The branch that moved into the rest of the world has the least genetic diversity, but populated the rest of the planet.
I think the northern group was smaller, got lighter skinned and more Asian in facial structure (like Kung! bushmen today). Weather cooled off, some of northern group moved south and intermingled as others moved along the Red Sea coast to India and Australia and others moved north into Asia.
About a fifth of the human gene pool belongs largely either to Indo-European or Dravidic speaking people inhabiting the Indian peninsula. The 'Caucasoid share' in their gene pool is thought to be related predominantly to the Indo-European speakers. A commonly held hypothesis, albeit not the only one, suggests a massive Indo-Aryan invasion to India some 4,000 years ago [1]. Recent limited analysis of maternally inherited mitochondrial DNA (mtDNA) of Indian populations has been interpreted as supporting this concept [2] [3]. Here, this interpretation is questioned. We found an extensive deep late Pleistocene genetic link between contemporary Europeans and Indians, provided by the mtDNA haplogroup U, which encompasses roughly a fifth of mtDNA lineages of both populations. Our estimate for this split is close to the suggested time for the peopling of Asia and the first expansion of anatomically modern humans in Eurasia [4] [5] [6] [7] [8] and likely pre-dates their spread to Europe. Only a small fraction of the 'Caucasoid-specific' mtDNA lineages found in Indian populations can be ascribed to a relatively recent admixture.
PMID: 10574762 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/pubmed/10574762
Full PDF File:
http://jorde-lab.genetics.utah.edu/elibrary/Kivisild_1999.pdf