We are family: some notes on common ancestors
Danny Dyer isn't special.
Normally I send out the extract from the archive on a Saturday morning. But there are still a few hours to go before Black Friday turns into Grey Saturday, so I thought I’d do it now and remind you of this special offer: '
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One of the most mindblowing things I have ever learned concerns the “genetic isopoint”. This concept, also known as the “identical ancestors point” (IAP), or “all common ancestors” (ACA), is the most recent point in a particular population’s past at which everyone then alive either has no living descendants left, or is the ancestor of everyone currently living.
In an individual family, this point is obviously fairly recent: the genetic isopoint for you and any siblings you happen to have is one generation back, since there’s no one in the direct line who isn’t an ancestor of everyone in your generation. For double first cousins – not as messed up as it sounds – it’s two generations back. For the human race as a whole, though, you’re obviously going to look back rather farther.
But not, perhaps, as far as you think, which is where it gets mindblowing. In 2004 a group of statisticians, led by MIT’s Douglas Rohde, calculated that the genetic isopoint was no farther back than 5300BCE, and possibly as recently as 2200BCE – a long time ago, sure, but recent enough that Egyptian civilisation had already been running for nearly a thousand years, had built the pyramids, run through half a dozen dynasties, and was now thinking about falling into the first of its nice, relaxing dark ages.
Remember, this was not merely the point at which everyone alive now shared a single common ancestor: whoever this most recent common ancestor was, the same team calculated that they lived much more recently, probably no earlier than 1400BCE (the time of the 18th dynasty; the one with Akhenaten and Tutankhamun in it), and possibly as recently as 55CE (the mysterious death of the Emperor Claudius). No: this earlier point was when everyone was everyone’s ancestor: if they weren’t it was only because they now have no living descendants left, either because they had no kids or because their line has died out.
I’m going to say that again, as this isn’t the easiest concept to grasp. To find a point in history at which everyone who still has living descendants is the ancestor of literally everyone on earth, you have to go back no farther than a few millennia. It may be as few as four. At the time that Egypt was chucking up its pyramids and the beaker people were doing their thing, it is quite possible that everyone involved was your ancestor, and mine, and Taylor Swift’s, and Kim Jong-Un’s, and Michelle Obama’s, and-
I did tell you it was mind blowing.
All this can do weird things to the brain. It seems at first to mean we all have the exact same set of ancestors, and yes, that is what it means, we do. But we don’t have them in the same quantities. Certain European royal dynasties like the Spanish Habsburgs collapsed because of their extensive-inbreeding, using uncle/niece marriages and so forth, as a way of keeping wealth and power within the family. After a few generations, this meant that its heirs were suffering from a severe shortage of ancestors, genetic defects and big chins.
This is a problem known as “pedigree collapse” and, for a gene, it can be fatal. On a longer timeline, though, we all have it, because there are only so many humans to go around. Go back 32 generations, and you theoretically have 4 billion 30th great grandparents: there simply weren’t enough people on the planet around 800 years ago to make that possible. Go back another couple of centuries, and you’re looking at more than 1 trillion 38th great grandparents, more than the number of people who have ever actually lived.
The reason this isn’t a problem is that, just as with the Habsburgs but comfortingly farther back, the same people start popping up at different points of our family tree. And it’s this which explains why the entire human race can have, ultimately, the same set of ancestors yet still look fairly different.
If you’re, say, Scottish, the odds are that most of your ancestry will be Scottish, too; a chunk will be from elsewhere in these islands, a fair bit will be Scandinavian, but probably relatively little will be Malaysian or Congolese. This means that, at the time of the genetic isopoint, everyone alive was your ancestor, but your ancestors in Africa or Asia probably only appear in your family tree once or twice. Those in the vicinity of Greenock, by contrast, likely pop up repeatedly. In the same way, if you’re Congolese today, your central African ancestors will be all over your family tree, while those from Glasgow or Guangzhou probably only appear once.
The smaller the geography you’re examining, incidentally, the more recent the last common ancestor was. A 2013 study found that Europeans reach that point a mere 1,000 years ago, meaning that we are all descended from Charlemagne (who lived 1200 years ago and definitely still has descendents). In a 2016 episode of Who Do You Think You Are?, TV’s Danny Dyer was genuinely excited to discover he was a direct descendent of Edward III, who lived around seven centuries back. Geneticist Adam Rutherford frowned a bit and did some maths, and thus calculated that it was “virtually impossible” that anyone with predominantly British ancestry isn’t descended from one of the 13 children of Edward III (apparently we’re all between 21 and 24 generations down, probably many times over). Which rather undercuts Dyer’s puppyish excitement.
In the same way, nearly everyone of Jewish descent will have ancestors who were expelled from Spain in the years immediately after 1492, even if their ancestry is predominantly Ashkenazi (that is, eastern European) rather than Sephardic (Iberian/Mediterranean).
This doesn’t mean that everyone has actual genes from any of our famous ancestors, of course: only half of our genes come from each parent, and over a few generations that wittles away to very little indeed. But it does mean that the entire idea of ethnic purity is pleasingly meaningless. Go back far enough, and it’s not merely that we’re all originally African: every one of us is, at some point, from everywhere.
These last common ancestors, incidentally, aren’t the same as “mitochondrial eve”: that’s an entirely different mindblowing theory, which states that if you track everyone’s genetic material back through the maternal line we all eventually converge on a single woman, who probably lived between 100,000 and 230,000 years ago. (She could, theoretically, lose the title: if she had two daughters, and one of their lines died out, the other would become mitochondrial eve.)
The use of Biblical names here has sometimes confused people: “Eve” was neither the first woman, nor the only woman of her time, and the geneticist Allan Wilson, one of those who came up with concept in 1970s, regretted the name, preferring “lucky mother”. Nonetheless, she has a male equivalent – Y-chromosomal Adam – who has been dated to the same extremely broad time period. There is, however, no particular reason to believe they were contemporaries, let alone actually dated.
Finally, you may have heard of “Luca”, the last Universal Common Ancestor: a cell, billions of years ago, from which every life form on Earth today descends, from you to my dog to the tree in your back garden to those mushrooms you had on your pizza. But Luca remains a topic of live research, it’s not entirely clear if it existed at all and even if it did it’s unclear where viruses fit in. We are all much more closely related to salmonella and legionnaire’s disease than we are to the coronavirus.
Which is very comforting, after the week I’ve had.
More on the genetic isopoint in this fascinating 2020 piece in Scientific American.
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