Using family archives, DNA tests, or genealogy websites, many people can trace their family history back several or even dozens of generations.
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The world record for the longest family tree is associated with the Chinese philosopher Confucius (551–479 BC). His lineage spans more than 80 generations – from ancestors living in the 8th century BC to descendants today. This is a family history spanning nearly 3000 years.
However, based on the scientific dating of the oldest known fossils, humanity has existed for about 300 thousand years. So how many generations back does our entire ancestral history go?
To calculate this, two things need to be known: how long modern humans (Homo sapiens) have existed and what the average length of one generation is, says population geneticist Matthew Hahn from Indiana University. In other words, the number of human generations can be calculated by dividing the time since the emergence of Homo sapiens by the length of one generation, also called the generation interval.
According to M. Hahn, the generation interval is usually defined as the average age at which people have children.
“It is usually longer for men than for women because men can have children at a later age,” explained the scientist.
The generation interval of humanity can vary depending on location and time period, so answers to the question of how many human generations lived before us also vary.
For example, a 2003 study of Icelandic residents published in the American Journal of Human Genetics calculated the generation interval based on detailed records from the country’s churches and other historical sources. Using this data, researchers from the company deCODE Genetics created a genealogical database of the entire Icelandic population.
The study showed that over the past 300 years, the average generation interval in Iceland was 30.3 years.
In another study published in 2005, data on the average age of European women giving birth from 1960 to 2000 was used. Researchers also calculated generation intervals for men. This study found that the average generation interval was 29.1 years.
However, these are estimates of the generation interval for a relatively recent period. In a 2023 study published in Science Advances, led by M. Hahn, the generation interval was calculated looking much further back – over the past 250 thousand years. This study combined data from two previous studies.
A 2017 study of the Icelandic population published in Nature, also led by deCODE Genetics, showed that parental age affects the nature of new mutations passed on to children. Based on this data, M. Hahn and his colleagues developed a model to predict what combination of new mutations could be expected in a human population depending on the specific generation interval of a given period.
“If you know what types of mutations people pass on to their children depending on their age, and you have a certain set of mutations, you can try to calculate the age of the parents in whom these mutations arose,” explained M. Hahn.
A 2020 study published in PLOS Biology evaluated when millions of mutations characteristic of modern humans first appeared. M. Hahn and colleagues categorized these mutations by their time of origin and determined what combination of new mutations formed in each period. Based on this data, the researchers could estimate the generation interval for each period.
Although the generation interval varied over approximately 250 thousand years, on average it was about 26.9 years. According to the scientist, based on such a generation interval, about 11,152 human generations would have lived over 300 thousand years.
Evolutionary biologist and population geneticist Moises Coll Macia, working at the Institute of Evolutionary Biology in Barcelona, Spain, told Live Science that a 26.9-year generation interval “is not unimaginable.” However, he said that when estimating such numbers, he would be inclined to provide not a single specific number but a possible range of generation intervals.
Humans and chimpanzees share a common ancestor who lived during the Miocene epoch, about 23–5 million years ago.
According to M. Coll Macia, the lower bound could be considered the generation interval of one of our closest living relatives today – chimpanzees (Pan troglodytes). Humans and chimpanzees share a common ancestor who lived during the Miocene epoch, about 23–5 million years ago. Therefore, according to M. Coll Macia, it is reasonable to assume that the generation interval of early humans could have been between that of modern humans and modern chimpanzees.
Based on a 2012 article published in PNAS, the generation length of chimpanzees is about 24.6 years.
Speaking about the upper bound applicable to modern humans, M. Coll Macia suggested a 26–30 year interval. This estimate is based on a 2016 study published in PNAS that analyzed Neanderthal DNA fragments found in ancient and modern human genomes. Using these, researchers sought to estimate the human generation interval over the past 45 thousand years.
Based on the upper generation interval estimate given by M. Coll Macia – about 30 years – at least 10 thousand human generations would have lived before us. Applying the lower estimate of 24.6 years, this number would increase to about 12,195 generations.
Whichever calculation we choose, one thing is clear – the human family tree stretches extraordinarily deep into the past.
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