October 27, 2010

Evolutionary trail

When my brother, a doctor, worked in rural Orissa several years ago, he had an attack of malaria. While it lasted, it was a frightening episode. He had very high fever and a bout of convulsions. Just as frightening was that the malaria did not respond to treatment with the drug chloroquine -- a standard prescription for malaria. When he realized this, he switched to two other drugs -- sulfadoxine and pyrimethamine -- and recovered.

But for me, his experience was a revealing glimpse into the constant battle modern medicine must fight to control disease. Why was his malaria resistant to chloroquine?

To answer that, we have to go all the way back to Charles Darwin and his
theory of evolution, or natural selection. In essence, the theory says that as species evolve over time, they retain and develop those characteristics which promote their survival and reproduction. This means that evolution also suppresses the characteristics that retard survival and reproduction. Only the individuals who survive can reproduce. Their descendants tend to retain their capacity to survive, and pass them on in their turn. At the same time and in the same way, whatever characteristics acted against survival tend to vanish.

So what happened with malaria and chloroquine? In India, malaria comes in two main strains, caused by two different microscopic parasites carried by mosquitos: vivax and falciparum. The falciparum strain of malaria can affect the brain: when that happens it is called cerebral malaria. This is what my brother suffered in Orissa.

Chloroquine was an effective treatment, used heavily and widely, against falciparum. But today, in certain parts of the country, and precisely because chloroquine was used heavily in those parts, falciparum has become resistant to chloroquine.

When it was first used, chloroquine killed falciparum parasites -- it prevented their survival in our bodies. True to Darwin's theory, falciparum, in an evolutionary sense, recognized the threat chloroquine posed to its survival. In each succeeding generation, only those falciparum individuals that were somehow able to survive the chloroquine onslaught managed to reproduce. Doing so, they passed on to their descendants the characteristics that helped them to survive. Over several generations, these characteristics got strengthened -- selected for, in other words -- and a greater and greater proportion of the falciparum population had them. Eventually, a strain of falciparum appeared that was totally resistant to chloroquine.

There's evolution for you.

What happened to falciparum is a perfectly natural process, simple and with an inexorable logic. It happens to every species on the planet. Humans included. For example, archaeological evidence shows that we are today a taller, stronger race than we were at the dawn of our history. Why has this happened?

You might look at it this way: In each generation, across the whole human population, it was generally the taller and stronger people who had the best chance to reproduce. Thus these were favoured characteristics that were passed on and strengthened; they were selected for. Each generation was just that much sturdier than its predecessor. So today, many generations later, we would seem like giants to our ancestors. "Goliaths!" they might call us in derision. (Of course, we could always shoot back: "Lilliputs!")

But we were discussing falciparum, remember? Natural selection applies in exactly the same way to humans and to falciparum parasites. There is one crucial difference, however, and that takes us to the heart of the tussle between disease and medicine.

In humans, evolution is a slow, measured process. Over a few thousand years, we are only a few inches taller, on average, than our ancestors were. It takes several generations for evolutionary changes to be noticed, and among us, that means hundreds of years. We procreate some twenty or thirty years after we are born. That is how long it takes for a desirable characteristic -- height, for example -- to be passed on.

In contrast, falciparum lives and breeds at a rate measurable in hours and minutes. All micro-organisms do. Those that cause the plague, for example, live for just half an hour. Generation follows generation at breakneck speed. Naturally, evolution also proceeds at breakneck speed, not at the stately human pace. Traits that contribute to survival -- here, the resistance to chloroquine -- are passed on and reinforced swiftly. In some cases, it is just a few weeks before resistance begins to appear.

Evolution, you see, has turned around to bite us -- and in the case of malaria and mosquitos, quite literally so. Whenever a new drug to treat a disease is discovered, it is only a matter of time before the disease, inevitably, stops responding to it. Natural selection ensures that, just as it ensures that our descendants will be generally taller than we are.

Medicine, therefore, is on a perennial treadmill. To date, it has managed to stay one step ahead of disease by the constant discovery of new drugs. But it is a precarious tightrope we walk. Who knows when we will lose the advantage of being that small step ahead?

Which is why, in the long run, prevention and precaution are better bets than cures and treatments. That means good health, exercise, regular
habits, cleanliness in our homes and outside: simple, basic ideas that hold the key to our survival.

Now, if only they get passed on to our descendants as well.


Anonymous said...

Nice informative post.

Re. modern man being taller and stronger than the ancient one, I'd always thought that was the other way round!

Not spent any time looking into it and if you have evidence you have it.

It had seemed that as man progressively could survive more by his wits and weaponry his evolutionary need to be big and brawny reduced. I expected our ancestors were rather hulky and strong.


Anonymous said...

This is all very well but where is the hand of God? Does He, with infinite detail, determine which member lives and which dies or this only applies to his chosen set made in His image? As a thinking man, D'souza, how do you define good and evil in this framework? This is not purely a theoretical question. Does one let the weak and disadvantaged fall, unfit, or....?

Anonymous said...

I guess you didn't want to touch that last comment re: God, recoiling as the bird who accidentally touches the best-surviving poisonous reptile.

There must be an evolutionary advantage to witholding your thoughts? I mean evolution says "it is what it is" -- how then shall we choose the correct path? Anthropomorphically, ethnocentrically or ...what is the guiding principle? Survival of the maximum of your local gene pool?

zap said...

Great post Dilip. Some of this easy flowing information will be used to my lil son..
When are the trolls on your posts evolving though? :)

maitreyi said...

hey i was aware of the evolutionary trail but your post enlightened me further! good one