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Modern humans—Homo sapiens—have existed for approximately 300,000 years. Determining exactly how many generations that represents requires knowing two things: how long the species has been around, and the average length of a generation.
According to Matthew Hahn, a population geneticist at Indiana University Bloomington, the number of human generations since Homo sapiens emerged equals the total time the species has existed divided by the generation interval.
This interval is the average age at which people have children. Hahn notes it tends to be longer for men than for women because men can father children later in life.
Several studies have attempted to calculate the generation interval, each yielding different estimates. A 2003 Icelandic study published in the American Journal of Human Genetics, based on extensive church and historical records compiled by the genetics firm deCODE Genetics, found the average generation interval in Iceland over the past 300 years was 30.3 years.
A separate 2005 study, using data on the average age at which European women gave birth between 1960 and 2000 and including estimates for men, arrived at 29.1 years.
These estimates, however, reflect recent history. A 2023 study led by Hahn and published in Science Advances extended the analysis across the past 250,000 years by combining data from two earlier works.
One was a 2017 Icelandic study published in Nature by deCODE Genetics, which showed that the mix of mutations passed from parents to children shifts as parents age.
Using that data, Hahn and colleagues modeled expected mutation patterns for a given generation interval. They then cross-referenced findings from a 2020 study in PLOS Biology, which estimated when millions of present-day human mutations first appeared.
This study grouped mutations by time period to determine the mutation mix in each era. This allowed the team to estimate the generation interval for each time window.
Across the roughly 250,000-year span, the average was 26.9 years, suggesting approximately 11,152 human generations have lived over 300,000 years.
Moises Coll Macia, an evolutionary biologist and postdoctoral researcher at the Institute of Evolutionary Biology in Barcelona, Spain, prefers to express the figure as a range rather than a fixed number.
He argues the lower bound of any estimate should correspond to the generation interval of chimpanzees (Pan troglodytes), humans' closest living relatives.
Since humans and chimpanzees share a common ancestor from the Miocene epoch (23 to 5 million years ago), it is reasonable to expect that past human generation intervals fell somewhere between those of modern humans and modern chimpanzees.
A 2012 paper published in PNAS estimated the chimpanzee generation interval at approximately 24.6 years.
For the upper limit among modern humans, Coll Macia points to a range of 26 to 30 years, drawing on a 2016 PNAS study that estimated the human generation interval over the past 45,000 years by analyzing fragments of Neanderthal DNA found in present-day human genomes.
Applying Coll Macia's upper limit of 30 years yields a minimum estimate of around 10,000 human generations.
Using the lower bound of 24.6 years produces a maximum of approximately 12,195 generations. By any measure, the human family tree is a long one.
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