I was told that Africa has more genetic diversity than the rest of the world combined. I guess Melanesia must be second. If you believe in molecular clocks, than the oldest population will have the DNA that the daughter populations will have variants of. Since it takes time to accumulate mutations, the more recent populations splits will have fewer variants and the older population splits will have more. Melanesians have more therefore they're older.
Yes...but you said above that Cavalli Sforza had the Melanesians the farthest away from the Africans. THAT doesn't seem to make sense if they are also old--they should be closest to the originals, the Africans.
The Toba eruption 75,000 probably is responsible for killing most of the other genes outside Africa. Albeit, the Neanderthals and apparently the 'Hobbits' on Flores suevived that eruption. It was the 'nuclear-winter' effect that did the killing, btw.