It was brought to my attention that even the increase from 20 years to 27.5 years was insufficient, given the scientific evidence updating the observed length of a historical human generation:
As early as 1973, archaeologist Kenneth Weiss questioned the accepted 20 and 25-year generational intervals, finding from an analysis of prehistoric burial sites that 27 years was a more appropriate interval, but recognizing that his conclusion could have been affected if community members who died away from the village were buried elsewhere. When assigning dates to anthropologically common ancestors 50 or more generations in the past, using the “accepted” 20 or 25 years as a conversion factor can produce substantial underestimates of the time interval. Nevertheless, those unsupported values continue in use, even in recent scientific papers.
In the first of the three more recent studies of generation length, sociologist Nancy Howell calculated average generational intervals among present-day members of the !Kung. These are a contemporary hunter-gatherer people of Botswana and Namibia whose life style is probably close to that of all our pre-agricultural ancestors in the dim past. The average age of mothers at birth of their first child was 20 and at the last birth 31, giving a mean of 25.5 years per female generation — considerably above the 20 years often attributed to primitive cultures. Husbands were six to 13 years older, giving a male generational interval of 31 to 38 years.
A second study by population geneticists Marc Tremblay and Hélène Vézina was based on 100 ascending Quebec genealogies from 50 randomly selected couples married between 1899 and 1974. The data came from BALSAC, an inter-university computerized research database at the University of Quebec at Chicoutimi, extracted from Quebec parish baptism and marriage registers going back to the 1600s. With an average depth of nine generations, but extending as far back as 12 or 13 generations, their sample included 10,538 generational intervals. They took as the interval the years between parents’ and children’s marriages, which averaged 31.7 years.
They also determined separate father-son and mother-daughter generational intervals, from lines that included at least five consecutive all-male or all-female generations. These averaged 35.0 years for male generations, 28.7 years for female years.
Biological anthropologist Agnar Helagason and colleagues, in the last of the three studies, used the Icelandic DeCODE genetics database, containing lineages of most Icelanders back two centuries, and much longer for many families. They computed separate patrilineal and matrilineal generation intervals over different lengths of time, to see if that produced a difference. The first values included only lines to ancestors who live in the 1848-1892 time frame, including three to five generations. Then they calculated interval lengths back to ancestors born between 1692 and 1742, extending them to a length of seven to nine generations. The results showed the most recent generations were a little shorter in length than more distant ones — the opposite of what the conventional view holds.
The female line intervals were 28.12 years for the most recent generations, 28.72 years for the whole lineage length. Male-line lineages showed a similar difference, 31.13 years for the recent generations, 31.93 overall. Based on their Icelandic findings and those of the Quebec study, they recommended using a female-line interval of 30 years and a male interval of 35 years.
This updated scientific evidence therefore requires a recalibration of the percentage of the known genetic distance for which evolution by natural selection could mathematically account.
CHLCA
Years: 9,000,000
UPDATE: Years per generation: 32.5
Generations per fixed mutation: 64
Years per fixed mutation: 2,080
Maximum fixed mutations: 4,327
Mutations required: 120,000,000
% attributable to evolution by natural selection: 0.000036, 1/27,733rd of the total required.