The web search results discuss general findings about dynamic human migration and genetic data in South America, but none specifically corroborate the claim that a gene study revealed migration as late as 720 AD. The Wikipedia evidence is general or irrelevant (e.g., 720 AD year). Since no independent sources confirm this specific date and finding, and the evidence is limited to general discussion, it cannot be corroborated.

The evidence provided does not contain any information regarding genetic traces associated with Indigenous Australasians dating to China's Tang dynasty. The web search and Wikipedia results are general or unrelated to this specific claim.

The web search results mention studies published in Nature Communications regarding rapid peopling of Australia and New Guinea, which relates to the topic of the claim. However, none of the provided evidence explicitly confirms the details (genes similar to Indigenous populations in Australia and the Pacific Islands) or the specific publication date (April 22) in the manner required for corroboration. The evidence is insufficient to confirm the claim's specific details.

The evidence provided does not contain specific sources that confirm the conclusion that much of the ancestry found in modern Indigenous South Americans did not primarily come from the continent’s earliest settlers. While the web search results discuss complex genetic histories in South America, they do not support this specific conclusion.

The web search results discuss the concept of 'ghost population' in the context of ancient humans and genetic studies. This supports the general concept mentioned in the claim. However, the evidence does not provide multiple independent sources confirming that the findings suggest an ancient 'ghost population' contributed genetic ancestry to Indigenous groups in the Amazon, nor does it confirm the exclusion of direct migration from Australasia. The evidence is limited to general discussions of 'ghost population' theory.