A Darwinian Knot
Disease, Genes and Henna
|Tattoo-like hand painting of a woman’s hands for her engagement party in Chennai, India.|
The brilliant red, orange, and gold hues of henna dyes have attracted humans for thousands of years. In the West we know henna main as a hair colouring, and in the East it is also a skin dye used to paint hands, arms and feet. Whether for skin dye or hair, the herb has been used cosmetically by every culture that possessed it. In certain regions of the world, however, the fascination with henna created a deadly confluence of disease and genes that is still with us today.
Our genetic makeup is a shifting tide that allows us to adapt to environmental challenges as they arise. Malaria, a deadly parasitic illness that has plagued us for 4,000 years, is one of the great killers of our species. When we humans encounter it, we adapt by changing genetically. Genetic change takes many years, but eventually the genes for a blood disorder inhospitable to the malaria parasite appeared, making those people with the genes more likely to survive infection than others.
Today this blood disorder, known as G6PD deficiency, is found in approximately 400 million people worldwide. Most affected are Africans and people of Mediterranean heritage, including Italians, Greeks, Arabs and Sephardic Jews. Because G6PD deficiency is carried on the X chromosome, the disorder is often silent in women, and manifests mostly in males. Usually there are no symptoms, but it can cause a serious, sometimes lethal anemia when carriers are exposed to triggers in certain foods or chemicals. There is a range of expression, with some people severely deficient and others less so. Girls carrying only one copy of the deficiency gene suffer no ill effect when exposed to triggers. Some males would sicken but survive.
|Henna (Lawsonia inermis) growing as a bush.|
Henna is one of those triggers. It is a shrubby tree native to North Africa and the Middle East, areas where malaria is common. It was a familiar and useful plant to the ancient cultures that grew up with it. The use of henna dye in ritual blessing dates back thousands of years, most often to celebrate the birth of a child, painting its hands and the soles of the feet. Sometimes the child would be decorated with a few dots of henna; other times, the patterns were more elaborate.
Henna expert Catherine Cartwright Jones believes henna is perfectly safe for most people; but the combination of malaria, G6PD deficiency and henna use set up a deadly triangle for many people. Mothers hoping to protect their children with a ritual blessing of henna must have watched despair as babies, usually males, sickened and died in days without apparent cause. Who would suspect a link between henna use and infant death if henna was used without ill effect on other children? To generations of mothers the sporadic and unpredictable deaths must have seemed to be the work of demons. Indeed, the ritual use of henna was probably hailed as a protection, not a culprit, and its use continued.
|Painting a child’s hands in Morocco.|
Thus, henna and malaria began a deadly tug-of-war in our species. Cartwright-Jones points out that because Jewish populations in antiquity used salt instead of henna to bless their children, the rate of G6PD deficiency is higher in their population than in those around them. In her paper "Henna, and the Evil Eye, Salt and Demons and the Geography of G6PD Deficiency," she notes that the incidence of G6PD deficiency in Sephardic Jewish populations runs as high as 60% today. Arab populations who lived side by side with them and used henna on their newborns have an incidence today of about 1.8 to 8.5%. Thus, the interaction between henna, malaria and genetics actually changed the genetic makeup of Arabs.
Why did henna use among Arabs take hold at all if the consequences were sometimes lethal? It turns out that henna confers a huge benefit to human health, one we overlook in the era of antibiotics and easily accessible, clean water: it is a potent anti-fungal and antiseptic. Skin infections were terribly feared in the Middle East for most of history. To get a serious skin infection meant you couldn’t work, marry, or interact socially until it healed. Both salt, which the Jews used on their newborns, and henna, which the Arabs used on theirs, have disinfecting properties. Henna’s ability to cleanse and heal sores and infections made it an important item in the herbal medicine chest.
To the early users, was applying henna to an infection, curing an illness or driving away a demon? With no knowledge of genetic illness, bacteria, or parasites, what else could they think? In any case, henna helped to ward off disease, and in so doing helped create a safe environment in which to live. But it also changed humans in a way henna users could not possibly have known: The genetic make up of an entire population of people was changed because of one plant, henna.
|Henna’s intensely fragrant flowers.|
How to Grow Henna
"My beloved is unto me a cluster of henna flowers in the vineyards of Ein Gedi."
– Song of Solomon 1:14
The intense fragrance of henna blooms evoke a romantic, sensuous atmosphere. A favourite hedging plant in ancient times, it is cultivated in private gardens and public spaces all over the world, thriving in areas where the climate is similar to the hot, dry conditions where it evolved. A shrubby tree that will grow to 3-6m (10-20ft) unless it is kept trimmed, it bears long spines and fragrant white flowers.
The seeds will germinate best in hot conditions, and can be pretreated by soaking in water for a week, ensuring water is changed daily. Seeds can then be sown in a lightweight sterilized soil mix, and kept evenly moist but not soggy. Germination should take place within 2 weeks, at which point seedlings can be gradually exposed to bright light. Bottom heat of 25°C (75°F) can help speed germination. If it is not to be planted right away, seed should be refrigerated to preserve its viability.
|Henna leaf detail.|
The plant prefers hot dry conditions, and cannot be grown where temperatures consistently go below 10°C (60°F). In temperate areas it is grown containers so it may be moved indoors during cool weather. It is the sharp, lanceolate leaves that contain the colouring agent lawsone, and the hotter and drier the growing conditions, the more concentrated the lawsone becomes in the leaves, producing the darkest red-oranges. Harvesting of leaves can begin when the tree is five years old. Henna is a heavy feeder in the garden, and although it is adaptable and tolerates poor soils, it will appreciate a well-prepared bed with good fertilizer. Areas with heavy rainfall are not conducive to growth, making this lovely shrub perfect for the xeriscaper.