A qpAdm weight is timeless: 47% steppe ancestry says nothing about whether the mixing happened in 3000 BCE or 300 CE. But the genome keeps time in a second channel — the lengths of the ancestry fragments recombination has been chopping since the admixture — and two related methods, ALDER and DATES, read a date out of it. A date is the cheapest strong corroboration a qpAdm model can get, which is why the two travel together in the literature and in our own reports.
The clock: recombination grinds fragments down#
At the moment of admixture, a first-generation offspring carries whole chromosomes from each parent population. Every generation after, recombination breaks parental blocks at roughly one crossover per Morgan per generation — so source-population fragments shorten, generation by generation, at a known statistical rate. Fragment length is therefore a clock: long intact blocks mean recent admixture, confetti means ancient.
Rather than calling fragments explicitly (hard in low-coverage data), both methods measure the statistical shadow of block structure: admixture linkage disequilibrium — the correlation between ancestry-informative alleles at pairs of sites — as a function of genetic distance d. That correlation decays as e^(−n·d) for admixture n generations ago: an exponential whose decay constant is the answer. Fit the curve, read off n, multiply by ~28–29 years per generation, subtract from the sample's own date (for ancients), and you have a calendar estimate with a standard error — jackknifed, as usual.
ALDER (Loh and colleagues 2013, descending from Moorjani's ROLLOFF) established the weighted form and its significance test for whether admixture occurred at all. DATES (Narasimhan and colleagues 2019; formalised by Chintalapati, Patterson and Moorjani 2022) adapted the machinery for exactly the regime ancient-DNA work lives in: it needs only the target plus two reference populations, tolerates pseudohaploid single samples, and was validated across the simulation range that matters for AADR-era datasets. In the Holocene survey work it dated the steppe-related admixture arriving in Central and South Asia and, in our own pipeline, the method behind era-dating work like the Çinamak analyses.
What a date adds to a weight#
A qpAdm model and a date check each other from independent channels — frequencies versus block lengths:
- Corroboration: a passing model whose implied timing matches the LD date is the strongest everyday evidence package available. The distal-protocol FDR improves further with orthogonal corroboration; a date is exactly that.
- Contradiction: a passing model whose source was extinct by the LD date — or a "recent" source against a 100-generation decay — is a proxy wearing the wrong era's clothes: the ancestry is real, the historical story is not. Choose a source consistent with the date.
- Discrimination: two sources qpAdm cannot split (below the resolution floor) sometimes imply different mixing eras — and the clock votes where the frequencies abstain.
Reading the caveats#
The clock's assumptions earn their own honesty box. It dates pulses: continuous mixing over centuries returns one intermediate date, not the interval's edges (extensions like DATES' multiple-pulse fitting help, within limits). Reference choice matters less than qpAdm's source choice but still matters. Very old admixture (hundreds of generations) decays into noise within a few centimorgans; very recent admixture in small samples is dominated by pedigree luck. And a generation time of 28–29 years is a convention — quote dates with their errors and that multiplier visible.
None of which dents the headline: for the price of one more run on data you already merged, the when column gets filled in beside the how much — and a model carrying weight, error, p-value and a compatible date is as close to closed as this field gets.
Terms used here are defined in the glossary.
References#
- Loh, P.-R. et al. (2013). Inferring admixture histories of human populations using linkage disequilibrium. Genetics, 193(4), 1233–1254. (ALDER.)
- Moorjani, P. et al. (2011). The history of African gene flow into Southern Europeans, Levantines, and Jews. PLoS Genetics, 7(4), e1001373. (ROLLOFF.)
- Narasimhan, V. M. et al. (2019). The formation of human populations in South and Central Asia. Science, 365, eaat7487. (DATES in the field.)
- Chintalapati, M., Patterson, N. & Moorjani, P. (2022). The spatiotemporal patterns of major human admixture events during the European Holocene. eLife, 11, e77625. (DATES formalised.)
