In this article,Race without Colour dated November 1994, Jared Diamond wrote :
“ Among tropical peoples, anthropologists love to stress the dark skins of African blacks, people of the southern Indian peninsula, and New Guineans and love to forget the pale skins of Amazonian Indians and Southeast Asians living at the same latitudes. To wriggle out of those paradoxes, anthropologists then plead the excuse that Amazonian Indians and Southeast Asians may not have been living in their present locations long enough to evolve dark skins. However, the ancestors of fair-skinned Swedes arrived even more recently in Scandinavia, and aboriginal Tasmanians were black-skinned despite their ancestors' having lived for at least the last 10,000 years at the latitude of Vladivostok. “
He goes onto in the article to argue hence it is sexual selection is the reason for our skin colour difference. It is covered here in a more succinct article.
I just like to point out though in Chapter 17 of Guns, Germs and Steel, Jared Diamond points out that the present residents of Southeast Asia, especially Indonesia and Malaysia had come from ancestors who arrived from Taiwan.
This is the reason why present Indonesians and Filipinos have light skins although the presence of the famous Java Homo Erectus fossils proved that humans have occupied at least western Indonesia for a million years – long enough to evolved tropical adaptations like dark skin. Now only the Philippines Negritos have dark skin like the New Guineans
Maybe it is just a case of updated knowledge since Guns. Germs and Steel was wrote many years later.
To answer why the Tasmanians remained dark despite 10,000 years at a high latitude, this page has some answers:
“John suggests that the low genetic variation common among island peoples might be the factor behind why the folks of Oz remained dark. Certainly in the New World there is some variation in skin color among the indigenous peoples, but the tribes of the north remained far darker than equivalent Old World groups (and similarly those of the equatorial regions do not rival Africans, South Indians or Melanesians in complexion).”