A Brief History of Everyone Who Ever Lived: The Stories in Our Genes

by Adam Rutherford

X chromosomes are only passed on by males half of the time because we also have a Y, but all of the time by women, who have two Xs. The observation that there is less Neanderthal DNA on our Xs implies that the first encounters we had with them that resulted in procreation were male Neanderthals with female Homo sapiens.

We’re not sure when we began cooking; the range of evidence is wide, but food cooked on fires was definitely part of our menu by 300,000 years ago, so before we were anatomically modern.

This pandemic extended its necrotic fingers all over Europe, to Germany, France, Italy, Spain and even North Africa. Historians speculate that the obliteration of so many souls over the waves that followed for the next two centuries made significant contributions to the end of Rome’s rule, and plague-weakened armies opened the door to people of the Middle East.

Because this horror happened during the modern age, with record keeping and an understanding of public health, and also during the period of rationing, the Nazis had effectively executed an experiment that we could or would never do. They had asked the question: ‘What happens to people when you profoundly starve them?’ Many of the results, though tragic, are not very surprising. People who lived through the famine suffered a collection of health problems, from the physiological to the psychiatric. Audrey Hepburn was fifteen at the time and endured the Hongerwinter like so many others, making flour for bread and biscuits out of dried tulip bulbs. As an adult she described surviving the starvation with anaemia, respiratory problems and oedema, and attributed health problems throughout her life to the malnutrition she endured in 1944. These conditions are all miserably predictable. Their DNA was already set in place. But the unborn would face a different set of problems. For children in utero, maternal malnutrition has an effect on the expression of their genes, which would be altered and disrupted. The result for the children of the Hongerwinter was a set of health problems that would persist throughout their lives. Compared to the general population, and compared to siblings who were conceived either before or after the famine, they were smaller and underweight. They were more likely to be obese, diabetic, schizophrenic, and suffer from cardiovascular diseases and breast ca...

Malnutrition can affect the body in many ways, but one of the ways the external world interacts with our inborn biology is via the domain of epigenetics, a word that literally means ‘in addition to genetics’. DNA does not change in a person’s lifetime (unless affected by random mutations from emerging cancers, or caused by attacks from radiation or other mutagens). But with all DNA being present in all cells, mechanisms are needed to make sure only the right genes are on at the right time. Epigenetics is one such system of gene regulation – a way of modifying DNA without altering the gene sequence itself. Think of DNA as an orchestral score, the notes on the page incorrigible and unchanging. If you buy the sheet music to Beethoven’s Seventh Symphony, it is the same notes whether it was published in 1816, 1916 or 2016. How it sounds when played by a full orchestra is another matter though. A conductor and each player in an orchestra will interpret the notes and place annotations on the manuscript to dictate how the music sounds, with crescendo and diminuendo and adagio. Each performance is unique, even when the original scores are identical. In DNA, the notes never change, but they get annotated in all sorts of ways. The most common is via small molecular tags called methyl groups; it’s simply a carbon atom attached to three hydrogens, and these stick in humans onto the nucleobase C (cytosine) in DNA. This has the effect of silencing that bit of DNA. If it’s in a gene, that...

In mammals, epigenetic modifications tend to get reset each generation, but some, very limited, rare epigenetic tags appear to be passed down from parent to child, at least for a couple of generations. We’ve seen a handful of these in mice, even fewer in humans. In rats, learnt behaviours such as fear or stress can be passed on to their children, and subsequently to grandchildren, also by proposed epigenetic mechanisms. This means, as far as scientists can tell, that DNA methylation of certain genes occurs in the mothers, is not reset as we would expect and persists after conception of their pups. The children of the Hongerwinter babies were not smaller, nor did they have an increased chance of cardiovascular disease. However they were fatter, or rather, they had increased neonatal adiposity. Later in life, this correlates with increased risk of diabetes and other health problems. This result was a first in humans, that showed that what happens to a mother can affect not only her children, but her grandchildren too.

The transgenerational studies in mice, rats and in those very few human experiments at first glance appear to contradict this hard core of evolutionary theory. But do they? The answer is probably not. If the changes are permanent, then we do have a puzzle. But given that in mice the changes have at best only lasted a few generations before fading, the effects appear to be intriguing but not revolutionary.

Some women might be tetrachromatic. They, through another random chance duplication, have acquired a fourth opsin on one of their X chromosomes. Around one in eight women are estimated to have this extra gene variant, but whether that bestows tetrachromacy is not yet known. The ones who do have this power see colours where we see monotones. It’s a new area of research, and the condition appears to be rare, and poorly accounted for.