My family tree doubles at every past ancestor generation, branching back into the past. At what point does it decrease to reflect the size of the smaller historical population?
Hillary Shaw
Newport, Shropshire, UK
Global population was around 2 billion in 1930, 1 billion in 1820, 0.5 billion in 1640 and 0.25 billion in 900. Meanwhile, if your parents were born in 1950, and 10 doublings equal 1000 ancestors, theoretically you would have 2000 ancestors from 1700 (assume average maternal age 25), 2 million from 1450 and 2 billion from 1200. So, these two series cross around the year 1300.
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However, your family tree numbers stabilise much later and smaller than that because, before railways, many people lived in isolated communities and married distant relatives.
As an extreme example, the Scottish archipelago of St Kilda had around 100 people for several centuries before mass emigration kicked in around 1800 – about the minimum number to avoid excessive inbreeding and genetic problems.
In the 1700s, chances are that most of your ancestors came from a village of a few 100. Before that, it’s almost all inbreeding
On the British mainland, before railways, there were travelling merchants, justices, preachers, soldiers and beggars, but people would marry in a similar social class, religion, et cetera.
In the cities, especially port cities, you might marry an outsider, but pre-railway villages were almost as isolated as St Kilda and of similar population size.
Around 1650, it took seven days to travel between York and London, due to atrocious roads, which would have been quagmires in winter, dust bowls in summer.
In 1700, just 15 per cent of the British population lived in urban areas, which were small. London then had 650,000 inhabitants, but other cities were much smaller: Bristol had 20,000, Manchester 10,000 and Liverpool 5000 people. So, chances are that most of your ancestors came from a village of a few hundred in the 1700s. Before that, it’s almost all inbreeding.
Simon McLeish
Lechlade, Gloucestershire, UK
It is actually more to do with the availability of unrelated partners than the wider population size. The availability of the bicycle has been cited as one factor that made it easier to find less closely related prospective partners. Parish records in Dorset, UK, show that, between 1887 and 1916, the proportion of marriages between people in the same village dropped from 77 to 41 per cent.
The royal families of Europe often married close relations for centuries to preserve the purity of the royal blood. The Habsburgs were the best known example, with the “Habsburg lip” a physical trait seen in many portraits of those related to the dynasty.
Robin Maguire
Hobart, Tasmania, Australia
The answer will vary when factors such as geography or social practices limit potential marriage partners to a particular segment of the population.
For an extreme example, consider the Ptolemaic dynasty of Egyptian rulers. After the death of Alexander the Great in 323 BC, the Macedonian general Ptolemy I Soter appropriated the throne of Egypt and his descendants kept power in the family through the practice of intermarriage.
The final ruler in the Ptolemaic line was Queen Cleopatra VII (born in 69 BC), the paramour of Julius Caesar and subsequently of Mark Antony. Going back through her family tree, she had about the usual number of direct forebears in the late 100s BC, but all of these were exclusively descended from the irreducible minimum of just two ancestors in the late 200s, King Ptolemy V Epiphanes and his consort, Queen Cleopatra I. There were six incestuous liaisons along the way. This unusual background did not seem to do the final Cleopatra any harm: she was almost as famous for her learning as for her vivacity.
John Davies
Haverbreaks, Lancaster, UK
Your family tree’s size just stops doubling when it reaches the size of the earlier population, and you discover that two of your great- to-the-X-grandparents are the same individual!
Anthony Woodward
Portland, Oregon, US
It never does. Tracing your ancestors backward in time for 8 million years, approximately 32,000 hominin generations, they will become the last common ancestors of chimpanzees and hominins.
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