Although filled with conjectures and hypotheses, this book does provide a fascinating glimpse into the current state of evolutionary biology. I got from this book a real sense that there is a lot underlying connectedness between various diseases and genetic and environmental factors that remains undiscovered, but which has been the source of speculation for ages.
Ch.1. Ironing it out
Hemochromatosis is an inherited disease in which the body continues to load up on iron because the mechanism regulating iron has broken down. The suggestion is that natural selection is maintaining this genetic defect because it had conferred some benefit in the past.
The body has several mechanisms that lock down access to iron because infectious bacteria thrive on iron. At first it would seem that people with hemochromatosis would be at an additional risk from infections because bacteria would multiply rapidly because of the extra iron. It turns out that people with hemochromatosis also have a problem with macrophages which are blood cells and part of the immune system. In a person with hemochromatosis, macrophages are deficient in iron even though there is an excess of iron elsewhere in the body because of the disease. This iron-deficiency in macrophages has unexpected benefits.
Macrophages normally carry off infectious agents to the lymphatic system. But if the infectious agents are able to use the iron in macrophages then the agents can feed and multiply. Tuberculosis and the bubonic plague are examples of infectious agents that exploit the iron in macrophages. During the bubonic plague, infected people would have painfully swollen lymph nodes bursting through their skin. However, people with hemochromatosis would have iron-starved macrophages that could resist the bubonic plague, and therefore be more likely to survive and reproduce. The gene for hemochromatosis would then be selected for and carried forward in future populations.
The long held tradition of bloodletting to fight disease may therefore have had a good basis in fact after all. Starving infectious bacteria of iron by reducing the amount of blood is the surprising benefit that might explain why people believed in the value of bloodletting for so long. There might have been enough situations when bloodletting resulted in recovery to explain why the belief in the procedure persisted for such a long time.
Ch. 2. A spoonful of sugar helps the temperature go down
Diabetes is a disease where the body fails to remove excess sugar from the bloodstream. Dr. Moelem suggests that excess sugar in the blood could have been selected for in the past because sugar can act like antifreeze in times of extreme cold.
About 13,000 years ago, the retreating glaciers had a sudden reversal that lasted a thousand years. This period was called the Younger Dryas because pollen from an Arctic wildflower called Dryas octopetala was found in mud cores dating back to 12,000 years ago. This wildflower only flourished during times of extreme cold. According to Dr. Moelem it’s possible that the human beings who survived the Younger Dryas did so because of excess sugar acting as an antifreeze in their bloodstream. This adaptation might be similar to that of the wood frog’s, which can freeze solid and then recover completely when the temperature rises to the right level. One interesting bit of evidence is that those people with a propensity for diabetes are descended from people who lived in exactly those places hit by the ice age 13,000 years ago.
Ch. 3. The cholesterol also rises
This chapter I found immensely interesting because of the news about Vitamin D (the “sunshine” vitamin) and its tremendous benefit in preventing many diseases, not just rickets. There is enough Vitamin D added to milk to prevent rickets, but not enough to bring about the extra benefit. Those living in northern latitudes are at a disadvantage because several minutes of directly overhead sunshine is what is needed to convert cholesterol in our bodies into enough Vitamin D to have an effect. Taking Vitamin D supplements is also an option, as is an ultraviolet B tanning salon or eating fatty fish as the Inuit do– cod liver oil will do the trick as well.
While the sun makes Vitamin D, it unfortunately destroys folic acid. Skin colour is the adaptation that balances out the effect of the sun. People with dark skin can protect themselves from folic acid depletion, but they need to carry higher amounts of cholesterol to maximize whatever sunlight gets through the skin. In northern climates, paler skin has the advantage of letting more sun through to produce Vitamin D. It only takes a 1000 years for a population to change its skin colour if it moves from one climate to another.
Avoid sunglasses while sunbathing. The pituitary gland is involved in helping produce melanin, which darkens the skin. Unfortunately, the pituitary gland gets its signal from the optic nerve, so if it’s covered by sunglasses then the body will not get the right signal about the direct sunlight.
While tanning and therefore making Vitamin D the natural way, make up for the lost folic acid. Spinach and cabbage are good sources of folic acid (also called folate).
Damaged celery produces psoralen, a toxin that can cause extreme sensitivity to sunlight. Don’t eat celery soup before sunbathing.
Ch. 4. Hey, bud, can you do me a fava?
Many plants produce toxins to protect themselves from predators. I remember reading with some amusement that the Greek philosopher Pythagoras warned about beans, particularly fava beans. It turns out the story may have had some basis in a real medical condition.
“Where there’s folklore smoke, there’s medical fire–in the case of the fava bean, a whole lot of it.”
People with favism cannot clear out the free radicals produced by fava beans because of a genetic deficiency. The free radicals attack red blood cells. It turns out that people with this genetic deficiency end up being more resistant to malaria. Malarial parasites find the red blood cells of people with favism not to their liking.
Ch. 5. Of microbes and men
The parasitic worm, Dracunuculus medinensis, which means little dragon, has been a common problem across Africa and Asia. Also known as the Guinea worm, it grows to 2 or 3 feet long in the infected person before making its way to the skin and burning through it with acid. The only effective treatment has been to wrap the worm around a stick and slowly pull it out–a process that could last for days or weeks. Some scholars think that the Rod of Asclepius was originally a drawing that early doctors used to advertise that they could remove the worm. Today we know that symbol as a snake wrapped around a staff.
“If you’ve come this far on our journey across the evolutionary landscape, you’ve probably gathered a good sense of the interconnectedness of–well, just about everything. Our genetic makeup has been adapting in response to where we live and what the weather’s like. The food we eat has evolved to cope with the organisms that eat it, and we’ve evolved to cope with that. We’ve looked at the way we’ve evolved to resist or manage the threat posed by specific infectious diseases, like malaria… At the end of the day, every living thing–bacteria, protozoa, lions, tigers, bears, and your baby brother–shares two hardwired imperatives: Survive. Reproduce.”
Another possible origin of folklore in this chapter– rabies being the basis for werewolf stories. One bite “transforms the victim into a possessed beast just like the biter.”
A suggestion to combat virulent infectious diseases that become resistant to drug treatments: change the nature of the infection such that the bacteria won’t need to incapacitate us. For example, protecting malaria patients with mosquito nets will mean the microbe will be more successful if infected people are able to walk around as if they had little more than colds. It would then be advantageous for the infectious agent to be less deadly, and by extension we would suffer less. Let evolution work in our favour.
Ch. 6. Jump into the gene pool
In recent years scientists have discovered that DNA can be modified in more than ways that than just by random mutations. A form of Lamarckism, or the inheritance of acquired traits, is making a comeback after being considered superceded by Darwin’s Origin of the Species. Barbara McClintock produced evidence in the 1950s of “jumping genes”–whole sequences of DNA that moved from one place to another during times of some environmental stress. Sometimes these jumping genes, or transposons, perform a “cut and paste” and other times they perform a “copy and paste.” Large portions of our junk DNA (DNA that does not code for proteins) are made of jumping genes.
Retroviruses are made of RNA, and can be written into DNA. HIV is a retrovirus, and the drug “cocktail” therapy used to combat HIV is aimed at stopping the enzyme that helps the retroviruses become part of DNA. The retroviruses that are part of our DNA are called HERVs, or human endogenous retroviruses, and they are believed to play a number of roles, such as producing a healthy placenta.
Ch. 7. Methyl madness : road to the final phenotype
Epigenetics is a whole new field in genetics that is concerned with the study of how children can inherit and express new traits derived from their parents without changes in the underlying DNA. What this means is that environmental conditions affecting mothers, and even sometimes fathers and grandmothers, can affect how genes are expressed in newborns. For example, a gene for thriftiness in storing fat may be expressed because the fetus detects the mother’s poor nutritional habits. This might explain in part the rise of obesity because the effects of overeating but eating poorly can be passed onto the children’s genes. The body will store fat more efficiently because it believes it is in a poor nutrition environment, when in fact a typical Western diet will result in obesity.
The Human Genome Project mapped out the sequence of nucleotide pairs in DNA, but the Human Epigenome Project will attempt to identify all the spots where methyl markers can attach and change the expression of a given gene.
Ch. 8. That’s life : why you and your iPod must die
A great quote on having a baby…
“Over the next nine months, millions of years of interaction with disease, parasites, plagues, ice ages, heat waves, and countless other evolutionary pressures–not to mention a little romance–will come together in a stunningly complex interaction of genetic information, cellular reproduction, methyl marking, and the commingling of germ lines to produce your little peanut.”
Another section in this chapter I found quite compelling. Elaine Morgan, a Welsh writer, challenged the male-centered savannah theory that explains why humans walk upright and have little hair. Her theory is the “aquatic ape theory”– the idea being that we are descended from a small population that lived in and around water, somewhere in what is now Ethiopia.
Points in favour of the aquatic ape theory:
- humans are the only land animals with fat attached to our skin (like hippos, sea lions, and whales)
- the ability to survive in both land and water environments has tremendous survival benefits against predators
- walking upright helps in venturing into deeper water and so this is how bipedalism may have come about
- we may have lost our fur so as to become more streamlined in the water
- we have downward-facing nostrils, which allowed us to dive
- the extra fat in babies help keeps them afloat
- newborns don’t start breathing until they feel air on their face, which makes birthing in water very safe, and actually easier on the mother
- newborns reflexively hold their breath, but they also make rhythmic movements that propel them through the water (a behaviour which lasts about four months)–this makes more sense in the aquatic ape theory than in the savannah theory