We have evolved to become smarter and taller than our ancestors, according to a study of global population conducted by the universities of Edinburgh and Tartu, in co-operation with the Estonian Genome Center.
The large international study was published in a recent Nature science journal. It concludes that the more genetically different the parents, the taller and more sharp-witted the children. “This means that throughout history the marriages between relatives have diminished the adaptability of humans”, said Tõnu Esko, one of the study’s authors, as well as the Director of Research at the Estonian Genome Center.
This means that for the first time ever, scientists have been successful in empirically testing the validity of general rules described by Darwin more than a hundred years ago. The leader of the group of scientists, James Wilson of the University of Edinburgh, added that it highlights the power of large-scale genetic analyses to uncover fundamental information about our evolutionary history.
The scientists analysed information about health and genes, bringing in over 100 research groups all over the world. The study included data on 350,000 individuals from all continents. This means people from cities, rural areas and groups that remain aloof for cultural or religious reasons, as well as isolated communities located in mountain villages and remote islands. “As we were interested in the effect that the parents’ genetic similarity has as a whole on the health risks of offspring – not exclusively in cases of marriages between relatives – we had to design the study as a truly global enterprise”, explained Peter Joshi, the chief author of the study.
It turned out that the more genetic similarities between the parents, the less the children grow in height, and the poorer mental capacities (cognition, memory, thinking) they have. Descendants of cousins turned out to be 1.5 centimeters shorter than average, with the time spent in education 10 months less than the average.
Despite past claims that genetic diversity is linked to high blood pressure and cholesterol level, the study at hand showed no such connections.
But what exactly was researched?
The human genome consists of 46 chromosomes, laid out in pairs of two, half of which are inherited from the mother, and the other half from the father. That means that we have 23 different chromosomes. Thus, on average, the overall genomic similarity between the parent and child, as well as siblings, is approximately 50 per cent. When it comes to the grandparent and grandchild, the similarity is 25 per cent, and with cousins 12.5 per cent.
But genetic similarity can be mapped, taking into account chromosomes or even smaller slices of the genome — this is because all individuals chosen at random share an ascendant in the past. Smaller ethnic groups, such as those of Estonia or Finland, as well as closed communities, have less generations leading to their common ascendants.
This study evaluated how much of the genome is homozygous, meaning: how long is the exact same part of the chromosome inherited from both the mother and the father that comes from their shared historic ascendant? “It could be compared to a tree, where every individual being researched is a leaf and the measured genetic distance between them could be seen as branches going in different directions”, Esko explained.
“If we compare the genomes of the parent and child, on average one mutation has occurred, meaning that the mechanism of DNA reproduction has performed a single mistake”, Wilson added. Usually, such modifications are harmful, changing the effectiveness of enzymes or turning certain genes off altogether. “This situation comes about only when there are identical copies of genes inherited from both the mother and the father. The more closely related the parents, the greater the likelihood of bad mutations manifesting”.
Although marriages between close relatives are a taboo almost everywhere in the world in order to avoid birth defects and major deformities, the present study offers a glimpse into how large a role the homozygous DNA regions inherited from a random ascendant play as a cause of disease.
“The methodology that has been developed makes it possible to find in a new way genetic risk factors that have been overlooked in previous intergenomic studies”, Esko said. “The era of so-called ‘clan genomics’ has begun”.
Katre Tatrik is the editor of the popular Estonian-language science portal Novaator.
Joshi PK, Esko T, Mattsson H, Eklund N, Gandin I, Nutile T, Jackson AU, Schurmann C, Smith AV, Zhang W, Okada Y, Stančáková A, Faul JD, Zhao W, Bartz TM, Concas MP, Franceschini N, Enroth S, Vitart V, Trompet S, Guo X, Chasman DI, O’Connel JR, Corre T, Nongmaithem SS et al. (2015). Directional dominance on stature and cognition in diverse human populations. Nature, 523 (7561), 459-62 PMID: 26131930