Looking for variations in Human generation times over the past 250 000 years.

Generation time, i.e. the difference in age between parents and their children, is thought to have played a major role in Human history, and it is common to refer to significant events, such as the reaching of new landmasses or exchanging DNA with other Hominids, as having happened a number of generations ago. When such statements are made, it is generally assumed that generation times will have remained constant throughout Human history, and that we can estimate the number of generations to have passed since a particular event by extrapolating from the generation times of modern hunter-gatherer populations. However, there is no particular reason to believe that either assumption is true; population times among modern Human societies are known to vary independently for males and females, and in response to a wide range of social and environmental factors, while generation times vary considerably between different groups of modern hunter-gatherers. Furthermore, generation time can be shown to have evolved since our separation from our nearest Ape relatives, as well as within the Great Apes as a group.

Another method used in previous attempts to estimate the generation time of past Humans has been to use the rate at which DNA mutates between generations, measured against samples of ancient DNA from 40 000-45 000 years ago. This probably gives a more realistic estimate of the number of generations that have passed, and therefore the length of each generation, but is averaged across both genders, and only looks a limited distance into the past.

In a paper published in the journal Science Advances on 6 January 2023, Richard Wang of the Department of Biology and Department of Computer Science at Indiana University Bloomington, Samer Al-Saffar, also of the Department of Computer Science at Indiana University Bloomington, Jeffrey Rogers of the Human Genome Sequencing Center and Department of Molecular and Human Genetics at Baylor College of Medicine, and Matthew Hahn, again of the Department of Biology and Department of Computer Science at Indiana University Bloomington, present the results of an attempt to estimate Human generation times, for both sexes and across several different populations, spanning the past 250 000 years.

As Humans age, they accumulate an increasing number of mutations in their genetic code, and pass this on to their descendants. Wang et al. used data from an Icelandic study, in which the ages of parents at the time of conception were known, to model the six potential types of single nucleotide mutations, establishing that some became significantly more common with age, while others did not, thus providing a clock by which to estimate the age of unknown parents at the time of conception. 

The mutation spectrum changes with human generation time. (A) Data on de novo mutations from 1247 Icelandic trios were used to train a model that predicts the effect of both maternal and paternal age on the mutation spectrum. (B) Data from 25.3 million segregating variants whose date of origin was estimated were used to assess the mutation spectrum at different periods in the past. The mutation spectrum from each time period (bin) was used as input to the model from (A) to estimate the generation interval for males and females. (C) Differences in the frequency of each of the six different mutation types through time, as compared to the most recent time period (smoothed lines from local regression). Wang et al. (2022).

All living Humans are thought to be descended from a single population living about 250 000 years ago. By combining this with their data on age related mutation rates, Wang et al. estimate that the average generation time over this period has been 26.9±3.4 years. Furthermore, they were able to record differences between the sexes in their data, giving an average male generation time of 30.7±4.8 years, and an average female generation time of 23.2±2.0 years. Furthermore, Wang et al. are able to detect changes in the signal over time, suggesting that the average Human generation span was 24.9±3.5 years about 250 generations (or approximately 6700 years) ago, having declined from a peak of 29.8±4.1 years about 1400 generations (or approximately 38 000 years) ago, which would have been just before the Last Glacial Maximum. These estimates are, however, across multiple Human populations.

The study shows a longer generation time for males than females across all timespans, something which tallies with our understanding of modern Human cultures, 99% of which typically see such a longer male generation interval. Wang et al. note that much of the variance in generation intervals seem over time is connected to male generation intervals, while female intervals remain more constant. They suggest that the reason for this is likely biological rather than cultural, with Male humans typically remaining fertile for about 20 years longer than females. This creates the possibility for social structures to create considerably more variation in male generation time than in female generation time. Thus the difference in average generation time is strongly connected to a difference in male generation time in all but the most recent period, during which an overall increase in generation time has been linked to an increase in both male and female generation intervals, and the female generation interval appears to be larger than at any point in the past 250 000 years.

Wang et al. also looked at variations in generation interval between Human populations, considering four major geographical groups, Africans, Europeans, South Asians, and East Asians, although they note that beyond about 2000 generations ago the ancestors of all of these groups were likely to have been living in Africa, while about 10 000 generations ago all of these groups would have been descended from the same ancestors.

It was found that over the past 1000 generations, the average generation time for European and South Asian populations had increased slightly, while that for East Asian and African populations had remained relatively constant. Over the past 40 000 years, Europeans have had an average generation time of 26.1 years, slightly lower than the East Asian average of 27.1 years, but beyond this, the generation time for all non-African populations grows progressively shorter further into the past. Over the past 10 000 generations all non-African populations have shown significantly lower average generation times than African populations, with an average of 20.1±3.9 for East Asians, 20.6±3.8 for Europeans, and 21.0±3.7 for South Asians, compared to 26.9±3.5 for Africans.

This strongly suggests that different time scales should be used to calculate generation length in Africa (roughly 27 years per generation) and outside Africa (20-21 years per generation). Wang et al. note that, since the generation times for non-Africans dating back more than 2000 generations, when they would presumably have been living in Africa, these differences are in fact between different groups living on the same continent, suggesting that considerable variation in the population on that continent (which is considerably genetically more diverse than the rest of the planet) may remain hidden.

Change in generation interval across different Human populations. Generation intervals were estimated in ancestors of four major continental Human populations included in the 1000 Genomes Project; sex-averaged generation intervals are shown here. Confidence intervals for each population were  obtained by bootstrapping. The inset shows results from including polymorphisms that  date back to 78 000 generations ago; note that  age estimates of mutations in the very distant past  have  decreased accuracy. Abbreviations: AFR, Africa; EAS, East  Asia; EUR, Europe; SAS, South Asia. Wang et al. (2023).

Wang et al.'s study improves considerably on our previous understanding of the spontaneity of genetic mutations, and uses this information to make estimates about Human generation times in the past which were previously impossible. Their results are compatible with previous studies which have estimated generation times over the past 40 000 years, but extend the period covered by the estimate back much further, to 250 000 years before the present. While they suspect that the convergence of the Human race back to a single population about 10 000 generations in the past will prevent this model from looking back any further, Wang et al. believe that larger data sets may in the future allow for a much more detailed understanding of generation times within individual populations over the past 100 generations, particularly as parts of this period can be covered by historical birth records.

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