Are most of the Muslims and Christians seen in Kerala descendants of Hindus who converted 50-200 years back?

Christians - probably. There are some Christians in Kerala who are descendants of people who were converted to Christianity by St. Thomas himself in the 1st century AD. Most Christians there however are the descendants of recent converts - those who were converted through the efforts of European missionaries. Muslims - No. Islam came to the North through conquest by Islamic invaders. In the South however, it spread much before that through Arab traders (India had trade networks with Arabs long before Muhammed or even Jesus). The http://en.wikipedia.org/wiki/Cheraman_Juma_Masjid is sometimes claimed to be the second oldest mosque - in the world!!

No. Christians in Kerala are a lot older than that. Christianity in Kerala had three major waves. The first wave was in 1st centiury AD (2000 years ago) when St Thomas, an apostle of Jesus, came to Kerala. He converted a lot of upper caste Hindus into Christians by virtue of his preaching. These Christians were later organised into a denomination by priests from Syria and the denomination was termed "Syrian Christian" (even if they were Indians). They are also called Nasranis. Note that, Christianity spread in Kerala, around the same time it spread to other countries in the world. A second wave of conversion happened when the Portuguese trader Vasco Da Gama landed in Kerala, and brought many Christian missionaries with him. The third wave was during the British time. They targeted mostly lower caste Hindus who were promised freedom from caste, since Christianity has no caste. Unfortunately, even the Christians who had descended from the upper caste Hindus, still kept their high status and did practise untouchability and other caste related stuff. They liked to associate themselves with their upper caste Hindu friends. And the lower castes who were converted still faced discrimination, although on a lesser degree. See another of my answer : I would say that Keralite Christians are some of the oldest Christians, not only in India, but in the world. As for the skin colour. Skin colour has nothing to do with caste. There are plenty of upper castes who are dark skinned and plenty of lower castes who are fair skinned, and the vice versa.