About Ancient North Indian (ANI)

From the group that coined the term:
"In our data, the hypothesis of mixture emerges naturally from 

PCA (Figure 3), which shows that nearly all the Indo-European and Dravidian speaking groups spread out on a one dimensional gradient in a plot of the first versus the second PC.

Modeling the history of many Indian groups as a mixture of two ancestral populations is an oversimplification. In reality, even if ancient mixture did occur, it is likely to have been between substructured populations instead of homogeneous populations, and it is likely to have occurred at multiple times and at multiple geographic locations. However, approximating the history of many Indian groups as a simple mixture of two homogeneous ancestral populations provides a good fit to the summary statistics of allele frequency differentiation, and we believe that in this sense it is a useful starting point for future analyses that can detect more subtle events."

This is in Note S2 of the supplementary material.

Razib Khan quotes (the main paper is behind a paywall):

"We warn that ‘models’ in population genetics should be treated with caution. Although they provide an important framework for testing historical hypotheses, they are oversimplifications. For example, the true ancestral populations of India were probably not homogeneous as we assume in our model, but instead were probably formed by clusters of related groups that mixed at different times. However, modelling them as homogeneous fits the data and seems to capture meaningful features of history."