Wisdom Of The Week

Consider microbes, those single-celled organisms that so vastly outnumber us on planet Earth and which live so plentifully inside our gut. When a typical microbial cell replicates, it copies its chromosomes – the DNA molecules that carry genetic information – then cuts itself in half to form two new cells, giving each a nearly identical copy of its original DNA, a form of vertical inheritance close to cloning. But microbes can also gain copies of genes from totally unrelated microbes through recently discovered and utterly unintuitive processes known as horizontal inheritance. Some microbes will scoop up loose DNA and incorporate it into their own; some will insert their own DNA into nearby microbes by building a tube and passing plasmids into the neighbouring cell; some gain DNA from viruses that act like ferries between microbes, even transporting microbial DNA between organisms of different species. This viral transfer of microbial DNA between species helps to explain the emergence of new strains of antibiotic-resistant bacteria. Horizonal inheritance accounts for 8 per cent of the human genome: part of your DNA is actually viral DNA from retroviruses that once upon a time inserted their DNA into human reproductive cells (sperm, eggs), thus becoming heritable. So whenever humans have a child, we’re passing down, via vertical heredity, viral genes inserted sideways into our genomes via horizontal heredity. In fact, we wouldn’t be able to reproduce at all without horizontal heredity. A crucial membrane between foetus and placenta exists thanks to a viral gene from one of those retroviral horizontal transfers. That viral gene makes all mammalian pregnancy possible. So at the level of DNA, humans are actually a mash-up of different species.

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Shine that light a little brighter, zoom out the lens as Zimmer does, and the view gets more weird. If you have a biological sibling, you share at least one parent, which means you are both likely to have large chunks of DNA from that parent in your own genome. With a more distant relative, like a fourth cousin, you have to go further back in the family tree to find a common ancestor – a great-great-great-grandparent. Over the generations, the DNA from that ancestor got cut up – essentially diluted – into smaller and smaller pieces as it made its way down the family tree, mixing with more and more DNA from ancestors that you and your fourth cousin don’t share. Zimmer cites research showing that out of any hundred pairs of third cousins, one pair wouldn’t share any identical segments of DNA. Out of any hundred pairs of fourth cousins, 25 pairs wouldn’t share any identical segments. And yet, we would never say these cousins are not kin. When you look at heredity in terms of genes, using genes alone to define kinship (or even to draw strict boundaries round what it means to be human) starts to seem a little dubious.

The question of who we are related to also bucks intuition on much broader levels of human ancestry. Leaving DNA aside, if we think of our ancestors simply as people who procreated with each other, we soon run up against an inescapable paradox:

We think of genealogy as a simple forking tree, our two parents the product of four grandparents, who are descended from eight great-grandparents, and so on. But such a tree eventually explodes into impossibility. By the time you get back to the time of, say, Charlemagne, you have to draw over a trillion forks. In other words, your ancestors from that generation alone far outnumber all the humans who ever lived. The only way out of that paradox is to join some of those forks back together. In other words, your ancestors must have all been related to each other, either closely or distantly … If you go back far enough in the history of a human population, you reach a point in time when all the individuals who have any descendants among living people are ancestors of all living people.

This is why, as has been repeatedly pointed out in recent years, every European alive today is a descendant of Charlemagne. Such ancestral tree-twisting is hard to keep up with, but it reveals that the obsession with being a ‘direct descendant’ of a celebrated historical figure has more to do with the way certain relationships are culturally valued – for example ‘legitimate’ v. ‘illegitimate’ children – than with science. In a sense, we are all royals, even if we don’t all have royal DNA in our genomes. And yet, we are obsessed with genealogies. ‘By one estimate,’ Zimmer writes, ‘genealogy has now become the second most popular search topic on the internet. It is outranked only by porn.’

- Race doesn’t come into it