some said north Indian womens were married by invaders and slowly they lost there identity. Ahh! This sounds hilarious to me. There was a practice called sati (உடன் கட்டையெரிதல்) in ancient India. When husband die, the women would burn herself with him. There were laws introduced to eradicate Sati in 17th century only. So women who were molastated in battle crimes would kill themselves. Whoever is the foreigner they will become a new caste in Ancient India. And they will be living alone side with other caste in Ancient India. 

So how do we relate to the dark skin question. We all know the race theory. Chinese and Europeans are different and white because they are in and above tropical line. Africans and Indians are different and black because they are closure to Equator line. So you can think why some tamils and mallus are black. What i learnt from history is Manu is a Dravidian and we know the story of how he saved the humanity from great flood. We have similar accounts which talks about 72 Pandiya Kingdom which was sunken in sea. Mudattirumaran, a Pandiya King had a past history of his ancestors lost their land becuase of some kind of tsunami. They got help from Cheras and Cholas and moved into southern part of India. Since they are from Equator region their clans are black in color or dark skin or dark brown. There are many people who believes in this dimensions. Because of this belief the movie Kandam was created by Canadian. You might search about British Indian Ocean territory, Mauritius, Maldives, Lakshadweep, Dwaraka and connect with Lumeria concept.