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TMRCA = Time to
Most Recent Common Ancestor

Estimating the time to the most recent common ancestor (abbreviated TMRCA) between two living descendants is an important aspect of interpreting DNA matches. This page is about that aspect.

TMRCA calculations help to focus documentary genealogical research as to time and place in order to discover that ancestor's identity by name, dates, places and other characteristics.

Importance of MRCA

It follows logically that, if two people share a most recent common ancestor (MRCA), they also share the preceding ancestors -- ancestors of the ancestor. Thus, the most recent common ancestor is the link to all the preceding generations.

TMRCA Results

Concept

The principle of TMRCA calculators is that the more similar area pair of haplotypes, the more likely that their donors share a common ancestor within a shorter period of time. It is assumed that a high degree of similarity is not happenstance.

Conversely, the less similar the haplotypes, the less likely and the longer the time for a common ancestor.

Bear in mind that -- if one goes back far enough in time -- we all share the same set of ancestors. Scientists estimate that the world's entire human population consisted of only a couple of thousand people 70,000 years ago. However, we aren't really looking for 70,000 year-old ancestors.

Molecular Clock

TMRCA estimates are based on the principle that mutations (on average) happen with relatively stable frequencies. For Y-STRs, imagine a room full of clocks (12, 25, 37, 67, or 111), all ticking independently.  At some time in the past, they all showed the same time, even if they now show different times. If we know their tick-rates and the times they show now, we can work out when it was they showed the same time.

For a layman's explanation of the molecular clock, see this article. It describes, mostly, the "big picture" of evolution but the same sort of thing also happens at the micro-level.  In genetic genealogy, the tick-rates have been mostly worked out by comparing Y-DNA to well-documented genealogies.

Probability

In essence, TMRCA calculators assess the similarity of a pair of compared haplotypes. All produce a probability (usually expressed in percentages) that a pair of people whose DNA is compared share a common ancestor within a specified number of generations. Usually given as a cumulative probability; it includes all generations from the present (1) to the specified number in the past.

For example: A TMRCA calculator indicates that Al & Bob have a 90% chance of sharing a direct paternal ancestor within 10 generations, but further research identifies the common ancestor as only 6 generations in the past. Was the calculation wrong? No; 6 generations falls inside the range of 1 to 10.

If the paper trail showed that the MRCA was 11 generations in the past, this reflects that a 90% probability is, on average, wrong once in 10 times.

Problems with TMRCA

Do not be fooled by apparent precision in TMRCA calculations. All results derived will be no better than probabilistic estimates, based on the input and the limitations of the method. They have a level of uncertainty which is unstated and sometimes unknown.

DNA mutations are, essentially, random events. Whenever randomness is present, certainty is unattainable and probability governs. Some things are more likely and others less, but none are either guaranteed or impossible.

Moreover, all probabilistic estimates -- to be worth their salt -- should come with confidence intervals. A statement such as "10 generation, 90% CL +/- 2" would clarify the level of uncertainty but are seldom present. TMRCA estimates likely have wide -- but undisclosed -- confidence intervals.

Generations vs. Transmission Events:

Results are often specified in "generations", but this isn't necessarily the appropriate time unit for two people with lineages diverging from a common source. Closer to the mark is transmission events, the number of times DNA is passed from a parent to a child.

Diverging lines from CMA

Assume Al is 7 generations removed from the MRCA he shares with Bob and Bob is 5 generations removed. The total number of transmission events is 11: Al's 7 plus Bob's 5 - 1 to avoid double-counting the MRCA. Sometimes, the calculator merely halves transmission events to report in generations.

Generation Length:

We eventually want to convert the TMRCA into years, so that we can research by dates. However, generations (times between transmission events) do not have a uniform length and their actual lengths can not be determined until the pedigrees are completed. Usually, a rough estimate for an average length is substituted.

You will see estimates for average generation lengths from 20 to 35 years. In general, paternal generations tend to run longer than maternal and those in economically-developed cultures longer than in less-developed.

Confidence:

TMRCA calculations do not have definable confidence levels. To decide how much confidence to place in the probabilities, it would be necessary to know, not only the average mutation rates, but also the variance within the data.

A rough estimate for variance in mutation rate may be the stated average rate for the marker -- a statistical technique often used for rare events. We haven't yet calculated the effect on confidence levels.

Exact matches

Exact matches (where there are no differences between haplotypes) present a unique calculation problem and require a different algorithm than close matches with at least one difference. The tools yield a hockey-stick shaped probability curve, which may or may not model the real world. (Other curves follow a slanted S shape.)

Coincidental Matches:

Certain haplotypes -- especially in the R1b1a2 (R-M269) subclade -- are very common. Those with these haplotypes may have hundreds of "close matches" at high comparison levels. For them, the assumption that all matches are causal rather than some accidental may not hold. SNP testing is recommended to help eliminate non-lineage matches.

TMRCA Methods and Tools

There are several types of TMRCA calculators available for Y-DNA. We classify them by their types of inputs:

Number of Markers

With these calculators, one inputs the number of markers to be compared, the number of markers which agree (or disagree) and an estimated average mutation rate.

Specific Marker Values

It really does make a difference which markers disagree and by how much, rather than merely how many markers disagree. Research since 2001 indicates that some markers follow a stepwise mutation model as opposed to the infinite alleles model used by the above calculators.

Another strength of these tools is that markers vary widely in their mutations rates, from 1:28 transmission events (CDY) to 1:11,111 (DYS426). Any overall average is a poor substitute. With marker-by-marker comparison, precision of TMRCA estimates is enhanced.


Sources and additional reading

We recommend these resources: