How evolution shapes our health and transforms medicine*
What has the theory of evolution to offer to modern medicine? Evolutionary insights are rarely used by medical practitioners when treating our cancers, fertility problems, allergies, dementias and so on. Jeremy Taylor’s book gives many examples of where evolution helps explain our modern patterns of disease and suggests new strategies for treatment.
Taylor starts with some encouraging facts. Since our gatherer-hunter past, mortality has decreased enormously so that life expectancy in several countries exceeds 80 years. Painless and sterile surgery, effective drugs, public health measures, vaccines, and organ transplants, among others, have transformed medicine from a fairly futile and even harmful practice into something approaching a science. So why, asks Taylor, do so many people suffer from autoimmune diseases (rheumatoid arthritis, multiple sclerosis, Type 1 diabetes, inflammatory bowel disease etc.), allergies (like eczema and asthma), heart disease, eye problems, bad backs, appendicitis, reproductive problems, cancers, mental illness, and dementia?
How can evolution have allowed this to happen? The problems are obvious to us but evolution is “blind and witless.” Like a politician, it focuses on the immediate problem – how are genes to be passed on. The problem for evolution to solve is not health but reproduction. Evolution selects for traits that in principle can lead to immortality – for genes! To genes, bodies are vehicles to get them safely into the next generation. It’s no surprise then that evolution has not eradicated disease from our bodies, particularly after reproductive age, but evolution also has something to do with the types of disease we get.
Taylor looks first, in “Absent friends”, at allergies and autoimmune diseases, becoming more common in richer countries. For instance, childhood diabetes is 200 times more common in the UK than in China. For most of our evolutionary history, humans have lived with parasites and micro-organisms (ticks, worms, protozoa, bacteria and viruses), in close proximity to domestic animals, with polluted water supplies, and in dirty dusty dwellings. This caused much poor health but our immune systems evolved with this background. In advanced societies, these parasites are no longer present and our immune systems are not challenged at an early age. According to this hygiene (or “old friends”) hypothesis , the immune system is “damped down” by early and frequent exposure to antigens: in these more hygienic times, it responds with inappropriate strength to harmless stimuli such as peanut proteins, grass pollen, or gluten. Autoimmune diseases, like Type 1 diabetes, coeliac disease and multiple sclerosis, seem linked to reduced exposure to bacteria in childhood.
There is evidence that deliberately infecting MS sufferers with parasitic worms alleviates their symptoms. Intriguingly, a case of severe autism seems to have been ameliorated by infection with parasitic worms or attacks by biting mites. Taylor quotes the example of an American boy, Lawrence, with a severe form of autism which led him to become very agitated and violently harm himself. His parents noticed that his symptoms seemed to go away when he had a fever. When Lawrence was older, his parents reluctantly agreed to put him into permanent care but, when he was attending a specialised summer camp, they were called by the staff to say that he was behaving … normally! It seemed that he had been severely bitten by chiggers, a type of mite found in grasslands and forests, and the powerful immune response this provoked had led to a total remission of his symptoms, returning when the reaction had subsided. To cut a long story short, Lawrence was eventually deliberately infected with an intestinal parasite, the pig whipworm and his symptoms completely vanished. This type of treatment has also been used on some sufferers from Crohn’s disease, an autoimmune disease of the bowel.
The make-up of one’s gut bacteria (microbiota) seems to be an important factor in some autoimmune conditions. Babies delivered by Caesarean section or not breast-fed do not acquire normal gut microbiota, while treatment with antibiotics can upset this. One answer lies in administering the correct bacteria after birth or via a “faecal transplant” later in life. Infection with parasites seems to work for several conditions but doesn’t seem a very nice idea so perhaps people could be treated with a extract of parasite antigens. It has also been found that early exposure to peanut proteins drastically cuts the chance of peanut allergy in children.
In “A fine romance,” Taylor attacks infertility and diseases of pregnancy from the standpoint of evolution. Why do some women have many miscarriages and some pregnant women get life-threatening pre-eclampsia, often leading to premature birth. Reproduction is rather inefficient in humans, with only about a fifth of ovulations with unprotected intercourse resulting in successful pregnancy. Some 30% of fertilised eggs fail to implant and another 30% are lost during the first six weeks. About 10% miscarry before 12 weeks. Of pregnant women, some 10% develop diabetes and another 10% very high blood pressure. This can lead to kidney and liver damage (pre-eclampsia), leading to seizures and convulsions (eclampsia). In 2013, 29,000 women died worldwide from pre-eclampsia. The treatment is induction of birth or Caesarean section.
The evolutionary setting for all this is that the foetus carries not only the mother’s genes but those of the father. The investment of the mother’s resources in one baby is set off against the investment necessary in all the future babies she can have during her reproductive life. On the other hand, the foetus, carrying the genes of the father as well, is vitally interested in getting the maximum investment of resources from the mother, even if that detracts from the interests of the mother’s future offspring. The mother’s body will tend to weed out any but the most viable embryos (hence the massive loss of embryos in the first twelve weeks).
The occurrence of pre-eclampsia is increased when pregnancy occurs quickly in a new relationship and this relates to another puzzle, that of how and why the mother’s body tolerates the presence of a foetus with a substantial proportion of “foreign” antigens that would normally lead to rejection. It seems that, in the course of a longer relationship, the mother’s immune system becomes habituated to and tolerant of the father’s antigens. This points towards a version of the “old friends” hypothesis in which pre-eclampsia is a sort of inappropriately strong immune response to the father’s antigens.
