Monday, September 13, 2021

How Long Is A Generation?

Genetic information gives you time in generations, rather than years. But most other methodologies measure age in years. Getting the conversion right is critical, isn't guaranteed to be stable in time, and is interesting.  The authors recommend using different generation lengths for populations in and outside of Africa to convert genetic data into historical calendar years.

Also note that an average generation runs from the average date at which someone has children who survive to reproduce from all of their children who survive to reproduce, and not the first date that person has children. The new data almost exactly match estimates made from prehistoric burial sites in 1973.
The generation times of our recent ancestors can tell us about both the biology and social organization of prehistoric humans, placing human evolution on an absolute timescale. We present a method for predicting historic male and female generation times based on changes in the mutation spectrum. 
Our analyses of whole-genome data reveal an average generation time of 26.9 years across the past 250,000 years, with fathers consistently older (30.7 years) than mothers (23.2 years). 
Shifts in sex-averaged generation times have been driven primarily by changes to the age of paternity rather than maternity, though we report a disproportionate increase in female generation times over the past several thousand years. 
We also find a large difference in generation times among populations, with samples from current African populations showing longer ancestral generation times than non-Africans for over a hundred thousand years, reaching back to a time when all humans occupied Africa.
Richard J. Wang, et al., "Human generation times across the past 250,000" years bioRxiv (September 7, 2021). doi:

The body text provides the statistical uncertainties:
We find the average human generation interval to be 26.9 ± 3.4 years (standard error) with an average for males of 30.7 ± 4.8 years and an average for females of 23.2 ± 3.0 years. The results show that human generation times have undergone a rapid increase in the recent past after declining for over a thousand generations. The average human generation interval was at a recent minimum of 24.9 ± 3.5 years at ~250 generations ago (6.4 kya), roughly concurrent with the historic rise of early civilizations. Before this, it had declined from a peak of 29.8 ± 4.1 years at ~1,400 generations ago (38 kya), just before the beginning of the Last Glacial Maximum. . . .
Our model estimates a longer generation interval for males than females across all analyzed time periods. These results are consistent with studies of contemporary cultures, more than 99% of which show a longer male generation interval. Overall, there is a high correlation between the average generation interval and the male-female difference (Pearson’s r = 0.88; P < 1e-10), likely due to a relatively constant generation interval in females (σ2 = 0.9 years) and a large amount of variation in males across time (σ2 = 6.8 years). Males and females reach puberty at approximately the same age, but the reproductive age in males can extend more than 20 years beyond that in females. . . .

[T]he average generation interval in each of the ancestral non-African populations grows progressively shorter into the past. The dominating pattern across the past 10,000 generations is a significantly shorter sex-averaged generation interval for East Asian, European, and South Asian populations, 20.1 ± 3.9, 20.6 ± 3.8, and 21.0 ± 3.7 years, compared to the African population, 26.9 ± 3.5 years (P < 1e-10, t-test). The estimated generation times do not converge between populations until we expand our analysis to at least 78,000 generations ago.
Hat tip to Razib Khan. More background here.

A Brief Review Of Long Term Demographic History

The Neolithic Demographic Transition

The data suggest that farming societies have shorter generation lengths than the hunter-gatherer societies that were predominant for most of human history. The difference between African and other regions may be that this transition to farming came later to Sub-Saharan Africa than East Asia, Europe and South Asia. 

This transition also resulted in a one time dramatic increase in population density, and a more gradual but steady increase in total population after this one time dramatic surge (with less dramatic surges up and down due to technological advances, climate shifts, and plagues). It also caused a more robust immune system to develop in people who had experienced the Neolithic Revolution.

The Pre-Modern World

In premodern times in the last few thousand years, the best evidence is that the average woman had about four children in a lifetime (with variation based upon the quality of local conditions), which implies at modern miscarriage rates an average of about six pregnancies per lifetime, and possibly more if miscarriage was more common in premodern times. 

About 2% of women died in each childbirth, with something on the order of 8% of women who lived long enough to have children dying in childbirth. 

The Early Modern World

In the early modern era (late 19th century on) and in Third World countries today, the number of children per lifetime surged and maternal and infant mortality fell, and life expectancies rose for a period of time, before the "demographic transition" associated with further economic development reduced the number of children per lifetime and increased generation length. The early modern phase is associated with rapid population growth.

The Modern Trend

The modern trend in developed countries, with men having children at the high end of the historical range of parental age, women having children at unprecedentedly late ages, very small differences between the ages of men and women who have children together by historical standards, and total fertility rates tending to fall below the replacement rate, is historically unprecedented. This trend is called "demographic transition" and is observed cross-culturally in almost every society that experiences significant economic development.

South Korea is the most extreme example of the modern trend in developed countries, with a total fertility rate of 0.84 (compared to 2.1 for a replacement rate and down from 6.1 in South Korea in 1960, which is an eightfold decline), and less in the biggest city, Seoul, where it is 0.64 as of 2020. 

The biggest factor is a surge in the percentage of South Korean women (and presumably a similar percentage of men) who never marry or have children which is currently in excess of 30% (see, e.g. here), something that is also unprecedented historically (at least for women). 

Secondarily, it is driven by postponed marriage and child birth. According to statistics for 2020 for South Korea, the average age of a woman at the birth of her first child was 32.3 years in 2020 in South Korea (57% of all births), the second child – 33.9 years (35% of births), and the third child – 35.3 years, with just 8% of births of a third or later in birth order child. In coming years, only about 61% of South Korean children have a sibling, and only 14% have two or more siblings. The median age of first marriage in South Korea in 2020 was 30.78 years for women and 33.23 years for men, and given that out of wedlock births are very rare there (1.9% of births as of 2014, the lowest in the world), the average age of a man at the birth of his first child is about 34.8 years. The percentage of women married for the first time who have a first child in the first two years has also fallen to a record low 55.5% as of 2020. 

Abortion is not particularly important in this trend. Abortion was first legalized in South Korea in 2021, although the law was often ignored before then as the penalty was only a misdemeanor. Abortion reduced the birth rate in South Korea as of 2017 by about 9-10% (after considering pregnancies that would have ended by miscarriage in any case), a low rate for the developed world. The abortion rate in South Korea has decreased greatly at about the same time as its most recently reached record low total fertility rates (implying that fewer South Koreans are getting pregnant in both planned and unplanned ways).

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