Take a closer look at the 20th parallel:
South of the 20th, India and Africa match pretty well, though south east Asia, Indonesia, the Philippines, and Latin America are noticeably lighter. North of the 20th parallel, even most of Africa is lighter than most of India.
My theory is that one of the first groups out of Africa (if not the first) was a dark-skinned group (all of the groups that left Africa were dark-skinned, of course,) that took a coastal route though (modern) Yemen and Oman, along the southern coast of Pakistan, and then spread into the abundant, hospitable lands around the Indus, Ganges, Narmada, and other rivers.
Some of them continued on, into Tibet, Indochina, and Indonesia. These guys were competent boat-builders, who made it to the Andaman Islands, Australia, Melanesia, and possibly up to Japan (the Ainu) and the New World (where we are now finding what looks like Melanesian DNA deep in the rainforests of Brazil.) This all happened somewhere between 100,000 years ago (out of Africa, maybe) and 40,000 years ago (people reached Australia.)
More-or-less meanwhile, other groups headed northward, then circled back around. By the time they came back, they’d lightened. Several groups back-migrated into Africa, (most recently the Arab conquests, but we also have evidence of migrations 3 thousand and 23 thousand years ago;) they probably conquered Iran and Indochina from the north. From Taiwan spread another group, the Polynesians, who might have partially displaced the Melanesians and settled many islands they hadn’t been able to reach. The Ainu got pushed out and even Australia got a small invasion, though in its case, by folks from India, so its skin color didn’t change.
But India is a tough place to conquer from the north. There are some awfully big mountains in thew way. India has certainly been conquered during the long expanse of history–I suspect this accounts for the violet blob that pierces central India, perhaps due to the Indo-European expansion–but they’ve probably been conquered less, overall, than places like China.
There are a lot of small, interesting groups that I suspect are ultimately descended from what was once a large group of people that stretched around the south eastern curve of Asia (and its islands), but are now quite isolated–
Not so obscure Australians