It is a commonplace that our upright bipedal stance, while freeing our hands for a variety of tasks, puts strain on our backs resulting in an increasing prevalence of back pain as we age. In fact, the Global Burden of Disease 2010, looking at 291 conditions, ranked low back pain worst for years lived with disability (approaching 10% of the world’s population). We also suffer from bunions, varicose veins, haemorrhoids, hernias, hip and knee problems.
Our bipedality, unique among mammals (including our closest relatives), has allowed our brains to expand but, since evolution doesn’t have a plan, this could not have been the reason for its evolution. Taylor points out, in “The downside of upright,” that there must have been an overriding reason for bipedality which outweighs the down side. Early fossils of hominids close to the split with chimpanzees’ ancestors are adapted for bipedalism and climbing trees, with opposable big toes. True bipedalism allows for a more energy-efficient two-legged gait and would have enabled our ancestors to expand their foraging territory from forests to savanna. Their hands would have been free to make and use tools and carry food. We are also adapted for running long distances, unlike our closest relatives, allowing our ancestors to run down prey by simply exhausting it. Taylor’s explanation for the biped’s health involves our different lifestyles – standing for long periods, sitting on chairs (rather than squatting or sitting on the ground), less physical activity, even our use of footwear when we do run, protecting our feet but jarring our joints.
The eye is often cited by creationists as so complex that it could only have been created by a supernatural being. In fact, all variations between light-sensitive patches and the primate eye with colour vision are found in nature, Taylor shows in “DIY eye.” Even some bacteria can focus light and use this to direct their movements. Calculations show that eyes can evolve in a virtual “blink of the eye” in the time since life evolved. Genetics shows that the eye only evolved once and that all subsequent eyes are modifications of this. It has been claimed that its layout, with the nerve cells in front of the retina, is a fault that evolution was unable to avoid, given the way the eye evolved. Taylor cites evidence that this layout is actually more advantageous to smaller animals since it maximises the distance from the lens to the retina, allowing more precise vision.
Our acute eyesight requires a high concentration of photoreceptor cells in the fovea, the centre of the retina where light is focused. Taylor thinks that this puts a lot of pressure on the blood supply and that this results in some people losing vision through macular degeneration in later life. This is an evolutionary trade-off for the benefit of having acute eyesight when young.
Next, in “Hopeful monsters,” Taylor explains to us “why cancer is almost impossible to cure.” Part of the problem is that, having mutated to become cancerous, the cells keep mutating. They form clones that compete with each other (and with normal cells) for food and oxygen. Unlike normal cells, cancer cells are “immortal,” achieving this through six steps, ending up as spreading or metastasising cancers. This is evolution in miniature, the first mutating to produce their own growth signals and last developing the ability to break away from a tumour and travel around the body. The other aspect of evolution and cancer cells is the development of drug-resistant cancer cells (as with antibiotic-resistant bacteria).
Heart disease is the major killer in the West: in “A problem with the plumbing,” Taylor explains how the evolution of the coronary arteries makes heart attacks more likely. Heart muscle needs oxygen but how is it to be supplied? Paradoxically, the oxygen-rich blood pumped by the heart passes through too quickly. Most vertebrates have coronary arteries to supply the heart with oxygen. These are very narrow and are prone to become narrowed even further by atherosclerosis, the formation of layers of plaque. When the muscle contracts, the branches of the coronary arteries are squeezed shut: they can only fill during relaxation. With increased exercise, the more rapid contractions reduce the time for the arteries to refill. If these are obstructed by plaque, the muscle is starved of oxygen causing pain (angina) and long-term damage. Pieces of plaque can break off, causing a blockage: the muscle supplied by that branch dies – a heart attack. Heart disease is usually attributed to lifestyle and diet but Taylor draws attention to another factor, the immune system. People who have had tonsils or appendixes removed in childhood are much more prone to heart attacks. This is due to disrupted development of the immune system (and shows that the appendix has some purpose). Also, people with autoimmune diseases are more prone to atherosclerosis (the “old friends” hypothesis).
Finally, in “Three score years – and then?” Taylor tackles Alzheimer’s disease. He convincingly argues that the focus on amyloid protein tangles in the brain cells of sufferers is misplaced. These are actually a symptom, rather than the cause, the underlying problem being inflammation. The genetic component of Alzheimer’s involves genes connected with the immune system. It seems that regularly taking anti-inflammatory drugs like aspirin or ibuprofen can delay onset. It may also be that a viral brain infection can cause the problematic inflammation. One candidate is Herpes simplex (cold sore) virus (HSV), found in 90% of people. Infecting the lips, it enters the trigeminal nerve and shelters from the immune system in the nerve ganglion, inside the skull next to the brain. Triggered by immune decline or by another infection, reactivated HSV can migrate back to the lips to cause more cold sores…or perhaps into the brain to start causing the changes in Alzheimer’s disease. Though not proven, this may indicate that Alzheimer’s is a consequence late in life, after genes have been passed on to offspring, of the early development of the nervous system which enables our success.
Jeremy Taylor has produced a meticulously detailed account of part of the growing field of evolutionary medicine which is going to affect treatments more and more.
Note: Taylor has produced science programmes for television, including The Blind Watchmaker and Nice Guys Finish First with Richard Dawkins for the BBC. His first book was Not a Chimp: the hunt to find the genes that make us human.
*University of Chicago Press, London (2015). £21.00 Hbk. ISBN 978-0-226-05988-4 (also e-book).